Intramolecular Interligand Interactions in Cu(I1) Ternary Complexes Involving Dipeptides and Amino Acids

Debjani Chakraborty and Pabitra K. Bhattacharya

Department of Chemistry, Faculty of Science, Maharaja Sayajimo University of Bar&a, Bar&a. 390 002 India

ABSTRACT

The formation constants of the mixed ligand complexes of the type (CuAL), where A refers to glycyl- glycine (A’), glycyl-L-ahtnine (A*), and glycyl-L-leucine (A’) and L refers to ar-alanine (L’), phenylala- nine (L’), tryptophan (L3), tyrosine (L4), and L-dopa (L5), were determined in 1: 1 (v/v) dioxan-water medium at 30°C and Z = 0.2 M (NaC104) using an SCOGS computer program. The influence of the noncoordinated aromatic side groups of the amino acids on ternary complex formation is discussed.

INTRODUCTION

Intramolecular interligand interactions in ternary complexes are analogous to the metal-enzyme-substrate interactions in metallo enzymes, resulting in the specific and selective nature of the metallo enzymes [ 1, 21. Hence, these intramolecular in- teractions in metal complexes have been studied extensively during the last decade.

Complexes of amino acids and oligopeptides are involved in the exchange and transport mechanism of trace metal ions in the human body [3]. Oligopeptides can be used as model compounds for such studies because they are able to mimic to a great extent the metal-binding site of much more complicated protein molecules. For a less specific but rather general study of the metal-binding ability of peptides, even studies of dipeptides can supply much information. Hence it is worthwhile to study various ternary systems involving dipeptides.

It is generally accepted [l, 2, 4-121 that initial complex formation between a dipeptide and copper(I1) in the binary system results in a chelate involving a terminal amino group and oxygen of the neighboring amide groups. At higher pH values, the dipeptide undergoes deprotonation of the amide group and becomes tridentate coordinating through N-amino, N-peptido, and 0-carboxylate groups [4, 81. Ternary

Address reprint requests to: Professor P. K. Bhattacharya, Department of Chemistry, Faculty of Science, M.S. University of Baroda, Baroda 390 002, India.

Journal of Inorganic Biochemistry. 39, l-8 (1990) @ 1990 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, NY, NY 10010 0162-0134/90/$3.5~

2 D. Chakraborty and P. K. Bhattacharya

complexes involving dipeptides have also been studied [4, 11, 13, 141. Sigel et al [4, 131 showed that in the ternary complexes involving copper( dipyridyl, and dipeptides, two species are formed, CuAL and CuA( - H)L. In the former dipeptide, A is coordinated from the amino nitrogen and peptido oxygen as a bidentate ligand.

The amide deprotonated dipeptide coordinates via N-amino and N-peptide groups in the CuA( - H)L complex. Crystal structure analysis [12] of the CuA( - H)L (A = glycylglycine, L = 1, lo-phenanthroline) ternary species indicates that the amide-deprotonated glycylglycine is tridentate. with a weak coordination from the third carboxylate end. Nagypal and Gergely 1111 studied mixed ligand complexes of copper(IIj with dipeptides and the amino acids glycine, cy-alanine, ol-aminobutyric acid, norvaline, /3-alanine, threonine, ornithine, lysine. serine, asparagine, glutamine, aspartic acid, and glutamic acid. They have also shown the existence of two types of coordination of the dipeptide ligand in a copper-amino acid-dipeptide ternary complex.

This paper deals with the ligand-ligand interactions in copper(H) ternary com- plexes involving dipeptides and amino acids with noncoordinating side groups such as phenylalanine, tryptophan, and tyrosine. The values have been compared with those of copper-dipeptide-alanine. The study has been further extended to the case of ternary complexes involving 3,4-dihydroxyphenylalanine (L-dopa). L-Dopa is im- portant as a neurotransmitter [ 151 in biochemical processes and is used for therapeutic purposes in cases of Parkinson’s disease [l&18]. It is also interesting because of its ambidentate nature [15, 19, 201.

EXPERIMENTAL

All the reagents used were of A.R. grade, and the titrations were carried out using a digital pH-meter with an accuracy of rtO.01. The proton-ligand formation constant of the dipeptides AH2 and AH and the formation constant of the binary complexes (CuA) and (Cu(A - H)) were determined in 1: 1 (v/v) water-dioxan medium at 30°C and I = 0.2 M (NaC104) using the SCOGS computer program [2 l-231 (charges on the species have been omitted for simplicity). Corrections for pH in 1: 1 (v/v) dioxan have been made by using the method suggested by Van Uitert and Haas [24]. In the case of amino acids, proton-ligand formation constants of the species LH2 and LH and formation constants of the binary complexes (CuL) and (CuL2) were also refined under identical conditions. The values have been tabulated in Tables 1 and 2. These refined values were used as fixed parameters for the refinement of the formation constants of the mixed ligand complexes CuAL and Cu(A-H)L. The formation constants for the ternary systems were computed from titrations in which total concentrations of the metal, ligand A, and ligand L were in 1: 1: 1 and 1: I:2 ratios. Titrations of each set were carried out twice to check the reproducibility of the data. The formation constants for the ternary species are shown in Table 3.

RESULTS AND DISCUSSIONS

The protonation constants and the constants for the binary copper(H)-dipeptide sys- tems were computed taking into account the species AH2, AH, A, Cu, CuA, and Cu(A - H). The equilibria corresponding to the binary constants are as follows.

Cu +A =CuA

TABLE 1.

Cu(I1) COMPLEXES OF DIPEF’TIDES AND AMINO ACIDS 3

Proton-Ligand Formation Constants of the Dipeptides and Their Corresponding Binary Constants in Dioxan-Water (1: 1, v/v) Medium at 0.2 M NaC104 and 30°C

(Standard Deviation a/3 in Parentheses)

Copper complexes

Ligand KP K; ‘cg K& Log KE:a”- H)

Glycylglycine (A’) 7.80 4.05 6.23 3.92

C.01) C.01) C.05) C.01)

Glycyi-L-alanine (A*) 7.91 4.56 6.48 3.92 (.02) (.03) (.07) (.02)

Glycyl-L-leucine (A’) 7.97 4.64 6.82 4.77 (.03) (w (.05) (w

KCU _ [CUAI aA - [Cu][A]

CuA +Cu(A-H)+H

KCUA [WA - WI[W Cu(A-H) = [CuAl

The evaluation of formation constants of the ternary copper(dipeptide-amino acid

complexes was done taking into account the species mentioned above plus LH2, LH, L, CuL, and the mixed ligand complexes CuAL and Cu(A - H)L.

In cases where the metal, ligand A, and ligand L were in the ratio 1: 1:2, the species CuL2 was also considered. The constants for the ternary species correspond to the

TABLE 2.

Ligand

Proton-Ligand Formation Constant and Binary Complex Formation Constants for the Amino Acids in Dioxan-Water Medium (1: 1, v/v) at 0.2 M (NaClOJ and 30°C

(Standard Deviation a@ in Parentheses)

Copper(B) complexes

Log K::L Log K::k,

Alanine (L ‘)

Phenylalanine (L*)

Tryptophan (L3)

Tyrosine (L4)

L-Dopa (L’)

9.56 (.Ol)

8.98 (w

9.24 (.@J)

9.06 (w

8.93 (.Ol)

3.18 (.Ol)

3.11

C.01)

3.20

C.01)

3.14

C.01)

3.13

C.01)

9.16

C.07)

8.96

(.W

9.13

C.04)

8.92

C.06)

8.99

(.Ol)

7.00

C.10)

7.37

C.05)

7.82

C.04)

7.35

C.07)

7.35

C.01)

4 D. Chakraborty and P. K. Bhattacharya

TABLE 3. Formation Constants of Mixed Ligand Complexes and A log K in (1: 1, v/v) Dioxan-Water Medium, Z = 0.2 M (NaClOJ at 30°C (Standard deviation u/3 in Parentheses)

Complex

(1)

CuA’L’

CuA ‘L*

CuA’L’

CuA’L4

CuA’LS

CuA’L’

CuA’L*

CuA2L3

CuA”L4

CuA*L’

CuA3L’

CuA2L2

CuA3L’

CuA3L4

CuA3Ls

following equilibria:

Log fc:,, ~0~ K;iFL-H'L A Log &.AL (2) (3) (4)

14.59 7.88 -0.90

(.05) (W

14.58 8.31 -0.71

(0.04) (0.06)

15.04 7.94 -0.42

(.02) (0. IO)

14.89 7.91 -0.36

(.05) (.07)

14.95 6.89 -0.37

(.10) (.lO)

15.24 8.53 -0.39

(.W (.ll)

15.48 8.19 + 0.05

(.06) (. 10)

15.81 8.19 +0.21

(W (.18)

15.60 8.27 +0.21

(.07) (. 10)

15.61 7.05 +0.16

(.05) (.06)

15.19 8.68 -0.80

t.09) (.ll)

14.95 8.28 m-O.83

(W (.08)

15.34 8.32 -0.61

(W (.20)

15.09 8.40 - 0.65

(.07) (W

15.18 6.81 - 0.63

(.lO) (.11)

Cu+A+L eCuAL

Kz;,, = ICuAL1 [Cul [Al IL1

CuAL i=Cu(A -H)L+H

Cu(I1) COMPLEXES OF DIPEPTIDES AND AMINO ACIDS 5

KCUAL tCu(A - HMW Cu(A-H)L = [CuAL]

The stability of the ternary complex CuAL can be quantified by calculating the values of A log K.

log KC,& = log Kz:AL - log KzzA

A log KC”AL = log K$$_ - log K&_

However, for the second species, Cu(A - H)L, only the protonation constant can be obtained. Hence the deprotonation of A in a ternary complex can be compared with the deprotonation in the binary complex CuA.

As stated earlier, in a CuAL complex the dipeptide A is coordinated from the amino group and peptide C=O, its carboxylate part remaining free. In the case of the CuAL complexes where L = cy-alanine or phenylalanine, A log K was found to be more negative or less positive. This can be accounted for by considering the electrostatic repulsion between the amino acid anion and the dipeptide anion. However, it was observed that for L = tryptophan or tyrosine, A log K is less negative or more positive. The amino acids are coordinated from the amino carboxylate end, whereas the indole and the hydroxyphenyl group of tryptophan and tyrosine, respectively, are free. The less negative or more positive A log K values for these complexes is due to hydrogen bonding between the indole NH of tryptophan or the hydroxyphenyl OH group of tyrosine and the free carboxylate oxygen of the dipeptides (Fig. 1).

Levodopa is ambidentate. For copper(H) complexes it has been observed that a low pH (pH 2-5) dopa coordinates via the amino carboxylate end, and above pH 6 both aminocarboxylate and pyrocatecholate are involved in binding with copper( resulting in polymeric species.

However, the present study shows that in the copper(dipeptide-L- dopa system,

FIGURE 1. bonds.

Proposed structures of (&AL) and (Cu(A -H)L) complexes involving hydrogen

0 HN-C H-t-6 ___. __ _ __x R

\ I

HC-C-&...x

X = CgH50H : C6H4(OH)2

6 D. Chakraborty and P. K. Bhattacharya

(1)

(2)

(31

(4)

Cu A’ L! Cud-HH!

cud L4

Cud-H)t

I 4 5 6 7 0 9

PH FIGURE 2. Distribution of ternary species with pH for Cu(II)-glycylglycine (A’)-amino acid (L) system.

coordination is from the aminocarboxylate end of dopa over the entire pH range 4.0-8.0. It is found that L-dopa forms a stable complex with copper(H) dipeptide. This can be attributed to hydrogen bonding between the OH of the hydroxyphenyl group of L-dopa and the free carboxylate oxygen of the dipeptide.

Rajan et al. [lS] observed that formation of metal chelates of dopa at the amino carboxylate end reduces the decarboxylation caused by pyridoxal and hence there is an improved transport of dopa to rat brain when it is administered in the form of Cu(I1) or Zn(I1). Since, in the first complex (GAL), dopa coordinates from the amino carboxylate end in the whole pH range 4-8, it can be suggested that dopa administered in the form of this ternary complex will not undergo decarboxylation easily and will help in providing a greater supply of L-dopa to the brain.

In the case of the formation of the ternary species Cu(A - H)L, it is found that de- protonation of the dipeptide N-H bond is very much reduced in the ternary complex. This is due to two reasons. First, the dipeptide on deprotonation forms a dianion. There is a strong repulsion between the amino acid anion and the dipeptide dian- ion. This inhibits the coordination of the dipeptide from the peptide nitrogen and hence reduces the deprotonation. Second, in the copper(dipeptide binary com- plex at high pH, the peptide group gets deprotonated and the species Cu(A - H) is formed, where the coordination is from amino nitrogen. peptido nitrogen, and carboxylato oxygen, the three atoms occupying the three equatorial positions. In the ternary species Cu(A - H)L, two of the equatorial positions will be occupied by the aminocarboxylate group of the amino acid so that the dipeptide has to occupy one

Cu(I1) COMPLEXES OF DIPEPTIDES AND AMINO ACIDS 7

60

3 Z

Z- 50 = Z

\ _ 40 W .u % :: 30

x

Z - 20

10

I_

/

I

-

4 5 7 8

FIGURE 3. Distribution of ternary species with pH for Cu(II)-dipeptide(A)-tyrosine(L4) system.

axial and two equatorial positions or the amino acid has to occupy one axial and one

equatorial position. Occupation of the axial position destabilizes the Cu(I1) complexes because of Jahn-Teller distortion. This may also inhibit the coordination from the peptide nitrogen.

It is interesting to compare the A log K values of the (CuAL) species when A = A’, A*, or, A3. It is found that A log K is more negative for the (CuA3L) species than for (CuA’L). This may be because the bulky CH#H(CH& group in the neigh- borhood of the free carboxylate in the dipeptide hinders its participation in the forma- tion of hydrogen bonds with the uncoordinated indole or hydroxyphenyl moiety of the amino acids. Again, comparison of A log K values for the (CuA’ L) and (CuA*L) species reveals that (CuA*L) species are comparatively more stable than (CuA’L) species. This is probably because the CHs group adjacent to the carboxylate group in A2 increases the electron density on the carboxylato oxygen, thereby making possible the formation of a strong hydrogen bond with the amino acid side groups.

The distribution of various ternary complexes (as percentages of total metal) as a function of pH is shown in Figures 2 and 3.

We are gmteful to the University Gmnts Commission for the departmental restnrch sup- port under which the work was mrried out.

8 D. Chakraborty and P. K. Bhattacharya

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Received March 22, 1989; accepted July 26, 1989