CL&I) Binary and Mixed-Ligand Binuclear Complexes Involving Histidine as Binucleating Ligand

Pushpa Pate1 and P. K. Bhattacharya

Department of Chemistry, Faculty of Science, M. S. University of Baroda, Baroda, India

ABSTRACT

Cu(I1) binuclear binary and mixed-ligand complexes involving histidine [A] as binucleating ligand and amino acids (L = glycine, cY-alanine, phenylalanine, tyrosine, tryptophan) or bipy [L] have been investigated by potentiometry, UV-VIS spectrophotometry, and cyclic voltametry in aqueous medium at 30°C and I = 0.2 M(NaCl0,). Experimental data have been obtained for the following species: [CuAH], [CuA], [CuACu], [L&AH], [LCuA], [LCuACu], and [LCuACuL]. The formation of new binuclear species has been confirmed by UV-VIS spectral studies and CV studies in aqueous solution. The stability of the complexes has been explained on the basis of intramolecular interligand interactions between the side groups over the ligands.

INTRODUCTION

In biological systems such as carboxypeptidase, carbonic anhydrase B and C, thermolysin, plastocyanin, and superoxide dismutase, the imidazole group of histidine is an important binding site for Cu(I1) [l-5]. Even for the stacking interactions with the other biomolecules, it is a potential site. It is also known that the ternary complex [L-histidine-Cu(II)-albumin) acts as an intermediate in the exchange of Cu(I1) in blood [6].

Histidine is a potentially tridentate ligand having imidazole, amino, and carboxylate groups as metal ion binding sites. In the binary metal complexes of histidine, there are two modes of coordination of histidine [7-101. The type of bonding mode is strongly dependent on the pH. It has been observed by Pettit et al. [9] that at low pH (1.8-3.2) there is coordination of amino nitrogen and carboxyl oxygen with the C&I) ion, and the imidazole nitrogen remains nonco-

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 Biochemtitry, 54,187-197 (1994) 187

0 1994 Elsevier Science Inc., 655 Avenue of the Americas, NY, NY 10010 0162-0134/94/$7.00

188 P. Pate1 and P. R Bhattacharya

ordinated and protonated. In the pH range (3.5-4.7) coordination from amino- nitrogen and imidazole takes place, resulting in a histamine type of bonding.

Many mixed-ligand systems [Cu-His-AA] where AA = asparagine, a-gluta- mine, a-serine, a-homoserine, Dl-Zaminobutyric acid, Dl-2,3 diamino propionic acid, Dl-Ornithine, glycine, cY-alanine, phenylalanine, tyrosine, tryptophan, thre- onine, valine, ATP, or citrulline have been reported. It has been observed that the formation of ternary complex is favored due to intramolecular interligand interactions [ll-141.

Histidine, being a polyfunctional ligand, can form a binuclear complex il.51 in the presence of more than one equivalent of metal ions. It may coordinate through the amino acid site with one metal ion and through imidazole nitrogen with a second metal ion, or it can coordinate through the histamine end (N-N) with the first metal ion and through free carboxylate [-COO-l oxygen with the other metal ion. In the presence of other ligands there may be formation of mixed-ligand binuclear complexes. It is interesting to study such complex sys- tems, as the study is useful from the point of view of cases where hyperaccumu- lated metal ions are present in the biofluids.

In the present study, solution study of complex systems Cu-His-L, where L = amino acids such as glycine, cw-alanine, phenyl alanine, tyrosine, tryptophan, or L = bipy have been carried out, taking Cu(I1) in the excess to allow the formation of binuclear complexes.

EXPERIMENTAL

All reagents used were of A. R. grade and titrations were carried out in aqueous medium. The summary of other experimental parameters for the Cu(II) binary and mixed-ligand systems has been given in Table 1.

The values of proton-ligand formation constants of histidine and the forma- tion constants of [CuAHl and [CuAl complexes were recalculated and are in good agreement with that reported in the literature [17].

The refined values in Table 1 and the protonation constants and formation constants of binary complexes of amino acids with Cu(I1) [20] have been used as ftxed parameters for the refinement of mixed-ligand complexes.

The values of the formation constants of the different species and A 1ogK have been presented in Table 2 and 3. The deviations in the values have been shown in brackets. These indicate random errors in the titration dependent on the precision of the instruments. The computer output of the species distribu- tion as a function of pH has been shown in Figures 1, 2, and 3.’

Electronic Spectral Study

The electronic spectra of the Cu-Hist system having concentration 0.004:0.002M was recorded on a UV-240 shimadzu spectrophotometer in aqueous solution at

PH N 3.0 and N 6.0. The spectra of the system is shown in Figure 4.

’ The percentage of Cu(II) in binuclear species is to be considered twice that of the percentage of respective species.

BINUCLEAR COMPLEXES 189

TABLE 1. Summary of Experimental Parameters

Soltuion Compositions Binary systems a) 0.02 M HClO, : 0.006 M Cu(ClO,), : 0.003 M ligand (A): 0.171 M NaClO, b) 0.02 M HClO, : 0.003 M Cu(ClO,), : 0.001 M ligand (A): 0.176 M NaClO, Mixed @and systems a) 0.02 M HClO, : 0.002 M Cu(ClO,), : 0.001 M ligand (A): 0.002 M ligand (bipy): 0.175

M NaClO, b) 0.02 M HClO, : 0.005 M Cu(ClO,), : 0.002 M ligand (A): 0.005 M ligand (bipy) : 0.168

M NaClO, c) 0.02 M HClO, : 0.01 M Cu(C10J2 : 0.004 M ligand (A): 0.008 M ligand (LH): 0.158 M

NaCIO, d) 0.02 M HCIO, : 0.012 M Cu(CIO,), : 0.004 M ligand (A): 0.008 M ligand (LH) : 0.158

M NaClO,

Ionic strength electrolyte 0.2 M dmW3, NaClO,

Experimental method

Calibration of electrode pH Range Temperature Method of calculations Species considered

pH Titration using ORION RESEARCH ion analyzer 1901, accuracy fO.OO1 pH unit Using Buffer solutions; pH = 4.00, 7.00, and 9.00 3.50 to 6.25 pH 30°C SCOGS [16] Binary system: AH3+‘, AH2+, AH, Am3, Cu(II), [CUAH]+‘, [CuA]+, and [CUACUI+~ Mixed-ligand system where L = Bipy: AHa+‘, AH,+, AH, A-, [LCUAH]~~, [LCuA]+, and [LCUACUL]+~ Mixed-ligand system where L = amino acid: AHa+*, AI&+, AH, A-, LH2+*, LH, L-, C&I), [&AH]+*, [CuA]+, [CUACU]+~, [CuL]+, [LCuAH]+*, [L&A], and [LCuACu]+.

TABLE 2. Proton-Ligand Formation Constants and Formation Constants of C&I) Binary Complexes of Histidine in Aqueous Medium, I = 0.2 M (NaClO,); T = 3O”C, with Standard Deviation in Parentheses

Cu(I1) Complexes

Ligand K,H K,H K,H IogK& LogK& LogK&c,,

Histidine 9.26 (0.00) 9.26a

a Reference 17 values.

6.49 1.48 8.11 10.50 12.67 (0.02) (0.05) (0.03) (0.05) (0.00) 6.49a 1.48” 8.01a 10.50”

190 P. Pate1 and P. K Bhattacharya

TABLE 3. Formation Constants of Mixed-Ligand Complexes and A log K in Aqueous Medium, I = 0.2 (NaClO,) at 3O”C, with Standard Deviation in Parentheses

Complex logK&, A IogK logK$& A IogK logK&,, A IogK

[Cu-His-Bipy] 17.043 +0.13 19.03 -0.27 29.75 - 0.52

(0.05) (0.03) (0.10)

Complex logK%w A 1ogK logK%,, AlogK logK&,,u A log K’

[Cu-His-Gly] 15.93 -0.10 (0.03)

[Cu-His-a-Ala] 15.89 -0.14 (0.04)

[Cu-His-Phe] 15.75 +0.15 (0.02)

[Cu-His-Tyr] 15.61 + 0.20 (0.04)

[Cu-His-Tryp] 16.26 +0.17 (0.05)

17.92 (0.01) 17.78 (0.09) 16.96 (0.06) 17.64 (0.02) 18.46 (0.08)

- 0.50 19.94 - 0.65 (0.08)

- 0.64 20.09 -0.50 (0.10)

- 0.48 20.00 -0.16 (0.07)

-0.16 21.22 + 1.25 (0.02)

- 0.02 21.80 + 1.1s (0.05)

- ---- --------l

20-/JTLff I ,

FIGURE 1. Species distribution for the [Cu-His] [0.006:0.003 M] system. (1) Free Cu(II), (2)

3.C LC PI- - 5:o ’ 6.0 (CUAH), (3) (CuA) (4) (CuACu).

C. V. Study

The electrochemical measurements were made with an EG&G PARC electro- chemical analyzer. Voltammograms were recorded on a model RE 0089 X-Y plotter. The working electrode for electrochemical measurements was a plat- inum electrode [model 303 A]. An aqueous Ag/AgCl electrode and platinum wire were used as the reference and auxiliary electrodes, respectively. The

BINUCLEAR COMPLEXES 191

80

60-

A’//, 3.0 4.0 5: 0

pH - 6-O

FIGURE 2. Species distribution for the [Cu-His-bipy] [0.005:0.002:0.005M] system. (1) Ku-bipyl, (2) [CuAHLl, (3) [CuAL], (4) [LCuACuL].

voltammograms were recorded in aqueous solution at different pH with NaClO, as supporting electrolyte. The voltammograms for the complexes at different pH are shown in Figures 5 and 6.

RESULTS AND DISCUSSION

In the present study it is observed that in the presence of an excess of metal ions, histidine can form binuclear complexes [CuACu] along with the complexes [&AH] and [CuA]. The formation of the binuclear complex [CuACul starts around pH N 3.5 (Fig. 1). At lower pH there is formation of [CuAHl species indicating the coordination from the amino acid (O--N) end, and at pH N 3.0 the formation of species [&A] starts, which increases with pH indicating (N-N) coordination. There is simultaneous formation of [CuACu] species after pH N 3.5, indicating that the coordination of the first Cu(I1) is through the (N-N) end and the free carboxylate end gets coordinated to the second (Cu(II), forming the complex [CuACul.

192 P. Pate1 and P. K Bhattacharya

FIGURE 3. Species distribution for the [Cu-His-Trypto] [0.012:0.004:0.008~] system. (1) [CuAHl, (2) [CuAl, (3) [Cull, (4) [CuL], (5) [CuACu], (6) free Cu(II).

f5 E 2 FIGURE 4. Electronic spectra of the [Cu- A-----+ His1 system. (1) pH = 3.0, (2) pH = 6.0.

BINUCLEAR COMPLEXES 193

POTENTTAl IN VOLTS

FIGURE 5. Cyclic voltammograms of [Cu-His] at scan rate = 0.05 Vs-’ and pH = 3.

The equilibria can be shown as follows (charges have been omitted for simplicity):

AH,$AH+2H+ AH=A+H+,

CuW + AH * [&AH] (O--N coordination),

Cu(I1) + A e [GA] (N-N coordination),

[CuA] + Cu(I1) P [CuACu].

The formation of binuclear complexes can be supported by electronic spectral and CV studies. At lower pH a band is observed at N 790 nm (Fig. 41, corresponding to the [CuAI-I] complex, which is comparable with the band at _ 770 nm as observed in the 1:l (Cu-Hist) system [9]. The band in the present system is broadened due to the presence of free Cu(I1) in the solution, indicating that one equivalent of Cu(I1) combines with the amino acid (O--N) end of histidine forming [CuAH] complex and another remains free. At higher pH (N 6.0) a band at u 690 nm is observed, different from the band at _ 645 nm due to the mononuclear [GA] complex, indicating the formation of a species different from [CuA]. This supports the formation of the new species [CuACu]. CV studies also further support the formation of binuclear species.

194 P. Pate1 and P. K Bhattachalya

FIGURE 6. Cyclic voltammograms of [Cu-His] at scan rate = 0.05 Vs- ’ and pH = 5.80.

In the cyclic voltammogram (Figure 5) it is observed that at low pH, N 3.00, one broad peak at - 0.2 I, and another at more negative potential at -0.38 v are observed during cathodic scans and corresponding oxidation potentials at + 0.06 v and - 0.12 v are obtained during reverse scans. The peaks at -0.2 v (broad) and + 0.06 v are attributed to the redox process:

Free Cu(I1) 2 Cu(1). -e

Whereas redox peaks observed at more negative potential are attributed to the electronic process:

[Cu(IIL4H] z [CLl(IMH]. -e

At higher pH, N 5.80, (Fig. 6) a broad cathodic peak at about -0.26 v and corresponding anodic peak at +0.12 v were observed, which are different from the free Cu(II1 at low pH, and are attributed to the redox process of Cu(II) bound to the -COOP end of histidine.

Another cathodic peak observed at abut -0.45 v, comparable with the [Cu-histarntre] (1:l) complex, is attributed to the reduction process

[Cu(IIlAl -+ [Cu(I)Al. The corresponding anodic peak observed at abut - 0.18 v is due to the oxidation of [Cu(I)Al species to [Cu(II)A], in which Cu(II) is bound to histamine end.

BINUCLEAB COMPLEXES 195

Formation of Mixed-Ligand Binuclear Complexes

(I) [Cu(ZZ)-A-Z.1 where L = bipyridyl system. Three mixed-ligand species [LCuAH], [LCuA], and [LCuACuL] were found in the solution. At lower pH, N 2.50, there is coordination of [Cu-bipy] with [O--N] end of histidine forming the complex [L-&AH]. At pH m 3.5 the formation of [L-CuA] starts and increases with pH. The binuclear mixed-ligand complex forms around pH N 3.75 [Fig. 21.

The equilibrium reactions can be shown as follows:

[Cu(II) + bipy(L) S [CuLl,

[CUL] + AH +t [LCUAHI,

[CuLl+ A ti [LCuA],

ECuLJ + [LCuA] s [LCuACuL].

As observed earlier 1181, the stability constant value of the complex [LCuAHl is higher, i.e., A 1ogK ( = logK$u, - logK&, - logK$,) values for the com- plex [L&AH] is positive. This is due to the rr-acidic nature of bipy and the presence of hydrophobic interaction of the side group in the axial direction of the metal ion in ternary complex [LCuAH]. In the case of complex [LCuAl the A 1ogK value is negative indicating absence of such hydrophobic interactions as CL&) coordinates through the (N-N) end. It is further observes that the

A log K’ = [ logK;;uACuL - (log KC,;,,, + 2 log KC,:, I]

for the binuclear mixed-ligand complex [LCuACuL] is negative, indicating that the complex is destablized, compared to the binary binuclear complex [CuACu].

(2) [Cu(ZZ)-A-L] where L = amino acid systems. In the mixed-ligand complexes of the type [LCuA] where L = amino acid, three mixed-ligand species [L&AH], [L&Al, and [LCuACu] could be refined. The plots of concentration of various species vs. pH [Fig. 31 indicate that at low pH the concentration of species [LCuAHl is higher than the [LCuA]. The formation of binuclear complex [LCuACul takes place at a lower pH range. The species [LCuACuLl could not be refined as the coordination of second amino acid L monoanion to the Cu(II) already coordinated to the monoanionic -COO- end may be very weak and will not allow the formation of the complex [LCuACuLl.

As observed earlier [17], it has been found that for the complex [LCuAHl where L = phenyl alanine, tyrosine or tryptophan, the A 1ogK is positive and where L = glycine or a-alanine the A logK values are negative. The positive values of A 1ogK of the complexes [CuAHL] where L = pheala, Trp or Tyr is due to the presence of stacking interactions between the phenyl, indol, or phenol groups over the amino acids and the imidazole group of the histidine. The A 1ogK values for the complexes [LCuA] are negative, but are less negative for the complexes where L = tryptophan or tyrosine, as expected, indicating the presence of hydrogen bonding of free-COO- group of histidine and -NH or

196 P. Fate/ and P. K. B~attacha~a

-OH groups of t~ptophan or tyrosine as suggested earlier [14] and can be represented as in Scheme I below:

r

;; /==N /C-O\cu,N / W Y./-

CH

i \ H2N’

1 \NH2- CH

‘32 I

LD

c=o

0 0 ___f.&__Q /

~

The values of A log K’2 of the [CuACuL] type of complexes where IL = tryptophan or tyrosine] are more positive than the complexes where L = glycine, cy-alanine, or phenylalanine. These more positive values of the A logK2 is probably due to the weak coordination of free -OH or -NH group of the secondary ligands, tyrosine or trytophan, to the second Cu(I1) ion, which is bound to the -COO- end of histidine, as shown in Scheme 11 below:

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* A log K’ = [log K&our_ - (log K&co + log K&)1.

BINUCLEAR COMPLEXES 197

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Received April 23, 1993; accepted June 11, 1993