Inorganica Chimica Acta 356 (2003) 210�/214

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Vanadium complexes with enamines having tyrosine constituents

Martin Ebel, Dieter Rehder *

Institut fur Anorganische und Angewandte Chemie, Universitat Hamburg, D-20146 Hamburg, Germany

Received 17 March 2003; accepted 21 March 2003

In honour of Prof. J.J.R. Frausto da Silva

Abstract

Four new oxovanadium(IV)-complexes of Schiff-base ligands with L-tyrosine constituents have been synthesised and

characterised. The crystal and molecular structures of [A -VO(nap-R -tyr)(H2O)] �/MeOH (nap is derived from 2-hydroxy-

naphthalene-1-carbaldhyde) have been determined. The biological significance of vanadium�/tyrosine interaction is addressed.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Vanadium complexes; Enamines; Tyrosine

1. Introduction

Among the biological functions of vanadium [1],

tyrosine and tyrosine derivatives play a distinctive role

in physiological interactions involving vanadium in the

oxidation states �/V, �/IV and �/III. A prominent

example is the potential of the tunichromes of vanadium

sequestering sea squirts (Ascidiaceae ) in the reduction of

vanadate (H2VO4�) to vanadyl (VO2�) and VIII

([VO(OH)(H2O)4]�) [2]. Tunichromes are polypeptide

pigments in the tunic and the morula blood cells of

ascidians, essentially built up of dopa and hydroxydopa

units. Vanadate, VO2� and VO2� also effectively

coordinate to transferrin, an efficient transport protein

for simple inorganic vanadium species [3] and competi-

tive ligand for vanadium coordination compounds in the

blood serum [4]. Two tyrosines both in the C- and the N-

terminal lobe of transferrin are involved in coordination

to vanadium. Protein phosphatases and kinases are

inhibited or stimulated by vanadate, and it has been

proposed that this activity of vanadates is related to the

presence of and vanadium coordination to tyrosine (or

serine or threonine) in the active site of these enzymes

[5], a view which is corroborated by the structure

* Corresponding author. Tel.: �/49-40-4123 6087; fax: �/49-40-4123

2893.

E-mail address: rehder@xray.chemie.uni-hamburg.de (D. Rehder).

0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0020-1693(03)00406-7

elucidation of a bovine low molecular weight phospho-

tyrosyl phosphatase [6], in which vanadate is covalently

attached to a serinate, giving rise to an overall trigonal

bipyramidal coordination array. Further, it has been

suggested that the insulin-mimetic behaviour of vana-

date may be due to vanadylation of the tyrosine residues

at the intracellular site of the insulin membrane receptor

[7,8].Tyrosine itself, as far as coordination is concerned,

interacts quite ineffectively with vanadate. The complexformation constants in water�/acetone are 1.8 M�1 forthe monoester HVO3(OTyr)�, and 1.1 M�1 for thediester VO2(OTyr)2

� [9]. Dipeptides containing tyrosineare sufficiently more effective ligands, although the maincomplexes formed exclude tyrosine from direct coordi-nation. Dipeptides such as Tyr�/Gly and Gly�/Tyrcoordinate in a tridentate fashion through the terminalcarboxylate, the terminal amino group and the depro-tonated amide-N [10,11]. Complex formation constantsare typically between 30 and 150 M�1 [10]. As aconsequence of the necessity to deprotonate the amide-N , rate constants for complex formation are slow, e.g.0.025 M�1s�1 for Gly�/Tyr [11]. Furthermore, redoxinteraction between vanadate and the dipeptide compli-cates the situation [11]. Tyrosine-containing dipeptidesbehave definitely different in this respect than dipeptideswith serine or threonine, which also form complexeswith direct coordination of the alcoholic function[10,12,13].

M. Ebel, D. Rehder / Inorganica Chimica Acta 356 (2003) 210�/214 211

The tridentate coordination mode of Gly�/Tyr derived

from multinuclear NMR studies in solutions containing

vanadate and Gly�/Tyr [10,11] has recently been estab-

lished for the solid state structure of the vanadylcomplex [VO(o -phen)Gly�/Tyr] [14], in which the tyr-

osine remains dangling, as in complexes of other metal

ions such as Cu2� in [Cu(Gly�/Tyr)(H2O)2] [15], or in

vanadium complexes with related ligands with the

tyrosine moiety, such as [VOL], where L is N ,N ?-ethylene(S )-histidine-(S )-tyrosine [16]. In this respect,

i.e. ‘avoiding’ participation of the deprotonated OH

function in coordination, serine containing systemsbehave similarly, as demonstrated by the complexes

[{VO(van-(L)-ser)(H2O}2m-O] [17] and [VO(sal-(R ,L )-

ser)}2m-O]� [18]. The present contribution, in which

Schiff bases containing tyrosine have been employed as

ligands, fortify the view that the amino acid side chain

function refrains from direct interaction with the

vanadium centre.

2. Experimental

2.1. Measurements

IR spectra were obtained in KBr pellets on a Perkin�/

Elmer IFT 1720, UV�/Vis spectra on a Carry 17

spectrometer. EPR spectra (X band mode) were re-corded in ca. 3 mM THF solutions at room temp. and

100 K with a Bruker ESP 300E spectrometer. Parameter

fittings were carried out with the program system

SimFonia.

The X-ray structure analysis was performed at 153(2)

K in the u /2u scan mode using Mo Ka irradiation (l�/

0.71073 A, graphite monochromator) on an Smart Apex

CCD diffractometer. Absorption corrections (SADABS)were applied. Hydrogens participating in hydrogen

bonding were found, all other hydrogens were calculated

into idealised positions and included in the last cycles of

the refinement. Data for the crystal structure determina-

tion and refinement of 3 �/MeOH: Empirical formula

C21H21NO7V, M�/450.33 g mol�1, orthorhombic space

group P212121. Unit cell dimensions: a�/7.2613(3), b�/

12.6250(6), c�/21.9597(10) A; V�/2013.13(16) A3, Z�/

4. rcalc�/1.486 g cm�3, m�/0.537 mm�1. F (000)�/932.

Crystal size 0.6�/0.1�/0.1 mm3. u range for data

collection 1.85�/30.008; index ranges: �/105/h 5/10,

�/175/k 5/17, �/305/l 5/30; reflections collected

50712; independent reflections 5846 (Rint�/0.1024);

completeness to u�/30.008: 99.8%. Data/restraints/para-

meters 5846/0/292; goodness-of-fit on F2: 0.932. Final R

indices [I �/2s(I0)]: R1�/0.0439, wR2�/0.0882; (alldata): R1�/0.0685, wR2�/0.1178; largest difference

peak and hole: 0.707 and �/0.518 e A�3. Absolute

structure parameter 0.00(3).

2.2. Preparations

Materials were used as obtained from the suppliers

(Sigma, Fluka), except of 2-hydroxy-1-naphthaldehyde,which was dried in vacuo prior to use. The reactions

were carried out using Schlenk-technique. Solvents were

deoxygenised but not dried, since most reactions were

carried out in a methanol/water mixture. For spectro-

scopic data, see Table 1 in Section 3.

2.2.1. [VO(sal�/tyr)(H2O)] �/2EtOH (1 �/2EtOH)

L-Tyrosine�/ethylester hydrochloride (1.23 g, 5

mmol), salicylaldehyde (0.62 g, 5 mmol) and vanadylsulfate pentahydrate (1.27 g, 5 mmol) were refluxed for 2

h in a mixture of ethanol (50 ml) and water (25 ml)

containing KOH (0.28 g, 5 mmol) as a scavenger for

HCl and sodium acetate trihydrate (1.5 g, 11 mmol) as a

general buffer. The brownish black reaction mixture was

left to cool overnight. Addition of 100 ml of water and

stirring for 1 h yielded a gray product, which was filtered

off, washed with water and pentane and dried in vacuo.The now silverish substance was dissolved in wet

(technical grade) ethanol, and water was added until

no more precipitate formed. The solid was then filtered

off, washed with water and pentane and dried in vacuo.

Anal . Calc. for C16H15NO6V �/2EtOH (M�/460.38 g

mol�1): C, 52.18; H, 5.91; N, 3.04. Found: C, 52.14; H,

5.25; N, 3.05%.

2.2.2. [VO(van-tyr)(H2O)] �/0.5EtOH �/H2O (2 �/0.5EtOH �/H2O)

L-Tyrosine�/ethylester hydrochloride (1.25 g, 2.5

mmol), o -vanilline (0.76 g, 5 mmol) and vanadyl sulfate

pentahydrate (1.27 g, 5 mmol) were refluxed for 2 h in a

mixture of ethanol (50 ml) and water (25 ml) containing

KOH (0.28 g, 5 mmol) and sodium acetate trihydrate

(1.55 g, 11 mmol). The greenish black reaction mixturewas left to cool to room temperature (r.t.) overnight. A

green solid formed which initially dissolved by addition

of 100 ml of water, but reappeared after several minutes.

After stirring for 1 h, the green solid was filtered off,

washed with water and pentane and and dried in vacuo.

Anal . Calc. for C17H17NO7V �/0.5EtOH �/H2O (M�/

429.32 g mol�1): C, 49.21; H, 5.05; N, 3.19. Found:

C, 49.01; H, 4.73; N, 3.15%.Alternatively, 2 �/THF �/H2O was prepared in the fol-

lowing way: L-tyrosine (0.45 g, 2.5 mmol), o -vanilline

(0.38 g, 2.5 mmol) and vanadyl sulfate pentahydrate

(0.63 g, 2.5 mmol) were refluxed overnight in a mixture

of methanol (25 ml) and water (10 ml) containing

sodium acetate trihydrate (0.75 g, 5.5 mmol). The

yellowish methanolic mixture turned dark green on

addition of water. After cooling, the solvent wasremoved in vacuo and the remaining dark green solid

extracted with several portions of THF. These extracts

were combined and half of the solvent evaporated.

Table 1

Selected IR and EPR data for compounds 1, 2, 3 and 4

nV�Oa nC�N

a gx gy gz Axb Ay

b Azb

1 1002, 993 1627, 1611 1.982 1.980 1.949 61.5 60.0 170.0

2 990 1630 1.983 1.980 1.952 61.5 63.0 170.0

3 1002, 990 1621, 1611 1.985 1.980 1.957 59.0 55.0 164.5

4 970 1620 1.985 1.971 1.951 62.0 65.0 170.5

a cm�1, in KBr.b 10�4 cm�1, in THF.

Scheme 1.

M. Ebel, D. Rehder / Inorganica Chimica Acta 356 (2003) 210�/214212

Addition of pentane to the solution gave a green solid,

which was again filtered off, washed with pentane and

dried in vacuo.

Anal . Calc. for C17H17NO7V �/THF �/H2O (M�/488.11g mol�1): C, 51.65; H, 5.57; N, 2.87. Found: C, 51.24;

H, 4.72, N, 2.89%.

2.2.3. [VO(nap-tyr)(H2O)], 3; and [VO(nap-

tyr)(H2O)] �/MeOH (3 �/MeOH)

L-Tyrosine (0.45 g, 2.5 mmol), 2-hydroxy-1-naphthal-

dehyde (0.47 g, 2.7 mmol) and vanadyl sulfate pentahy-

drate (0.63 g, 2.5 mmol) were refluxed overnight in a

mixture of methanol (25 ml) and water (10 ml) contain-

ing sodium acetate trihydrate (0.75 g, 5.5 mmol). The

yellowish methanolic mixture turned dark green at theaddition of water. After cooling, a heterogeneous

precipitate consisting of white (sodium acetate) and a

green (3) constituent formed. The solid was filtered off,

washed with 50 ml of water for removal of NaAc, and

10 ml of toluene, and dried in vacuo to provide solvent-

free 3.

Anal. Calc. for C20H17NO6V (M�/418.05 g mol�1):

C, 57.43; H, 4.10; N, 3.35. Found: C, 57.02; H, 4.94; N,3.38%.

From the filtrate, dark green crystals of 3 �/MeOH

suitable for X-ray analysis formed after standing at r.t.

for 3 days.

2.2.4. [VO(nap�/glytyr)(H2O)] �/MeOH (4 �/MeOH)

Glycyl�/L-tyrosine �/0.5H2O (0.25 g, 1 mmol), 2-hy-

droxy-naphthaldehyde-1-carbaldehyde (0.19 g, 1.1

mmol) and vanadyl sulfate pentahydrate (0.25 g, 1

mmol) were refluxed overnight in a mixture of methanol

(20 ml) and water (10 ml) containing sodium acetatetrihydrate (0.28 g, 2 mol). The solution was cooled to r.t.

and concentrated to half of its volume. The green solid

which formed was filtered off. Layering of the filtrate

with pentane gave additional product. The filter residues

were combined, dissolved in 50 ml of THF and filtered.

Addition of approximately 10 ml of pentane to the

filtrate gave a green precipitate, which was recovered by

filtration, washed with pentane and dried in vacuo.Anal. Calc. for C22H20N2O7V �/MeOH (M�/507.10 g

mol�1): C, 54.45; H, 4.77; N, 5.52. Found: C, 54.85; H,

4.80; N, 5.51%.

3. Results and discussion

The complexes have been synthesised from aromaticaldehydes, L-tyrosine (or L-tyrosine�/ethylester) andvanadyl sulfate under anaerobic conditions in aqueous/alcoholic solutions containing sodium acetate as de-picted in Scheme 1. The formulation of the complexes 1to 3 has been adapted from the structure of thenaphthaledene complex 3 (for details see below), theformation of which is accompanied by chiral conversionat Ca, i.e. in 3, the tyrosine moiety attains the Rconfiguration. The chiral vanadium centre is in the Aconfiguration. The glycyl-tyrosine complex 4 exhibitsthe same spectral features as 1 to 3 (see Table 1 forcharacteristic spectral parameters). We therefore assumea corresponding coordination mode via the phenolate-O , the enamine-N and the carboxylate-O , rather thanparticipation of deprotonated amide-N as found incomplexes with dipeptides [11,14,19�/21]. The IR spectradepict bands around 990 and 1620 cm�1, typical of thevanadyl group and the coordinated enamine function,respectively. The n(V�/O) is doubled in the case of 1 and3, which points towards different environments due topacking phenomena in the polycrystalline powders. TheEPR spectra of frozen THF solutions show a slightlyrhombic pattern. Employing the additivity relationshipfor the parallel (z ) component of the anisotropichyperfine coupling EPR parameter A , and the partialcontributions to Az [22] for the four ligands in thetetragonal plane, we arrive at a calculated Az �/168(3) �/10�4 cm�1, which compares well with the experimentalvalues of 165�/171 �/10�4 cm�1 (Table 1), corroboratingthe formulations proposed for 1, 2 and 4. Replacing the

Fig. 2. Section of the lattice of 3 �/MeOH, including hydrogen bonds.

Table 2

Selected bond lengths (A) and bond angles (8) for 3 �/MeOH

Bond lengths

V�/O1 1.582(2) O3�/C9 1.241(3)

V�/O2 1.988(2) C8�/C9 1.513(4)

V�/O4 1.909(2) O5� � �O3 2.659, 2.704

V�/O5 2.001(2) O5� � �O2 3.096

V�/N 2.016(2) O6� � �O7 2.756

N�/C10 1.299(3) O7� � �O2 3.016

O2�/C9 1.285(3)

Bond angles

O1�/V�/O2 106.55(11) O4�/V�/N 87.29(9)

O1�/V�/O4 107.30(11) O5�/V�/N 88.90(9)

O1�/V�/O5 108.50(12) O2�/V�/O4 145.91(9)

O1�/V�/N 106.63(11) O2�/V�/O5 84.24(10)

O2�/V�/N 79.31(9) O4�/V�/O5 88.90(9)

M. Ebel, D. Rehder / Inorganica Chimica Acta 356 (2003) 210�/214 213

imine-N or carboxylate-O in 4 for amide-N [20] wouldresult in an expected Az of 159(3)�/10�4 cm�1. In theUV�/Vis region of ethanolic solutions there is a strongCT band at 370 nm. Two weak, broad bands around530�/580 (band II) and 690�/810 nm (bands IA�/IB) areassociated with the d�/d transitions d(xy )0/d(x2�/y2)and d(xy )0/d(xz ,yz ) [25].

Compound 3 (Figs. 1 and 2) crystallises with onemolecule of methanol of crystallisation in the orthor-hombic space group P212121. Selected bonding para-meters are provided in Table 2. The Schiff base ligandacts in the usual tridentate fashion. The three donorfunctions (N, O2, O4) together with an aqua ligand O5are in an about ideal plane of an overall square-pyramidal array. Vanadium is 0.587 A above this plane.The N�/O2�/O4�/O5 plane is almost coplanar with thatof naphthalene (dihedral angle 12.98). The dihedralangle between the planes spanned by the naphthaleneand tyrosine moieties amounts to 37.68. Bond distancesand angles fall within the range commonly found in thisfamily of vanadium complexes, such as [{VO(van�/

ser)(H2O)}2m-O] [17], [VO(sal�/ser)}2m-O]� [18],[VO2(nap�/his)] [23], [VO(sal�/ala)] [24] and [{VO(sal�/

val)}2m-O] [25], where van, sal and nap stand for thevanillin, salicylaldehyde and 2-oxo-naphthaldehyde-1moieties of the Schiff bases of serine (ser), histidine(his), alanine (ala) and valine (val).

The water molecule O5, the carboxylate oxygens O2

and O3, the tyrosine oxygen O6, and the O7 of the

methanol of crystallisation are involved in an intermo-

lecular hydrogen bonding net work (Table 2) leading to

the supramolecular structure shown in Fig. 2. Structure

elements in the lattice are two�/dimensional layers

consisting of double-stranded ribbons, formed by inter-

linking adjacent molecules via the aqua ligand and the

Fig. 1. XSHELL plot (50% probability level) of the molecular structure of 3 �/MeOH.

M. Ebel, D. Rehder / Inorganica Chimica Acta 356 (2003) 210�/214214

non-coordinated oxygen of the carboxalato ligand O3 in

such a manner that each water links to an O3 of the

neighbouring as well as to its own strand. The two

strands are in the anti conformation with respect to thepositions of the doubly bonded oxo groups O1. A

‘molecular’ view thus is to refer to pseudo dimers anti -

[{VO(nap-tyr)}2(m-H2O)] as the building blocks. The

dangling OH of the tyrosine is hydrogen bonded to

methanol above/below the layers (Fig. 2), which also

interlinks the layers by forming H bonds to the

coordinated O2 of the carboxylate (not shown in Fig. 2).

4. Conclusion

Despite of the established ability of tyrosine itself [9],

and tyrosine binding sites in proteins [3�/6], to coordi-

nate to the oxovanadium cation, this appears not to be

the case if competing binding modes of a ligand are

available, such as in tridentate dipeptides or the Schiff

bases employed here, which prefer the thermodynami-

cally more efficient coordination through NH2, amide-N and carboxylate (dipeptides) [11,14], or phenolate,

enamine-N and carboxylate (Schiff bases of salicylalde-

hyde derived carbonyl compounds), leaving, essentially

for steric reasons, the tyrosine-OH dangling at the

periphery. In so far, Schiff bases with tyrosine moieties

resemble those with serine moieties [17,18], although, in

the latter case, the alcoholic side-chain function of serine

is sterically accessible and binding of alcoholates tovanadium is by no means unusual [26]. Although the

tyrosine in the complexes introduced here is not directly

involved in coordination, it participates to the supra-

molecular bonding situation by bridging to a carbox-

ylate of an adjacent molecule via the methanol of

crystallisation, again resembling serine [17]. Under

physiological conditions, the communication transfer

inherent to the methanol in the model compound maywell be taken over by interstitial water molecules.

Acknowledgements

This work was supported by the Deutsche For-

schungsgemeinschaft and the Fond der Chemischen

Industrie.

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