Q U A N T I T A T I V E S T R U C T U R E - - A C T I V I T Y R E L A T I O N S H I P S OF

O R T H O - A M I N O M E T H Y L P H E N O L S AND T H E I R DERIVATIVES*

L. A. Kudryavtseva, A. B. Mirgorodskaya, Zh. V. Molodykh, S. M. Gorbunov , R. A. Shagidull ina, and Sh. M. Yakubov

UDC 615.281:547.564.4].074

It is known that the activity of mediators and coenzymes is determined by their chemical structure, small variations of which may lead to a serious change of biological effect [1]. The analogous dependence of biological action on the structure

of fragments in the molecule of compounds is also found in synthetic preparations. Therefore, the establishment of simple quantitative associations between structure and biological properties assists in the search for chemical groups responsible for

the given type of activity. Phenols and their derivatives are known widely as antimicrobial and antifungal agents. A series of quantitative

relationships for structure--biological activity was established for substituted phenols [4, 12, 14, 17], also including substituted

ortho-aminomethylphenols (APs) [4].

In the present paper, we continued the investigation into the dependence of biological activity on the structure of APs

and their derivatives and synthesized a series of quaternary ammonium salts of ortho-aminomethylphenols (QAPs) of the general

formula (I)

_ 3 1 7 " ,

where R:

Ia: H; 11o: 4-C1; Ic: 4-F; Id: 4-Br; Ie: 4-NO2; Ig: 4-CH3; Ih: 4-n-C4H9; Ii: 4-t-C4H9; Ij:

4-n-CsH17; Ik: 2-CH3; I/: 2,3 (CH3) 2.

Data on the biological activity of the compounds (I) toward St. aureus and C. albicans are presented in Table 1.

The dipole moments (DMs) were determined in hexane at 25~ for a series of APs, including those which we previously studied, of the general formula (II) [4]

A

where R:

IIa: H; l/b: 4-Cl, IIc: 4-F; lid: 4-Br; IIe: 4-NO2; IIf: 4-CH3; IIg: 4-C2H5; llh: 4-n-C4H9;

IIi: 4-t-C4H9; lIj: 4-n-C8H17; IN: 2-CH3; IIl: 2,3 (CH3)2; llm: 2,3,5 (CH3)3; lIn: 4-CH30; IIo: 2,4-Br2; IIp: 3,5 C12.

Since the DMs are a quantitative measure of the charge distribution in the molecule and may determine the electrostatic interactions of the substance with the receptor, there is interest in showing the influence of the polarity of the APs on their

*Communication 2. For Communication 1, cf. [4].

A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan' Scientific Center, Academy of Sciences (AN) of the RF. Translated from Khimiko-farmatsevticheskii Zhurnal, Vol. 27, No. 8, pp. 27-30, August, 1993. Original article submitted June 24, 1991.

566 0091-150X/93/2708-0566512.50 �9 Plenum Publishing Corporation

TABLE 1. Bacteriostatic and Fungistatic Activity (minimal concentration, M) of the Compounds of the Formula (Ia-m) and

the Physicochemical Parameters of the Substituents*

1 , pound st. aureus C. albicans

I a 3,4 l 3,41 164 8,60 0 0 Ib 1,53 1,53 218--219 8,10 0,7l --0,97 I c 6,43 12,85 207--209 8.62 O, 14 --0,46 I d 1,34 t .34 209--212 8, l 0 0,86 -- I, t 6 I e -- 7,94 255 5,80 --0,28 -- I f 1,63 1.63 114--115 9,19 0,56 --[,24 I g 1,56 1,56 120 9 ,10 1,02 - - 1,31 I h O, 179 0,090 125 9,00 2,13 -- 1,63 I i 0,143 0,090 210 (with decomp. ) 8,99 1,98 --2,78 I j 0,0012 0,0025 128 8,90 4.13 -- 1,57 I k 3,26 3,26 200 (with decomp. ) 8,83 0,56 --l,24 [ .~ 1,56 0,78 180-- 182 8,94 I, 12 --2,48 Im 1,49 1,49 200 (with decomp.) 9,20 1,68 --3,72

MR.O,I

O, 103 0,603 0,092 0.888 0,736 0.565 1,030 1,959 1,959 3,82

*Here and in Table 2, physicochemical parameters are taken from [15].

**All compounds were recrystallized from ethanol.

biological activity. Data on the bacteriostatic and fungistatic action of the compounds (II) and the values of the DMs are

presented in Table 2.

Computer analysis of the experimental data was performed in order to calculate the association between the structure

of the APs synthesized and their derivatives and the types of activity studied, since it is considered that the biological activity

of the compounds may be presented in the form of linear combinations of physicochemical parameters - - the principle of the

quantitative structure--activity relationship (QSAR) [7, 16]. Multiple regression analysis applied to the experimental data [6]

allowed verification of the possible association between the activity and different parameters of the substituents including those

such as the hydrophobicity constants (70, the steric Taft constants (Es), and electrostatic properties which are expressed by the

pK a values. These factors were considered to be the main ones responsible for the biological action of the compounds [1].

It can be seen from the data in Table 3 that the activity toward St. aureus and C. albicans for the QAPs with para

substituents in the benzene ring is associated with the factor 7r by linear dependences with the correlation coefficients r =

0,982-0.986 (cf. Table 3, equations 1 and 2), which also remain practically high for a wider series of QAPs with substituents

at different positions of the benzene ring (equations 3 and 4). On the basis of this, it can be assumed that the penetration to

the primary sections of action is determining for the biological activity of the compounds of the formula (I). Moreover, the

good correlation between the biological activity toward St. aureus and the molecular refraction (MR) (equation 5) indicates that

this form of activity is associated with the polarizability of the QAPs. Since the coefficients a and b in the equations 1 and 5

are virtually the same, the equation obtained with the application of the MR instead of a- gives the analogous correlation

qualitatively. This is determined by the fact that the 7r and the MR are almost collinear in the series of compounds studied.

At the same time, there is virtually no correlation between log 1/C (St. aureus and C. albicans) and the E s for the QAPs (r <

0.4).

For the APs, the bacteriostatic and fungistatic activity also show good correlation with the hydrophobic properties of

the substituents in the benzene ring, but the r = 0.916-0.974 (cf. Table 3, equations 6-9), i.e., it is somewhat lower than for

the QAPs. No dependence of these types of activity on the steric constants of the substituents E s (r < 0.4) is observed for the

compounds of the formula (II). Since the E s and the 7r have low correlation (cf. Table 2, r = 0.528), the steric constants of

the substituents were utilized as a second descriptor or to obtain the two-parameter equations 11 and 12 with higher correlation

coefficients than those in the equations 6 and 7.

St. aureus: log 1/C=0,740(-4-0,14):~4_0,15(• s 4- 4-1,743(+_0,270) (n= 13, r=0,959, s=0,251, F2,~o=57), (11)

C. albicans: logl/C=O.707(+O,15)n4-O,196(+_O,17)E~4- 4-1,809(_+0,30) (n= 13, r=0,942, s=0,281, F2.m=39,5). (12)

However, as can be seen from these equations, the most significant influence on the value of the activity is shown by

the hydrophobic properties of the substituents in the benzene ring of the APs.

The utilization of the DM as a second parameter in the dependence of the bacteriostatic activity of the compounds of

the formula (II) on the ~r (equation 6) is statistically insignificant. However, the inclusion of the DM in the equation 7 (for C.

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TABLE 2. Bacteriostatic and Fungistatic Activity (Minimal

Concentration, M) for the Compounds of the Formula (II)

and the Physicochemical Parameters of the Substituents*

Com- i02.C, M pound (exp.) ~ .~ a E,

a u r e u s r c a n s f ~ , ~

l l a 3,30 1,66 11,20 8,51 0 0 2,56 l i b 0,676 1,35 10,80 7,93 0,71 --0,97 3,24 l l c 0,741 1,48 - - 8,21 0,14 --0,36 3,12 l i d 1,10 1,10 I0,70 7,66 0,86 --1,16 3,19 l ie** 5,01 5,01 - - 5,60 --0,28 - - - - [ I f 1 ,51 0,759 11,30 8,65 0,56 --1,24 2,93 I lg 0,562 0,708 11,30 8,60 1,02 --1,31 2,92 l l h 0,120 0,240 - - 8,45 2,13 --1,63 - - l I i 0,240 0,479 11,40 8,51 1,98 --2,78 - - I l j 0,0019 0,0019 - - 8,60 4,13 --1,57 2,95 l lk 1,82 0,607 - - 8,07 0,56 --1,24 2,45 It ~ 0,562 0,282 - - 8,18 1,12 --2,48 2,31 Ilm 0,240 0,479 - - - - 1,68 --3,72 - - l ln 0,832 1,38 11,40 8,80 --0,02 --0,55 2,76 llo** 0,0204 0,0204 - - 5,82 1,72 --2,32 - - liP** 0,0575 0,115 6,90 1,42 --1,94 - -

Note. **Not utilized in the correlations.

albicans) is statistically justified (F 1,8 = 5.24 -- the partial criterion of Fisher for the inclusion of the DM in the equation)

and leads to an increase in the correlation coefficient

log !/C=0,73(• ( • 1 7 7 (13) ( n = 1 0 , r=0,985, s=O,171, F2,7=115,7 )

The descriptors of hydrophobicity ~r and the DM for the series of APs considered are thereby mutually independent. Although

the main contribution to the equation 13 is introduced by the hydr0Phobicity of the substituents, the influence of the polarity

of the APs on their activity is present. The equation 13 describes 97 % of the dispersion of the fungistatic activity of the APs.

Therefore, for the compounds studied in the given work and for substituted phenols [12], the biological action is

determined practically by the hydrophobicity of the substituents in the benzene ring. There are close observed linear

dependences for log 1/C = f(Tr) for the same values of the 7r for the APs and their quaternary ammonium salts (QAPs) in

relation to St. aureus and C. albicans (cf. Table 3).

It is interesting to note that the absence of the association of the biological activity with the pK a of the phenolic

hydroxyl of the investigated compounds is probably observed due to the fact that the content of the phenolate form at neutral

pH values is insignificant, judging from the pK a values (cf. Tables 1 and 2), apart from the compounds with low pK a values

(Ie), (IIe), (IIo), and (IIp). At the same time, we previously showed the possibility of utilizing the pK a values of phenols and

the APs as a second descriptor in the two-parameter equations log 1/C = f(vr, PKa), in which the factor of hydrophobicity is

predominant [4]. However, it should be noted that the APs occur in two tautomeric forms at neutral pH values of the aqueous

solutions: the neutral (a) and the zwitterion (b), since the highly polar media favor the transfer of the proton from the phenolic

hydroxyl to the amino group situated in the ortho poskion, i.e., the charge separation [8].

a) ~ ' b) R o

where K T is the constant of tautomeric equilibrium.

The betaine forms (b) with the charge on the oxygen atom may probably participate in the mechanism of the biological

action of the APs, introducing the specific into the interaction of these compounds with the biotarget. It seems to us that this

may explain some difference in the slopes (a) of the dependences log 1/C = f i r ) for the compounds (I) and (II) and the lower correlation coefficients in these equations for the compounds (II) (cf. Table 3). However, in spite of the difference in the

structures of the APs, the QAPs, and phenols, their biological action may be described by the general equation log 1/C = f(Tr),

with the correlation coefficient r = 0.9 for 39 compounds according to the data of [4] and the Tables 1 and 2.

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TABLE 3. Association of the Biological Activity with the Parameters of the Substituents for the Compounds (I) and (II); the

General Form of the Equation log 1/C = ax + b (x = 7r; MR) I I

No. of Com- Descri- I [ a b equation pound Descriptor ptor . r s .,~

i

l I St. aureus n 9 0,986 0,199 0,859-t-0,050 1,17:t:0,096 2 I C. albicans n 10 0,982 0,221 0,835-+-0,050 1,205-t-0,094 3 I St. aureus n 12 0,958 0,299 0,845• 1 ;08+-0,131 4 I C. albicans n 13 0,958 0,295 0,815m_0,073 1,44+0,116 5 I St. aureus MR.O,I 10 0,986 0,188 0,930~0,060 1,13• 6 II St. aureus ~ 13 0,944 0,277 0,670• 1,598• 7 II C. abticans a 13 0,916 0,322 0,617__+0,082 1,623• 8 II St. aureus ~ 10 0,953 0,289 0,710-f-0,079 1,61-1-0,1.90 9 [I C. albicans .-t l0 0,974 0,2l I 0,713• 1,658m_0,085

It is proposed that compounds of the phenol type change the penetrability of bacterial membranes, although it is not

excluded that they may also cause the physical disorganization of the latter [5]. It can be concluded from the results obtained

that the activity for each series of derivatives (the APs or the QAPs) is practically the same toward St. aureus and C. albicans. Consequently, the mechanisms of their action on fungi and bacteria are close and are determined, in the first instance, by their

penetration to the biotarget. The utilization of the regression equations obtained wilt allow the prognosis of the biological activity of new derivatives

of the APs and their quaternary ammonium salts.

EXPERIMENTAL

In the study of biological activity, test objects utilized with Staphylococcus aureus 209-P and Candida albicans-622.

The bacteriostatic and fungistatic properties were studied by the method of serial dilutions in liquid nutrient medium according

to the method of [3].

Compounds of the formula (II) were obtained by the reaction of bisdimethylaminomethane with the corresponding

phenol according to the method of [10, 18]. Their constants are presented in [11, 12]. Compounds of the formula (I) were synthesized analogously [9]; their physicochemical parameters are presented in Table 1. The acid--base dissociation constants

(pKa) of the compounds of the formula (I) and (II) synthesized were determined by potentiometric titration in aqueous solutions

on the pH-340 instrument; the pK a values averaged from three to four measurements are presented in the Tables 1 and 2. The

experimental determination of the DMs for the APs was performed by the Debye method of diluted solutions in hexane at 25 ~ according to [2]. The data of the elemental analysis (C, H, N) for the compounds (Ia-m) correspond with the data calculated using the empirical formulas.

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