1996 caffeic acid j electroanal chem 405 169
TRANSCRIPT
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8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
1/8
E L S E V I E R J o u r n a l o f E lec t r n an a lyt i ca l C h emis t r y 4 0 5 ( 1 9 9 6 ) 1 6 9 - 1 7 6
JOURN L OF
x idat ion o f ca f fe ic ac id and re la ted hydroxy c innam ic ac ids
P h i l i p p e H a p i o t a A n d r e a s N e u d e c k a J e a n P i n s o n
a
H 6 1 ~ ne F u l c r a n d b
P e d a t s u r N e t a c C h r i s ti a n R o l a n d o d
a L a b o r a t o i r e d ' E l e c t r o c h i m i e M o l ~ c u l a i r e d e l " U n i v e r s i t ~ P a r i s 7 , U n i t ~ d e R e c h e r c h e A s s o c i ~ e a u C N R S N o . 4 3 8 , 2 p l a c e J u s s i e u ,
7 5 2 5 1 P a r i s C e d e x 0 5 , F r a n c e
b L a b o r a t o i r e d e s P o l y m b r e s e t d e s T e c h n i q u e s P h y s i c o - c h i m i q u e s , l n s t i t u t d e s P r o d u i t s d e l a V i g n e , 9 , p l a c e V i a l a , 3 4 0 6 0 M o n t p e l l i e r C e d e x , F r a n c e
c C h e m i c a l K i n e t i c s a n d T h e r m o d y n a m i c D i v i s io n , N a t i o n a l I n s t i t u te o f S t a n d a r d s a n d T e c h n o l og y , G a i t h e r sb u r g , A I D 2 0 8 8 9 , U S A
a L a b o r a t o i r e d e l ' A c t i v a t i o n M o l ~ c u l a i r e , E c o l e N o r m a l e S u p ~ r i e u r e , U n i t ~ A s s o c i ~ e a u C N R S N o . 1 1 1 0 ,
U n i t~ A s so c i ~ e ~ l ' U n i v e r si t ~ P i e r r e e t M a r i e C u r i e, 2 4 r u e L h o m o n d , 7 5 2 3 1 P a r i s C e d e x 0 5 , F r a n c e
Receiv ed 29 Septem ber 1995; in revised form 16 Octobe r 1995
A b s t r a c t
Oxidation of caffeic acid (3,4-dihydroxycinnamic acid) 1H a has been studied by electrochemical methods and by pulse radiolysis in
aqueous and organic solvents. The results h ave been c omp ared with the behaviour o f 4-coumaric acid 2 H 2 and ferulic acid 3H 2. The first
oxidative intermediates have b een characterised by the ir UV spectra and oxidation potentials. In the ca se o f 2H 2 and 3H 2, the initial
radicals d ecay by a second order process indicating a radic al-rad ical coupling mechanism. On the contrary, fo r caffeic acid the oxidation
leads to the form ation of the corresponding o-quinone through disproportionation of the initial semiquinone radical.
K e y w o r d s : Electrooxidat ion; Pulse radiolys is ; Caffeic acid ; 4-Couma ric acid ; Ferul ic acid
1 I n t r o d u c t i o n
P o l y p h e n o l s a x e t h e m a i n s e c o n d a r y m e t a b o l i t e s i n
p l a n ts . T h e y p a r t l y d e t e r m i n e t h e q u a l i t y o f t h e p l a n t - d e -
r i v e d f o o d p r o d u c t s , c o n f e r r i n g u p o n t h e m b e n e f i c i a l o r
d e t r i m e n t a l e f f e c t s . I n p a r t i c u l a r , t h e b r o w n i n g p r o c e s s
w h i c h a l s o t a k e s p l a c e i n w i n e s , m a y b e d e s i r a b l e f o r s o m e
o f t h e m ( s h e r r y , M a d e i r a , M a r s a l a ) w h e r e a s i t m a y b e
u n d e s i r a b l e f o r o t h e r s ( w h i t e w i n e s ) [ 1 ]. T h e b r o w n i n g i s
r e l a t e d t o t h e e n z y m a t i c o x i d a t i on o f p h e n o l s t o t h e c o r r e -
s p o n d i n g h i g h l y r e a c t i v e o - q u i n o n e s [ 2 ,3 ] . E n z y m a t i c o x i -
d a t i o n i s t h e m o s t i m p o r t a n t r e a c t i o n i n t h e p r e s e n c e o f
p o l y p h e n o l o x i d a s e [ 4 , 5 ] . H o w e v e r , n o n - e n z y m a t i c a u t o -
o x i d a t i o n c a n a l s o c a n t a k e p l a c e i n t h e p r e s e n c e o f
o x y g e n , m o r e e s p e c i a l l y w h e n t h e m e d i u m i s a l k a l i n e [ 6 ] .
T h e d i f f e r e n t s t e p s l e a d i n g t o t h e f o r m a t i o n a n d t o f u r t h e r
r e a c t i o n o f q u i n o n e s t o p r o d u c e c o n d e n s a t i o n p r o d u c t s a r e
n o t f u l l y u n d e r s t o o d a n d o n l y a f e w h y p o t h e s e s h a v e b e e n
p r o p o s e d c o n c e r n i n g t h e i r m e c h a n i s m [ 7 , 8 ] .
E l e c t r o c h e m i s t r y [ 9 - 1 1 ] a n d p u l s e r a d i o l y s i s [ 9 , 1 2 ]
t e c h n i q u e s p r o v i d e p o w e r f u l t o o l s f o r th e s t u d y o f r e a c t i o n
m e c h a n i s m s i n v o l v i n g e l e c t r o n t r a n s f e r a n d a f f o r d c o m p l e -
m e n t a r y i n f o r m a t i o n [ 9 , 1 3 - 1 6 ] . T h e f i r s t t e c h n i q u e a l l o w s
t h e c h a r a c t e r i s a t io n o f t h e i n t e r m e d i a t e s f o r m e d u p o n e l e c -
t r o n t r a n s f e r b y t h e i r r e d u c t i o n - o x i d a t i o n p a t t e r n [ 1 0 , 1 1 ] .
0 0 2 2 - 0 7 2 8 /9 6 / 1 5 . 0 0 © 1 9 96 E l s ev ie r S c ien ce S .A . A l l r ig h t s r e s e r v ed
S S D 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 4 1 2 -4
I t a l s o c a n p r o v i d e s p e c t r o s c o p i c i n f o r m a t i o n o n i n t e r m e d i -
a t e s w i t h l i f e t i m e s f r o m 1 s t o m i n u t e s b y c a r r y i n g o u t
e l ec t ro ly s i s i n a t h in l ay e r sp ec t ro e l ec t ro ch em ica l ce l l [1 7 ] .
I n s p i t e o f th e w i d e r a n g e o f l i f e t i m e s c o v e r e d b y c o m b i n a -
t i on o f t h e a b o v e e l e c t r o c h e m i c a l t e c h n i q u e s , i t i s d e s i ra b l e
t o h a v e a d d i t i o n a l i n d e p e n d e n t m e t h o d s t h a t a l lo w v e r i f i c a -
t i on a n d c o m p a r i s o n o f t h e r e s u l ts . F o r t h i s p u r p o s e , w e
u t i l i s ed p u l se r ad io ly s i s w h ich p e rm i t s t h e ch a rac t e r i sa t i o n
o f s h o r t - l i v e d i n t e r m e d i a t e s b y r e c o r d i n g t h e i r U V - v i s
s p e c t r a . T h e i r r e a c t i o n k i n e t i c s c a n a l s o b e f o l l o w e d s t a rt -
i n g in t h e m ic ro seco n d r an g e [1 2 ] .
W e h a v e u s e d t h e s e t e c h n i q u e s t o i n v e s ti g a t e t he o x i d a -
t i o n o f c a f f e i c a c i d 1 H 3 ( 3 , 4 - d i h y d r o x y c i n n a m i c a c i d ), a n
a n a l o g u e o f c a f t a r i c a n d c o u t a r i c a c i d s w h i c h a r e r e s p o n s i -
b l e f o r t h e b r o w n i n g o f w h i t e w i n e s . W e a l s o h a v e e x a m -
i n e d t h e o x i d a t i o n o f 4 - c o u m a r i c a c i d 2 H 2 ( 4 - h y d r o x y c i n -
n a m i c a c i d ) a n d f e r u l ic a c i d 3 1 -1 2 ( 3 - m e t h o x y - 4 - h y d r o x y -
c i n n a m i c a c i d ) . C o m p o u n d s o b t a i n e d b y c h e m i c a l o x i d a -
t i o n ( s o d i u m p e r i o d a t e ) o f c a f f e i c a c i d i n a c i d i c c o n d i t i o ns
( p H = 2 - 7 ) h a v e b e e n p r e v i o u s l y c h a r a c t e r i s e d a s t w o
s t e r e o i s o m e r s o f 2 , 5 - ( 3 , 4 - d i h y d r o x y p h e n y l ) t e t r a h y d r o -
f u r a n - 3 , 4 - d i c a r b o x y l i c a c i d [ 1 8 ,1 9 ] . T h e h a l f - w a v e p o t e n -
t ia l o f t h e o - q u i n o n e f o r m e d i n t h e o x i d a t i o n o f c a f f e ic
a c i d h a s b e e n d e t e r m i n e d b y p o l a r o g r a p h i c r e d u c t i o n i n
w a t e r b e t w e e n p H 1 a n d 9 [ 2 0 ] . T h e o n e - e l e c t r o n r e d o x
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8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
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17 P. Hapiot et al. / Journal o f Electroanalytical Chemistry 405 1996) 169-176
p o t e n t i a l o f c a f f e i c a c i d h a s b e e n m e a s u r e d b y p u l s e
rad io ly s i s 0 . 0 8 4 V S H E ) ) [2 1] an d th e r eac t i o n o f 4 -h y -
d r o x y c i n n a m i c a c i d w i t h s e v e r a l r a d i c a l s h a s b e e n e x a m -
ined [22].
H O
H O ~ C O O H
C a f f e i c a c i d 1 H 3
H O ~ C O O H
C o u m a r i c a c i d 2 H z
H C o H ~ C O O H
F e r u l ic a c i d 3 I t 2
p u r p o s e - b u i l t p o t e n t i o s t a t e q u i p p e d w i t h a p o s i t i v e f e e d -
b a c k c o m p e n s a t i o n d e v i c e [ 2 3 ] . T h e d a t a w e r e a c q u i r e d
w i th a 3 1 0 N ico l e t o sc i l l o sco p e .
S p e c t r o e l e c t r o c h e m i c a l e x p e r i m e n t s w e r e p e r f o r m e d
w i t h a c a p i l l a r y - s li t - c e l l f o r U V - v i s s p e c t r o s c o p y u s in g
g o l d - L I G A - s t r u c t u r e s a s a n o p t i c a l t r a n s p a r e n t e l e c t r o d e
[ 1 7] . T h e g o l d - L I G A - f o i l s u s e d w e r e 1 0 0 / x m t h i c kn e s s
a n d o f a h o n e y c o m b s t r u ct u re w i t h 2 0 / z m d i a m e t e r h e x a g -
o n a l h o l e s w i th a w a l l t h i ck n ess b e tw een th e h o l e s o f 1 5
/ z m . T h e g o l d - L I G A - s t r u c t u r e w a s f i x e d i n a c a p i ll a r y s li t
o f 1 5 0 / z m . T h e c o m p l e t e e l e c t r o c h e m i c a l c o n v e r s i o n i n
th e cap i l l a ry s l i t o ccu r r ed i n l e s s t h an 2 s . A s i l v e r w i r e
c o a t e d w i t h s i l v e r c h l or i d e w a s u s e d a s r e f e r e n c e e l e c t r o d e .
I n o r g a n i c s o l v e n t s , t h e c o r r e s p o n d i n g p h e n o l a t e s w e r e
p r e p a r e d i n s i t u b y n e u t r a l i s a t i o n a f t e r r e m o v a l o f o x y g e n
b y b u b b l i n g a r g o n t o p r e v e n t a i r o x id a t i o n o f t h e p h e n o l a t e
[ 24 ]. T h e b a s e w a s a 2. 7 M s o l u t io n o f N M e a O H - 5 H 2 0
i n m e t h a n o l .
2.3 . Radiolyt ic experiments
2 Ex p er im en ta l
2.1. Chemicals
A c e t o n i t r il e A C N ) w a s U v a s o l q u a l i ty M e r c k ) a nd
w a s u s e d a s - r e c e i v e d . D i m e t h y l s u l p h o x i d e w a s f r o m
A l d r i c h . T h e s u p p o r t i n g e l e c t r o l y t e N E t 4 B F 4 an d th e b ase
N M e 4 O H - 5 H 2 0 w e r e f r o m F l u k a p u ri s s) . C a f f e ic a c id
a n d t h e d i f f e r e n t h y d r o x y c i n n a m i c a c i d s w e r e c o m m e r -
c i a l l y a v a i l a b l e A l d r i c h ). C a f f e i c a c i d w a s p u r if i e d b y
f l a s h c h r o m a t o g r a p h y b e f o r e u s e . p H s i n h y d r o - o r g a n i c
s o l v e n ts w e r e m e a s u r e d w i t h a g l a s s e l e c t ro d e s t a n d a r d i s e d
i n a q u e o u s b u f f e r s .
2.2. Cyclic voltamm etry an d spectroelectrochemical exper
iments
A l l t h e c y c l ic v o l t a m m e t r y e x p e r i m e n t s w e r e c a r r i e d
o u t a t 2 0 + 0 .1 ° C u s i n g a c e l l e q u i p p e d w i t h a j a c k e t
a l l o w i n g c i r c u l a t i o n o f w a t e r f r o m t h e t h e r m o s t a t . T h e
c o u n t e r e l e c t r o d e w a s a P t w i r e a n d t h e r e f e r e n c e e le c t r o d e
a n a q u e o u s s a t u r a t e d c a l o m e l e l e c t r o d e E ° S C E ) = E °
S H E ) - 0 . 2 4 1 2 V ) w i th a s a l t b r id g e co n ta in in g t h e su p -
p o r t i n g e l e c t r o d e . T h e S C E e l e c t r o d e w a s c h e c k e d a g a i n s t
t h e f e r r o c e n e / f e r r o c e n i u m c o u p l e E ° = + 0 . 40 5 V S C E ) )
b e f o r e a n d a f t e r e a c h e x p e r i m e n t .
F o r l ow s c a n r a te c y c l ic v o l t a m m e t r y 0 . 0 5 - 5 0 0 V
s - 1 , t h e w o r k i n g e l e c t r o d e w a s e i t h e r a g l a s s y c a r b o n d i s k
3 m m d i a m e t e r T o k a i C o r p .) o r a 1 m m d i a m e t e r g o l d
d i s k . T h e y w e r e c a r e f u l l y p o l i s h e d b e f o r e e a c h v o l t a m m o -
g r a m w i t h 1 / x m d i a m o n d p a s t e a n d u l t r a s o n i c a l l y r i n s e d
i n a b s o l u t e e t h a n o l . E l e c t r o c h e m i c a l i n s t r u m e n t a t i o n c o n -
s i s te d o f a P A R M o d e l 1 75 U n i v e r s a l p r o g r a m m e r a n d a
F o r r a d i o l y s i s e x p e r i m e n t s , f r e s h s o l u t i o ns o f t h e p h e n o -
l i c c o m p o u n d s u n d e r a n a e r o b i c c o n d i t i o n s w e r e p r e p a r e d
b y b u b b l i n g N 2 0 i n w a t e r p u r i f ie d w i th a M i l li p o r e
S u p e r - Q s y s t e m ) a n d t h e n a d d i n g N a N 3 a nd K O H a n d
f i n a l l y t h e p h e n o l . T h i s p r o c e d u r e a v o i d s c o n t a c t b e t w e e n
th e p h en o la t e a n d 0 2 an d p rev en t s au to -o x id a t io n . T h e
so lu t i o n w as t h en t r an s fe r r ed i n to t h e i r r ad i a t i o n ce l l w i th -
o u t b e i n g e x p o s e d t o a i r . T h e p u l s e r a d i o l y s i s a p p a r a t u s
h as b een d esc r ib ed b e fo re [2 5 ] . I t u t i l i s e s 5 0 n s p u l ses o f 2
M e V e l e c t r o n s f r o m a F e b e t r o n 7 0 5 a c c e l e r a t o r . T h e d o s e
p e r p u l s e w a s v a r i e d b e t w e e n 8 a n d 8 0 G y , t o p r o d u c e
b e t w e e n 5 a n d 5 0 m M o f o x id i s i n g r a d i c a ls , a s d e t e rm i n e d
b y S C N - d o s i m e t r y [ 26 ]. T h e k in e t i c s p e c t ro p h o t o m e t r i c
d e t e c t i o n s y s t e m c o n s i s t e d o f a V a r i a n 3 0 0 W x e n o n l a m p ,
a 2 cm o p t i ca l p a th l en g th i r r ad i a t i o n ce l l , a K ra to s h ig h
i n t e n s i t y m o n o c h r o m a t o r , a n R C A 4 8 4 0 p h o t o m u l t i p l i e r ,
an d th e ap p ro p r i a t e sh u t t e r s , l en ses , an d o p t i ca l f i l t e r s . T h e
s ig n a l s w ere d ig i t i s ed w i th a T ek t ro n ix 7 6 1 2 t r an s i en t
r e c o r d e r a n d a n a l y s e d b y c o m p u t e r . A l l e x p e r i m e n t s w e r e
car r ied ou t a t ro om tem per atu r e 22 __+ 2°C.
3 Resu l t s
3.1. O xidation of caffeic acid
3.1 .1 . Cycl ic vol tammetry
T h e o x i d a t i o n o f c a f f e i c a c i d 1 H 3 w a s f i r s t s t u d ie d b y
c y c l i c v o l t a m m e t r y i n w a t e r a t d i f f e r e n t p H v a l u e s a n d i n
sev e ra l w a te r + o rg an ic so lv en t m ix tu res . A ty p i ca l
v o l t a m m o g r a m o f t he o x i d a t i o n o f a 1 0 - 3 m o l 1 - 1 c a f f e i c
ac id so lu t i o n i n w a te r a t p H = 4 a t a s can r a t e o f 0 . 2 V s -
i s s h o w n i n F ig . 1 . A c h e m i c a l l y r e v e r s i b l e v o l t a m m o g r a m
E p~ = + 0 . 2 7 ,
E p a =
+ 0 . 3 9 V S C E ) ) i s o b s e r v e d t h a t
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8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
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P . Ha p io t e t a l . / J o u rn a l o f E l e c t ro a n a ly t i c a l C h e mis t ry 4 0 5 1 9 96 ) 1 6 9 - 1 7 6 1 71
20 ~.A
~ r
I
- 0 . 4 0 . 0 0 . 4
E / V SC E)
F i g . 1 . C y c l i c v o l t a m m e t r y o f 1 0 - 3 t o o l 1 - J s o l u t i o n o f c a f f e i c a c i d in
w a t e r a t p H = 4 o n a 3 m m g l a s s y c a r b o n e l e c t r o d e . S c a n r a t e v =
0 . 2 V s - I .
can be cycled without noticeable decrease of the current.
The number of electrons per molecule was determined to
be two by comparison with the intensity of the peak
current of ferrocenemethanol under the same conditions.
The same general pattern is observed in (50/50) water
(pH = 4) + dimethyl formamide (Er~ = + 0.21, E v a =
+ 0.36 V (SCE)) and (50/50) water (pH = 4) + methanol
(E ~ = +0. 15, Eva = +0 .2 6 V (SCE)) mixtures. The elec-
trochemical behaviour was studied as a function of pH in a
(5 0/5 0) water + methanol mixture. By increasing the pH
(of the water component) above four, the oxidation poten-
tial becomes gradually more positive and the peak poten-
tial separation increases. This behaviour is in agreement
with what is expected for a quinone + hydroquinone sys-
tem. At pH > 10 the reverse reduction peak disappears,
indicating that under these conditions the generated o-
quinone is not stable.
In dimethylsulphoxide (D MSO) solutions containing 0.1
M NEtaBF4 as supporting electrolyte, on a glassy carbon
electrode, 1H 3 shows an irreversible oxidat ion peak at 0.2
V s -l (E p= + 0.8 5 V (SCE)) (see Fig. 2(a)). Upon
addition of two equivalents o f base (NMe4OH • 5H2 0) to
neutralise both the COOH group and one of the phenol
groups, to form 1 a 2 - , the oxidation wave appears at a less
positive potential (Fig. 2(b)). This wave becomes totally
reversible at scan rates higher than 0.5 V s- l and irre-
versible for scan rates lower than 0.05 V s -~. The height
of the anodic peak for the oxidation of 1H 2- corresponds
to the transfer of one electron, as shown by comparison
with t r i - t e r t - b u t y l phenolate under the same experimental
conditions. The E°potential for this oxidation can be deter-
mined from the reversible voltammogram as the midpoint
between the anodic and cathodic peak potentials (E ° =
-0 .3 1 V (SCE)) which can be ascribed to the reversible
couple 1 HE -/ 1H - . After the addition of a third equiva-
lent of base in order to achieve complete neutralisation of
the acid, the trianion 13- behaves as a quasi-reversible
system on a gold or G.C. electrode A E p = 90 mY, I v c / l p a
= 1), with E ° = - 0 .8 6 V (SCE), down to a scan rate of
50 mV s -~ (Fig. 2(c)). This first reversible system is
followed by a second partially reversible one (see Fig.
2(c)). These two peaks were found to be monoelectronic
and were ascribed to the reversible couples 1 3 - / 1 2 - . a n d
1 2 - / 1 -. Similar studies performed in ACN exhibit the
same general features with minor differences: the oxida-
tion of 1H a presents an irreversible peak around + 1.2 V
(SCE) (more positive than in DMSO) and the second
oxidation of 13- is completely irreversible. The electro-
chemical characteristics are summarised in Table 1.
I ~ (a)
I I I
- 0 . 7 0 . 1 0 . 9
o.5~ ~ b)
- 0 . 7 - 0 . 3 0 . 1
o . 4 ~ j ~
(c)
I [ I
- 1 . 2 - 0 . 4 0 . 4
E / V S C E )
F i g . 2 . C y c l i c v o l t a m m e t r y o f t h e o x i d a t i o n o f 1 0 - 3 m o l 1 - ~ s o l u t i o n o f c a f f e i c a c i d i n d i m e t h y l s u l p h o x i d e o n a 1 m m g l a s s y c a r b o n e l e c t r o d e + 0 . I M
N E t 4 B F 4 a s s u p p o r t i n g e le c t r o l y t e ) . a ) I H 3 , b ) I H 2 - , c ) 1 3 - . S c a n r a t e v = 0 . 2 V s - t .
-
8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
4/8
-
8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
5/8
P . Ha p io t e t a l . / J o u rn a l o f E l e c tro a n a ly t ic a l C h e mis try 4 0 5 1 9 9 6 ) 1 6 9 - 1 7 6
173
O . 2
0.16
0.12
0 . 0 8 •
• I
0 . 0 4
0 . . . . | * i i . . . . i . . ~ - . . ~
3 8 0 4 2 0 4 6 0 5 0 0 5 4 0
~ , / n m
Fig. 4. D ifferential absorption spectra after pulse radiolysis of
1 0 - 3
mol
1-t of caffeic acid in 0.1 M NaN3 aqueous solution saturated with N20
(pH = 5.7). Monitored af ter 1 gs ( I ) and 10 ms (0) .
c o m p l e t e i n 1 / xs o r l e s s . D i f f e r e n t i a l a b s o r p t i o n s p e c t r a
m o n i t o r e d a t v a r i o u s t i m e s a f t e r t h e p u l s e a r e p r e s e n t e d i n
F i g . 4 f o r s l i g h t l y a c i d i c p H ( p H = 5 .7 ) . T h e i n i ti a l s p e c -
t r u m ( 1 / z s a f te r t h e p u l s e ) c a n b e a s s i g n e d t o th e c o r r e -
s p o n d i n g o - s e m i q u i n o n e ; t h e s p e c t r u m o b t a i n e d a f te r 1 m s
i s a s c r i b e d t o s u b s e q u e n t p r o d u c t s . T h e o - s e m i q u i n o n e
e x h i b i t s a b r o a d p e a k w i t h a m a x i m u m a r o u n d 4 8 0 n m
( 6 = 1 9 0 0 1 m o l - ] c m - 1 ) s i m il a r t o t he s p e c tr a o f o t h e r
c o n j u g a t e d p h e n o x y l r a d i c a l s [ 1 5 , 1 6 ] . T h i s a b s o r p t i o n i s i n
t h e s a m e w a v e l e n g t h r a n g e a s w e f o u n d b y s p e c t r o e l e c t r o -
c h e m i s t r y f o r th e s a m e i n t e r m e d i a t e i n A C N . H o w e v e r , i n
t h e r a d i o l y s i s e x p e r i m e n t s a t p H = 6 - 1 2 , t h e s p e c t r u m
s h o u l d c o r r e s p o n d t o t h a t o f t h e d e p r o t o n a t e d f o r m o f t h e
s e m i q u i n o n e 1 2 - . a s th e p K a o f o - s e m i q u i n o n e r a d i c a l s
a r e k n o w n t o b e a r o u n d f i v e [ 2 8 ] . A f t e r 1 0 m s a b r o a d
p e a k i s o b s e r v e d w i t h A n ~x = 4 1 0 n m , w h i c h c a n b e
a s c r i b e d t o t h e o - q u i n o n e 1 H b y c o m p a r i s o n w i th a n
a u t h e n t i c s a m p l e u n d e r i d e n t i c a l c o n d i t i o n s [ 1 9 ] . B y v a r y -
i n g t h e d o s e p e r p u l s e , t o p r o d u c e d i f f e r e n t i n i t i a l c o n c e n -
t r a t io n s o f t h e s e m i q u i n o n e r a d i c a ls , 1 2 - ' , i t w a s f o u n d t h a t
t h e d e c a y o b e y e d a s e c o n d - o r d e r r a t e l a w , w i t h 2 k ] = 8 . 4
X 10 7 1 m o l - 1 s - 1 . ( s e e F i g . 4 ). A t t h is p H , s o m e o t h e r
a b s o r p t i o n s c o r r e s p o n d i n g t o d i f f e r e n t s p e c i e s w i t h d i f f e r -
e n t k i n e t i c e v o l u t i o n s w e r e a l s o o b s e r v e d a t l o w w a v e -
l e n g t h s ( A < 4 0 0 n m ) . T h e s e a r e a s c r i b e d t o t h e i n t e r m e d i -
a t e f o r m e d b y t h e r e a c t i o n o f c a f f e ic a c i d w i t h a s m a l l
f r a c t i o n o f t h e s o l v a t e d e l e c t r o n s ( i n c o m p e t i t i o n w i t h t h e
r e a c t i o n o f e~q w i t h N 2 0 ) . T h i s w a s d e m o n s t r a t e d b y
a d d i t i o n o f i s o p r o p a n o l t o t h e s o l u t i o n t o r e a c t w i t h t h e
O H r a d i ca l , t o p r e v e n t f o r m a t i o n o f t h e o x i d i s i n g ra d i c a l
N 3 r a d i c a l , a n d t o l e a v e t h e s o l v a t e d e l e c t r o n a s th e o n l y
r e a c t i v e s p e c i e s r e m a i n i n g i n t h e s o l u t i o n .
S i m i l a r s p e c t r a w e r e o b s e r v e d a t h i g h e r p H b u t , b e c a u s e
o f t h e s t r o n g a b s o r b a n c e o f t h e p a r e n t m o l e c u l e s a t t h e s e
p H v a l u e s , i t w a s n o t p o s s i b l e t o r e c o r d s p e c t r a a t w a v e -
l e n g th s b e l o w 4 2 0 - 4 3 0 n m . T h e a b s o r b a n c e w a s t h e r ef o r e
m o n i t o r e d a t 4 8 0 n m t o s t u d y t h e d e c a y o f t h e in i ti a l
s e m i q u i n o n e . L a r g e v a r i a t i o n s o f t h e s e m i q u i n o n e r a d i c a l
l i f e ti m e w i t h p H w e r e o b s e r v e d ( s e e T a b l e 2 ) . I n b a s i c
s o l u t i o n s ( p H = 1 1 . 3 ) th e s e m i q u i n o n e w a s r e l a t iv e l y s t a-
b l e , w i t h a l i f e - t i m e l o n g e r t h a n 1 0 0 m s , w h e r e a s a t
p H = 5 . 7 t h e r a d i c a l d e c a y e d a b o u t 2 0 0 0 t i m e s m o r e
r a p i d l y . T h e s e f i n d i n g s a r e i n a g r e e m e n t w i t h t h e u s u a l
b e h a v i o u r o f q u i n o n e / h y d r o q u i n o n e c o u p l e s an d c o rr e -
s p o n d t o a d i s p r o p o r t i o n a t i o n r e a c t i o n l e a d i n g t o t h e f o r -
m a t i o n o f th e o - q u i n o n e f r o m t h e s e m i q u i n o n e . F o r e x a m -
p l e , f o r t h e w e l l k n o w n
p - b e n z o q u i n o n e / p - h y d r o q u i n o n e
c o u p l e , t h e s e m i q u i n o n e i n t e r m e d i a te b e c o m e s u n s t a b le a t
p H < 1 0 in w a t e r [ 2 9 ]. A s i m i l a r b e h a v i o u r w a s f o u n d f o r
t h e s e m i q u i n o n e f o r m e d b y o x i d a t i o n o f t h e m e t h y l e s t e r
o f t h e c a f f ei c a c i d , w h i c h h a s a n a b s o r b a n c e p e a k a t 5 2 0
n m ( 6 = 2 8 0 0 1 m o l - t c m - ] ) a n d a l a rg e a b s o r b a n ce i n
t h e U V b e l o w 4 2 0 n m . R a d i o l y t i c o x i d a t i o n s w e r e a l s o
p e r f o r m e d b y c o n t i n u o u s i r r a d i at i o n o f a q u e o u s s o l u t i o n s
t o m o n i t o r s p e c t r a o f s t a b le p r o d u c t s . S o l u t i o n s s i m i l a r t o
t h o s e u s e d i n t h e p u l s e r a d i o l y s i s e x p e r i m e n t s w e r e i r r a d i -
a t e d i n a g a m m a s o u r c e a n d t h e s p e c t r a w e r e m o n i t o r e d
a f t e r v a r i o u s i r r a d i a t i o n t i m e s . T h e r e s u l t s o f th i s t e c h n i q u e
w e r e c o m p a r a b l e w i t h t h o s e o f s p e c t r o e l e c t r o c h e m i s t r y : i n
t h e r a d i o l y s i s t h e o x i d i s i n g s p e c i e s a r e p r o d u c e d c o n t i n u -
o u s l y i n a n h o m o g e n e o u s f a s h i o n , w h i l e i n t h e s e c o n d c a s e
a c o n t i n u o u s o x i d a t i o n i s p r o v i d e d b y t h e e l e c t r o d e . I n
b o t h m e t h o d s t h e r e a c t i o n i s f o l l o w e d b y r e c o r d i n g a b s o r p -
t i o n s p e c t r a . T h e s p e c t r u m o b t a i n e d u p o n i r r a d i a t i o n o f
c a f f e i c a c i d a t p H = 6 . 3 ( F i g . 5 ) s h o w s a d e c r e a s e o f t h e
3 1 0 n m p e a k a n d a n i n c r e a s e o f a b r o a d a b s o r p t i o n a t
3 4 0 - 3 5 0 n m . A f t e r l o n g e r i r r a d i a t i o n t i m e s a l o n g t a i l
g r o w s a t h i g h e r w a v e l e n g t h , a p p a r e n t l y i n d i c a t in g p o l y m e r
f o r m a t i o n .
Table 2
Pulse radiolysis: characteristics of the radicals in w ater
Compound Radical
hma t
¢rnax pH 2k/1.m ol- 1 s- 1
/ n m / 1 m o l -
1 c m i
Caffeic acid (1H 3) 12 . 480 1900 5.7 8.4 × 107
1 1 . 6 2 . 6 X
104
12.6 stable
Caffeic acid methyl ester (4H 2) 4 - 520 2800 8.0 (8 + 3) × 107
4-Coum aric acid (2H 2) 2 - 600 1900 6.3 7.3 × 10s
7.3 6.8 × lOs
12.05 7.5 × l0 s
Ferulic acid (3I-I2) 3 - 550 900 7.1 7.0 × 108
-
8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
6/8
1 7 4 P. Hapiot et al . /Jo ur na l of Electroanalytical Chemistry 405 1996) 169-1 76
3 2 Oxidation of monohydroxycinnamic acids
3 2 1 Cyclic voltammetry
F o r c o m p a r i s o n w i t h t h e r e s u l t s o f c a f f e i c a c id , t w o
m o n o h y d r o x y c i n n a m i c a c i d s w i th t h e s a m e g e n e r a l s k e le -
t o n w e r e e x a m i n e d b y c y c l i c v o lt a m m e t r y u n d e r t he s a m e
c o n d i t io n s : 2 H 2 ( 4 - c o u m a r i c a c i d ) a n d 3 H 2 ( f e r u l ic a c i d ).
T h e c y c l i c v o l t a m m e t r y o f 2 H 2 a n d 3 H 2 i n 5 0 / 5 0 w a t e r
+ m e t h a n o l m i x t u r e s w a s i n v e s t i g a t e d a t d i f f e r e n t p H
v a lu es . A t n eu t r a l o r s l i g h t ly ac id i c p H , t h ey ex h ib i t an
i r r e v e r s ib l e o n e - e l e c t r o n o x i d a t i o n p e a k . T h e p e a k p o s i t i o n
v a r i e s l i n e a r l y w i t h t h e p H w i t h a s l o p e c l o s e t o 5 8 m V
p H - l ( t h e t h eo r e t ic a l s l o p e f o r 1 H ÷ a n d 1 e - ) . A t h i g h e r
p H , t h e v a l u e o f t he p e a k p o t e n t i a l r e m a i n s c o n s t a n t u p t o
p H = 1 3 . T h i s p H d ep e n d en ce in d i ca t e s t h a t i t i s t h e
d e p r o t o n a t e d f o r m s 2 2 - o r 3 2 - w h i c h a r e o x id i s e d , e v e n
a t p H b e l o w t h e p K a , to g i v e t h e p h e n o x y l r a d i c a ls
[ 9 , 1 3 - 1 6 ] . T h i s e l e c t r o c h e m i c a l b e h a v i o u r i s v e r y s i m i l a r
to t h e r e su l t s o b t a in ed fo r t h e a l co h o l s [9 , 1 6 ] o r a ld eh y d es
[ 1 5 ] d e r i v e d f r o m h y d r o x y c i n n a m i c a c i d .
I n n e u t r a l A C N , 2 H 2 a n d 3 H 2 e x h i b i t i r r e v e r s ib l e
b r o a d p e a k s a t 1 .0 a n d 0 .9 5 V ( S C E ) r e s p e c t i v e l y . O n t h e
r e v e r s e s c a n ( i f th e o x i d a t i o n o f 3 H 2 h a s b e e n s c a n n e d
b e f o r e h a n d ) a b r o a d r e d u c t i o n p e a k i s v is i b l e a r o u n d 0 . 5 V
( S C E ) w h i c h c a n b e a s c r i b e d t o t h e f o r m a t i o n o f d i m e r s o r
h i g h e r o l i g o m e r s . T h e o x i d a t i o n p e a k p o t e n t i a l w a s f o u n d
to in c rease l i n ea r ly w i th t h e l o g a r i t h m o f t h e scan r a t e ,
w i t h a s l o p e a r o u n d 4 0 m V p e r l o g ( v ) , w h i c h i n d i c a t e s a
m ix ed co n t ro l o f t h e g lo b a l k in e t i c s , i .e . b y th e e l ec t ro n
t r a n s f e r s t e p a n d b y a s u b s e q u e n t c h e m i c a l r e a c t i o n . T h i s
i s i n ag reem en t w i th t h e o x id a t io n o f 3 I - I 2 t o t h e p h e n o x o -
n i u m c a t i o n 3 H 2 + w h i c h i s l i k e l y t o b e a s t r o n g a c i d a n d
to d ep ro to n a t e r ap id ly .
I n o r d e r t o a v o i d t h e r e a c t i o n s o w i n g t o t h e p r o t o n
t r a n s f e r, t h e o x i d a t io n o f t h e c o r r e s p o n d i n g p h e n o l a t e s w a s
i n v e s ti g a t e d . A f t e r a d d i t io n o f t w o e q u i v a l e n t s o f b a s e
( N M e 4 O H - 5 H 2 0 ) , t he p h e no l a te 2 2 - e x h i b i t s i n D M S O
0
0.5
. ~ t
i
|
300 400 500
k / n m
F i g . 5 . U V - v i s s p e c t ra l c h a n g e s d u r i n g i rr a d i a ti o n o f a 4 . 4 × 1 0 . 4 m o l
- 1 o f c a f f e i c a c i d i n 0 . 0 3 M N a N 3 a q u e o u s s o l u t i o n s a t u r a t e d w i t h N 2 0
( p H = 6 . 3 ). N u m b e r s o n e a c h s p e c t r u m a r e t h e i r r a d i a t i o n t i m e s i n
m i n u t e s .
- 0 .0 8
O.S ~tA
I
-0 .8 -0 .4 0 .0
E / V ( S C E )
a )
~'~ -0 .12
- 0 . 1 4
(b)
-0. 16 , 11 t t , i
- 1 .5 - - 0 .5 0 0 .5
l o g (v )
F i g . 6 . C y c l ic v o l ta m m e t r y o f b a si c 3 2 - i n D M S O ( + 0 . 1 m o l 1 - 1
N E t 4 B F 4 ) o n a 1 m m d i a m e t e r g o l d e l e c t r o d e . ( a ) c ° = 1 0 - 3 t o o l 1 - i ,
v = 0 . 2 V s - ~ . ( b ) V a r i a t i o n o f t h e p e a k p o t e n t i a l a s a f u n c t i o n o f t h e
l o g a r i t h m o f t h e s c a n r a t e ( v ) .
a n d i n A C N a w e l l - d e f i n e d m o n o e l e c t r o n i c i r r e v e r s i b l e
o x i d at i o n p e a k ( E p = - 0 . 0 2 V ( S C E ) in D M S O a n d
- 0 . 0 3 V ( S C E ) i n A C N a t 0 . 2 V s - l ) . S i m i la r r e s ul ts
w e r e f o u n d f o r t he o x i da t i on o f 3 2 - ( E p = - 0 . 1 5 V
( S C E ) i n D M S O a n d i n A C N a t 0 .2 V s - 1 ). F o r b o t h
p h e n o l a t e s , a r e d u c t io n p e a k c o u l d b e o b s e r v e d o n t h e
r e v e r s e s c a n a r o u n d - 0 . 4 V ( S C E ) ( F i g . 6 ( a )) . T h e v a r i a -
t i o n o f t h e o x id a t io n p eak p o ten t i a l w i th t h e scan r a t e w as
i n v e s t i g a t e d i n D M S O w h e r e r e p r o d u c i b l e r e s u l t s c a n b e
o b t a i n e d . T h e p e a k p o t e n t i a l w a s f o u n d t o v a r y l i n e a r l y
w i th t h e scan r a t e b e tw een 0 . 0 5 a n d 1 0 V s - ~ w i th a s lo p e
o f 2 1 m V d e c a d e - ~ o f t h e s c a n r a t e . T h i s s l o p e i s c o n s is -
t e n t w i t h a m e c h a n i s m i n v o l v i n g a f a s t e l e c t r o n t r a n s f e r
b e t w e e n t h e e l e c t r o d e a n d t h e p h e n o l a t e f o l l o w e d b y a n
i r r e v e r s i b l e c o u p l i n g o f t w o r a d i c a l s ( D I M 1 m e c h a n i s m )
[111.
3 2 - , . 3 - - + e - ( E o )
2 k l
3 - + 3 - ~ d i m e r
S i m i l a r r e s u l t s l e a d in g t o t h e s a m e m e c h a n i s t i c c o n c l u -
s i o n s w e r e o b t a i n e d w i t h t h e n e u t r a l i s e d 4 - c o u m a r i c a c i d
2 2 - . T h e s e r e s u l ts a r e v e r y s i m i l a r t o t h o s e p r e v i o u s l y
o b t a i n e d f o r t h e c o r r e s p o n d i n g a l c o h o l s [ 16 ] a n d a l d e h y d e s
[ 1 5 ] w h e r e a c o u p l i n g b e t w e e n t w o p h e n o x y l r a d i c a l s w a s
f o u n d t o b e t h e k e y s t e p i n t he f o r m a t i o n o f t h e d i m e r .
-
8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
7/8
P. Hapiot et al. / Journal of Electroanalytical Chemistry 405 1996) 169-176 1 7 5
0 . 0 2
0 . 0 1 5
~ O . O 1
<
0 . ~ 5
. . . . i . . ° . - . . . . i . . . L
3 9 0 4 3 0 4 7 0 S l 0 5 5 0 5 9 0 6 3 0
k / n m
0 . 0 2 .
0 . 0 1 5 [ ( b )
0 . 0 0 5
0 - * - - ' - - * - • ' - - * - '
3 9 0 4 3 0 4 7 0 5 1 0 5 5 0 5 9 0 6 3 0
k / n m
F i g . 7 . D i f f e r e n t i a l a b s o r p t i o n s p e c t r a a f t e r p u l s e r a d i o l y s i s o f 1 0 - 3 m o l
I - I o f a ) 2 H z a n d b ) 3 H z a c i d s i n 0 . 1 M N a N 3 a q u e o u s s o l u t i o n
s a t u r a t e d w i t h N 2 0 p H = 1 2 ) . M o n i t o r e d 1 / . ~ s a f t e r t h e p u l s e .
3 2 2 P u l s e r a d i o l y s i s
P u l s e r a d i o l y s i s e x p e r i m e n t s w e r e p e r f o r m e d w i t h t h e
m o n o h y d r o x y c i n n a m i c a c i d s 2 H 2 a n d 3 H 2 u n d e r t h e s a m e
c o n d i t i o n s a s t h o s e u s e d f o r t h e o x i d a t i o n o f c a f f e i c a c i d .
T h e s p e c t r a o f t h e f i rs t i n te r m e d i a t e s r e c o r d e d 1 g s a f t e r
t h e p u l s e , a s c r i b e d t o t h e g e n e r a t e d p h e n o x y l r a d i c a l , a r e
p r e s e n t e d i n F i g . 7. T h e p h e n o x y l r a d i c a l o f 4 - c o u m a r i c
a c i d s h o w s a l a r g e a b s o r b a n c e b e l o w 4 0 0 n m a n d a n o t h e r
o n e i n t h e v i s i b l e r a n g e , w i t h a p e a k a t h = 6 0 0 n m
( 6 = 1 9 0 0 1 m o l - 1 c m - 1 ) . T h e p h e n o x y l r a d ic a l o f f e r u l ic
a c i d s h o w s a b r o a d w e a k e r a b s o r b a n c e i n t h e v i s i b l e r a n g e
w it h a m a x i m u m a t A = 5 1 0 - 5 5 0 n m ( 6 = 9 0 0 1 t oo l - 1
c m - l ) . B o t h r a d i c a l s e x h i b i t e d a n o th e r in t e n s e a b s o r p t i o n
a t l o w e r w a v e l e n g t h s w h i c h w e r e n o t d e t e r m i n e d b e c a u s e
o f i n t e r f e r e n c e b y t h e a b s o r p t i o n o f t h e p a r e n t p h e n o l . T h e
d e c a y o f t h e p h e n o x y l r a d i c a l d e r i v e d f r o m 4 - c o u m a r i c
a c i d w a s s t u d ie d i n d e t a il a t d i f f e r e n t p H v a l u e s , p h e n o l a t e
c o n c e n t r a t i o n s ( 0 . 7 t o 7 m M ) , a n d d o s e s ( r a d i c a l c o n c e n -
t r a t i o n s b e t w e e n 5 a n d 5 0 m M ) . I n a l l c a s e s , t h e d e c a y o f
t h e r a d i c a l f o l l o w e d a s e c o n d - o r d e r r a t e l a w , w i t h 2 k = 7
× 1 0 8 1 m o l - 1 s - 1 ( c a l c u l a t i o n b a s e d o n t h e m o l a r a b -
so rp t iv i ty i n T ab le 2 ) . T h e f in d in g th a t t h e d ecay r a t e
c o n s t a n t o f t h e p h e n o x y l r a d i c a l d o e s n o t c h a n g e w i t h t h e
in i t i a l co n cen t r a t i o n o f p h en o la t e i n d i ca t e s t h a t t h e r eac t io n
p r o c e e d s v i a a r a d i c a l - r a d i c a l m e c h a n i s m a n d t h a t t h e
p o ss ib l e a d d i t i o n o f t h e r ad i ca l t o t h e p a ren t m o le cu le i s
t o o s l o w t o b e o b s e r v e d u n d e r t h e s e c o n d i t i o n s . F e r u l i c
a c i d e x h i b i t e d a s i m i l a r b e h a v i o u r a n d i ts s e c o n d o r d e r
d e c a y r a t e c o n s t a n t i s r e p o r t e d i n T a b l e 2 . ( T h e s e d i m e r i s a -
t i o n r a te c o n s t a n t s a r e o f t h e s a m e o r d e r o f m a g n i t u d e a s
th o se r ep o r t ed in t h e l i t e r a tu re [9 , 1 5 , 1 6 ] . )
4 . D i s c u s s i o n a n d c o n c l u s i o n s
T h e e l e c t r o c h e m i c a l o x i d a t i o n o f 4 - c o u m a r i c a n d f e r u l i c
a c i d s i n w a t e r + o r g a n i c s o l v e n t m i x t u r e s t a k e s p l a c e b y
e l e c t r o n t r a n s f e r f r o m t h e c o r r e s p o n d i n g p h e n o l a t e i o n
e v e n a t p H v a l u e s l o w e r t h a n t h e p K a . T h e p h e n o l a t e i s
o x i d i s e d t o t h e p h e n o x y l r a d i c a l w h i c h t h e n d i m e r i s e s b y
r a d i c a l - r a d i c a l c o u p l i n g , a c c o r d i n g t o t h e g e n e r a l p a t t e r n :
P h O H . P h O - + H +
P h O - • . P h O + e -
P h O + P h O ~ d i m e r
R a d i o l y t i c o x i d a t i o n b y t h e a z i d y l r a d i c a l i n a q u e o u s
so lu t io n s i s r ap id w i th b o th t h e p h en o la t e i o n an d th e
n e u t r a l p h e n o l s u n d e r s t u d y . T h e d i m e r i s a t i o n o f t h e p h e -
n o x y l r ad i ca l s i s f a s t , w i th r a t e co n s t an t s o f th e o rd e r o f
1 0 8 1 m o 1 -1 s -1 , s im i l a r t o t h e v a lu e s fo u n d fo r t h e
a l c o h o l s a n d a l d e h y d e s d e r i v e d f r o m h y d r o x y c i n n a m i c
acid .
T h e c h e m i c a l b e h a v i o u r o f c a f f e i c a c i d i s v e r y d i ff e r e n t .
C y c l i c v o l t a m m e t r y i n w a t e r s h o w s a r e v e r s i b l e t w o e l e c -
t r o n w a v e , s p e c t r o e l e c t r o c h e m i s t r y a t p H - ~ 4 s h o w s t h e
s p e c t r u m o f o - q u i n o n e , a n d d e c a y o f th e r a d i c al o b s e r v e d
b y p u l s e r a d i o l y s i s a l s o r es u l t s i n f o r m a t i o n o f t he o -
q u in o n e . In n eu t r a l o r ac id i c p H th e o -q u in o n e i s r ap id ly
f o r m e d b y d i s p r o p o r t i o n a t i o n o f t h e o - s e m i q u i n o n e w i t h
r e a c t i o n t i m e s l o w e r t h a n 1 0 - 4 s .
Q - ' + Q - ' + 2 H + , . Q + Q H 2
T h e o b s e r v a t i o n o f a r e v e r s i b l e t w o - e l e c t r o n t r a n s f e r i n
e l e c t r o c h e m i s t r y c o n f i r m s t h a t t h e f o r m a t i o n o f t h e o -
q u i n o n e i s f a s t a n d a l s o e q u i l i b r a t e d . T h e s a m e g e n e r a l
b e h a v i o u r i s o b s e r v e d w i t h t h e m e t h y l e s t e r o f c a f f e i c a c i d.
I n a p r o t i c s o l v e n t s , t h e s a m e o - q u i n o n e i s a l s o o b ta i n e d b y
a t w o - s t e p o x i d a t i o n i n v o l v i n g p r o t o n e x c h a n g e p r e - e q u i -
l i b r i u m p r i o r t o t h e e l e c t r o n t r a n s f e r . T h e s e m i q u i n o n e
i n t e r m e d i a t e s h a v e b e e n c h a r a c t e r i s e d b y t h e i r r e d o x p o -
t e n t i a l s a n d U V s p e c t r a .
T h e f a c t t h a t t h e o - q u i n o n e i s f o r m e d d u r i n g t h e o x i d a -
t i o n o f c a f f e i c a c i d i s i n a g r e e m e n t w i t h w h a t w a s p r e v i -
o u s l y d e s c r i b e d f o r t h e h o m o g e n e o u s o x i d a t i o n o f c a f f e i c
ac id b y d i f f e ren t o x id an t s [1 8 , 1 9 an d r e fe ren ces c i t ed
t h e re i n ]. T h e f o r m a t i o n o f t h e c o u p l i n g p r o d u c t s i s o b -
s e r v e d i n w a t e r a n d o r g a n i c s o l v e n t s d u r i n g c o n t i n u o u s
r a d i o l y s i s o r e l e c t r o l y s i s e x p e r i m e n t s ( e x p e r i m e n t a l t i m e
o f t h e o r d e r o f s e v e r a l m i n u t e s ) . T h i s i n d i c a t e s t h a t t h e
o - q u i n o n e i s f o r m e d p r i o r t o t h e c o u p l i n g p r o d u c t s a n d i s
i n v o l v e d i n t h e f o r m a t i o n o f t h e s e p r o d u c t s .
I n a p r e v i o u s p u b l i c a t i o n , i t w a s p r o p o s e d t h a t t h e
o x i d a t i v e c o u p l i n g o f c a f f e i c a c i d a t p H = 4 - 5 i n v o l v e d
-
8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169
8/8
176 P. Hapiot et aL / Journal o f E ectroanalytical Chemistry 405 1996) 169-176
t h e d i r e c t d i m e r i s a t i o n o f t h e s e m i q u i n o n e r a d i c a l [8 ]. T h i s
d i r e ct m e c h a n i s m i s u n l ik e l y t o o c c u r b e c a u s e o f t h e f a st
a n d d i re c t f o r m a t i o n o f t h e o - q u i n o n e f r o m t h e s e m i q u i n o n e
a t t h e s e p H v a l u e s ( d i s p r o p o r t i o n a t i o n r a t e c o n s t a n t 2 k = 8
X 1 0 7 1 m o l - 1 s - 1 . A v a r i a n t o f t h is m e c h a n i s m h a s b e e n
p r o p o s e d w h i c h c o n s i d e r s t he c o m p r o p o r t i o n a t i o n e q u i l ib -
r i u m b e t w e e n t h e o - q u i n o n e a n d t h e c a f fe i c a c i d t o g iv e
s e m i q u i n o n e , w h i c h t h e n l e a d s to t h e f o r m a t i o n o f t he
d i m e r . T h e a b o v e c o m p r o p o r t i o n a t i o n e q u i l i b r i u m i s c e r -
t a i n l y fa s t a n d r e v e r s i b l e , a s i n d i c a t e d b y t h e e l e c t r o c h e m i -
c a l d a ta , b u t t h e f o r m a t i o n o f t h e s e m i q u i n o n e i s n o t
t h e r m o d y n a m i c a l l y f a v o u r e d [ 2 9] a t s l ig h t l y a c i d ic p H . I n
a d d i t io n , u n d e r t h e c o n d i t i o n s w h e r e t h e s e m i q u i n o n e d o e s
n o t r a p i d l y d i s p r o p o r t i o n a t e t o t h e o - q u i n o n e ( i n w a t e r a t
b a s i c p H o r i n o r g a n ic s o l v e n t s ) n o e v i d e n c e f o r a c o u p l i n g
r e a c t i o n w a s f o u n d . O n t h e c o n tr a r y , u n d e r s u c h c o n d i t i o n s
t h e s e m i q u i n o n e w a s u n r e a c t i v e .
I t t h e r e f o r e a p p e a r s t h a t th e f o r m a t i o n o f d i m e r s ( o r
o l i g o m e r s ) f o l l o w s v e r y d i f f e re n t p a t h w a y s s t a rt i n g f ro m
t h e m o n o p h e n o l s ( d e r i v e d f r o m 4 - c o u m a r i c o r f e r u li c a c id s )
a n d f r o m c a f f e i c a c i d . I n t h e fi r s t ca s e , t h e p h e n o l a t e i s
o x i d i s e d t o a p h e n o x y l r a d i c a l w h i c h u n d e r g o e s f u r t h e r
d i m e r i s a t i o n , w h i l e i n t h e s e c o n d c a s e , c a f f e i c a c i d i s
o x i d i s e d to th e c o r r e s p o n d i n g o - q u i n o n e , w h i c h u n d e r g o e s
f u r th e r c o n d e n s a t i o n s l e a d i n g t o b r o w n i n g p r o d u c t s .
A c k n o w l e d g e m e n t
M s . B ~ n ~ d i c t e G r e n e t te i s g r a t e fu l l y a c k n o w l e d g e d f o r
h e r h e l p w i t h t h e c y c li c v o l t a m m e t r y e x p e r i m e n t s .
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