Interligand Interactions in Ternary Copper ( II) Complexes of Dipepticks and Auxins

Debjani Chakraborty and Pabitra K. Bhattacharya

Dc:partment of Chemistry, M. S. University of Baroda, Baroda, India

~-

ABSTRBCT

The formation constant of the mixed ligand complexes of the type (CuAL) and (CUA_~L). where A refers to glycylglycine, glycyl-L-alanine, and glycyl-L-leucine; and L refers to indole acetic acid, indole propionic acid, and indole butyric acid. They were determined in 1: 1 (V/V) dioxan-water medium at 30°C and I = 0.2 M (NaCIO,), using SCOGS computer program. The astatistical stabilization of the ternary complexes has been attributed to intramolecular i*:+ogen bond formation between the two ligands .

INTRODUCTION

A dipeptide coordinates to copper(H) via its terminal amino group and oxygen of the neighboring amide group at low pH [l- 111. At higher pH values, the peptide-NH undergoes deprotonation and the ligand becomes tridentate coordinating through N-amino, N-peptide and 0-carboxylate groups [3, 71. In ternary Cu(II)-dipeptide- ligand systems involving bidentate coordination of the dipeptide, the free carboxylate of the dipeptide can have intramolecular interligand interactions with the noncoordi- nated groups of the other ligands. Such intramolecular noncovalent interactions may be either electrostatic as in (Cu-dipeptide-histidine) 1121 or of hydrogen bonding type as in (Cu-dipeptide-tyrosine) [ 13 1.

Sigel et al. [14] studied stability constants of mixed-ligand [M(phen)(phe CA)]+ complexes (M = Cu+*, Znf2; phen = 1, lo-phenanthroline; phe CA-= benzoate, 2-phenyl acetate, 3-phenyl propionate, 4-phenyl butyrate, 5-phenyl valerate. 6-phenyl caproate). These complexes have been found to be stable due to intramolecular stacking between the phenyl residue of pheCA_ ligands and the phen molecule. In continuation with their studies of Cu-phen-phenylacetate and analogous, Sigel also

Address reprint requests to: Professor P. K. Bhattacharya, Department of Chemistry, Faculty of Science. M. S. Universily of Baroda. Baroda 396062, India.

Journal of Inorganic Biochemistry, 41, 57-62 ( 199 1) 57 0 1991 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, NY, NY 10010 0162-0134/91/%3.50

58 D. Chukraborty and P. K. Bhattcrcharyu

studied the system (CuAL), where A = 1, 10 phenanthroline and L = auxins such as indole acetic acid and indoIe propionic acid. Stacking interaction between coordi- nated phenanthroIine and the noncoordinated indole part of the auxins has been attributed to the stability of these compIex=s.

Auxins are the growth hormones in plants which regulate cell enlargement. In the biological systems, there is the possibility of the formation of ternary complexes (M-dipeptide-auxin). The present paper deals with the ligand-ligand interactions in copper[II; ternaq complexes involving dipeptides, such as giycyiglycine {A’), giycyl-L-alanine (A’) and glycyl-L-leucine (A3) anri auxins such as iridole acetic acid (IAA = L’), indole propionic acid (IPA = L’), and indole butyric acid (IBA = L3).

EXPERIMENTAL

All the reagents used were of analyrlcal reagen:: grade an, 4 the. litralionr; 1h.el.e carrizj OUI using the ciigital pH meter with an accuracy of kiii.T!i. The prz>tcrn-iiga,-td formation constant of the dipeptides AIf,, AH, and the fmTIIarit-Jfl cOI’is;i:;;tS Of rr,c

binary complexes CUA and CuA_ H were determined in i: L (V/Vj ~~;1a:er-j!~3.~.7

medium at 30°C and I = 0.2M (NaClO,) using the SC’OGS computer !I51 (charges 3n the species have been omitted for simplicity). Corrections for pH in I:1 (V/V) dioxan were made by using the method suggested by Van Uitert and Hass [ 161. 19 the case of auxins. proton-ligand formation constants of the species LH, azd LX, and tormation constants of the binary complexes CuL and CuL, , tV-:it a& rekz?. under identical conditions. The values have been tabulated in Table i. Tizsc IAXJ

values were used as tixed parameters for the refinement of rhe formErion .zonstant of the ternary complexes CuAL and CuA _ ;-. L.

For the determination of the formation constant of the ternary complexes CuAL and CuA _“L, the following sets of solutions (50 c.c.) having Cu:,4:L in the ratio

TABLE 1. (a) Proton-Ligand Formation Constants or’ the Dipeptides ancl ‘1 Acir Corresponding Binary Constants in Dioxan-Water (1: 1, k’ ;VJ Meaiun: UI 0.2 M NaClO, and 30-C with Standard Deviation in Parenthesis: (5) Pro,:Jn- L&and Formation Constants of the Auxins and Their Corresponding Diary Constants in Dioxan-Water (1: 1, V/V) Medium at 0.02 M NaC’iO, ul?d 30°C with Standard Deviation in Parenrli:.::<

-- ---.w.__---

(a) Ligands K h’

I KY

Glycylglycine [A’) 7.63 4.05 !O.Ol! 10.01)

Glycyl-L-alanine (A’) 7.91 4.54 (0.02) (0.03)

Glycyl-L-leucinc f,AS) i.07 4.5% (0.03) (0.01)

(b) I .igands icy log KC,;, e--m

lndole acetic acid (IAA = L’; 5.95 I.% (0.03) (0.03)

Indole propionic acid (IPA = L*) 6.00 2.73 (0.09) (0.08)

lndolc butyric acid (IBA = L’) 6.15 2.69 (0.02) (0.07)

CopFrlII 1 Complexes log Kg& PK?,A ----

6.23 3.92 tO.05) (0.01)

6.47 3.92 , ,371 ‘3 (0.02 j 6.82 4.77

(0.05) W.W) log Kcc”,k,

----- z.5;’

(0.02) 2.68

!O.ci’#) 2.91

(iJ.cu)

COPPER(I1) COMPLEXES OF DIPEPTIDES AND AUXINS 59

: : 1:2 and 1: I:3 were prepared and titrated against standard alkali:

1. a.02 M HCIO,, 0.006 M metal perchlorate, 0.006 M ligand Iigand (L), and 0.156 M NaClO,

2. 0.02 M HCIO,, 0.004 M metal perchlorate, 0.004 M Iigand ligand (L), and 0.160 M NaCIO, .

(A), 0.012 M

(A), 0.012 M

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 2.

The standard deviations in the formation constants of binary ana ternary com- plexes are given in parenthesis. The value of standard deviation for A fog Ii was obtained from the corresponding values of log K& log KS”,, , and iog ?: &,.

The protonation constants and the constants for binary copper(H)-dipeptide sys:cnis were computed taking into account the species AH,. AH. A, Cu, CL!... nncl Cu4 ..if. The evaluation of the formation constants of the ternary (copper(dipeptide-eusin? complexes was done by takin, 0 into account the species mentioned above plus LH ?, LH, L, and CuL, CuL,, and the mixed ligand complex CuAL and CuA_ H L. The constants for the ternary species correspond to the following equilibria:

Cu+A+L=CuAL,

KC” CUAL

CuAL = (Cu)(A! (L> ’

CuAL=CuA_&+K.

K;,,_kL = __&3.&

TABLE 2. Formation Constants of Mixed-Ligand Complexes and Alog K in ( !: 1. V/VI Dioxan-Water Medium: I = 0.2 M (NaClO,) at 30°C: Standard Dcviationj arc Given in Parenthesis

-_--- ----_-- Complcxc?; lOI2 K L;u,AL A Il-lgK io17, K $;:I: 11 I”

i-

CIJA’ L’ 9.82 +0.73 :; .:! s (o.lSr :r0.16: irr.:Ey

CuA’ L’ 10.13 +).!a 5 49 (0.!1) trO.14) (0.09)

CuA’ L’ 9.s +0.4-l 3.50 (0.141 (k0.16) (0.09)

CuA' L’ 11.26 + 1.93 5.85 (0.07) (2-o. 10) (0.09)

CuA' L’ 1 i.2:. +2.O-I 6.13 ;o.cl?) (“0.12) (0. IO)

Cm., Ai L” II.17 +2.02 6.30 :‘I. 10) ( c 0. !J) (0.17)

Cu A’L’ I .i 1 L ,w._.. +o.t33 s.91 c0.i II I kO.12) (0.13)

C!‘r..’ L’ 11 :O.!2 -to.57 5.60 (0.08) (*o.lz) (0.06)

CuA3L’ 10.19 +0.69 6.01 (0.07) (kO.11) (0.07)

60 D. Chakraborty and P. K. Bhattacharya

The stabilities of the ternary complexes values of Alog K.

CuAL can be quantified by calculating the

Alog K = log KC,“,;, - log K’,$_

= log KEkL - log KC,;,

= log KC,*, - log KC,;, - log KC,;, .

It was found that the concentration of the species CuAL, did not exceed 0.1% of the total metal concentration in all the ternary systems under study. Hence, this species has been neglected in the present study.

For the species CuA, u L only the protonation constant can be obtained. Hence, the deprotonation of A in ternary colnplex can be compared with the deprotonation in the binary complex CuA.

As the dipeptides coordinating from the oxygen of the peptide group have one negative charge, it can be expected that AlogK should be negative due to electro- static repulsion between the two ligands in the ternary complex. Formation constant values in Table 2 reveal that the ternary complex (Cu-dipeptide-auxin) exhibit

FIGURE 1. Species distribution for the copper(U)-glycyl L-alanine(indole butyric acid(L) ternary system at a metal:A:L ratio 1:1:2; (0.006 M:0.006 Mt0.012 M). (1) unbound copper( (2) CuA, (3) CuA_ u, (4) CuL, (5) CuAL, (6) CuA_ “L.

COPPER COMPLEXES OF DIPEPTIDES AND AUXINS 61

positive Alog K values. The greater stability of the present ternary complexes can be explained as follows.

The dipeptide A in the CuAL complex is coordinated from the amino group and peptide C = 0, the carboxylate group remains noncoordinated. There is a possibility of hydrogen bond formation between the indole group of the auxin and free carboxylate of the dipeptide which stabilizes the ternary CuAL complex, as in Cu(II)-dipeptide-tryptophan [ 181. This accotints for the positive Alog K values of the present ternary complexes.

In the case of formation of the ternary species CuA _ “L, it was found that deprotonation of the dipeptide N-H bond is reduced in the ternary complex. The dipeptide on deprotonation forms a dianion and there is a strong repulsion between the auxin monoanion and the dipeptide diznion.

This inhibits the coordination of the dipeptide from the peptide N and hence reduces the deprotonation of peptide-NH in the ternary complex. As the deproto- nated A_, is tridentate and has two negative charges, there is no possibility of the formation of CuA _ H L2 species.

Comparison of the AlogK values of (CuAL) species where A = A’, A2, or A3 shows that Alog K is more positive for (CuA2L) species than (CuA’ L). This is

FIGURE 2. Species distribution for the copper(glycyl-L-leucine(A)-indolo butyric acid(L) ternary system at a metal:A:L ratio of 1:1:2; (0.006 M:0.006 M:0.012 M). (1) unbound copper (II), (2) CuA, (3) CuA, “I (4) CuL, (5) CuAL. (6) CuA _,., L.

62 D. Chakraborty and P. K. Bhatracharya

probably because the -CH, group, adjacent to the carboxylate group of A2. increases the electron density on the carboxylate oxygen, thereby making possible the forma- tion of a strong hydrogen bond with the indole moiety of the auxins. (CuA3L) species are relatively less stable compared to (CuA’L) because the bulky CH,CH(CH,), gro;lp in the neighborhood of free carboxylate in dipeptide hinders its participation in the formation of hydrogen bond with noncoordinated indole moiety of the auxins.

The diszibution of various binary and ternary complexes (as percentage of total metal) as a tinction of pH has been calculated for the ternary (copper(II)-glycyl-L- alanine-indole butyric acid) (CuA’Lj) and (copper(U)-glycyl-L-leucine-indole bu- tyric acid) (CuA3L3 j systems as shown in Figures 1 and 2. In both cases, the ternary species CuAL nttains its maximum concentration at pH 5.0. The concentration of CuAL species for the CuA2 L3 system reached a maximum of 60% whereas that for the CuA’L” system did not exceed 30%, thereby showing the stability of the former species compared to the latter. The formation of CuA_,L species in all the ternary systems begins near pH = 5.5 and there is a steady increase in its concentration with the rise in pH.

One of the authors (DC) is thankful to the University Grants Commission, New Delhi, for the award of Senior Research Fellowship.

REFERENCES

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Received March 15. 1990; accepted May 21, 1990