On the K-absorption spectra of copper complexes as models of biologically active metallobiomolecules

Specrrochimica Acro, Vol.4lA, Printed in Great Britain.

No. 3, pp. 495498,

1985 0

0584-8539/85 ~3.00 + 0.00 1985 Pcrgamoa Press Ltd.

On the K-absorption spectra of copper complexes as models of biologically active metallobiomolecules SUJATAPATIL,* ALOK KUMAR,* B. D. PADALIASand S. V. DEsHPANDEt *Department of Physics, Indian Institute of Technology, Bombay 400076, India and THindustan Lever Research Centre, Andheri, Bombay, India (Receioed for publication 20 September 1984) Abstract-The results of an X-ray absorption spectroscopic study of the models of some biologically active metallobiomolecules, such as Cu(II) dipeptide complexes with imidazole, 2-methylimidazole, 2,2’-bipyridine and l,lO-phenanthroline, are reported here. The present XAS study provides information on the nature of chemical bonding in these compounds.

EXPERIMENTAL

INTRODUCFION

Metallobiomolecules are metal ions associated with biomolecules, which in biologically active forms contain integral stoichiometries of one or more metallic elements. These metallobiomolecules are involved in some fundamental biological processes such as electron storage and electron transfer (Fe, Cu), dioxygen binding, storage and activation (Fe, Cu) and substrate activation and catalysis (Mg, Mn, Fe, Cu, Zn and MO). The metal containing sites (coordination sites/active sites) are usually the loci of the above mentioned biological process&. There has been considerable research in recent years to model the active sites of these metallobiomolecules by synthetic metal complexes [ 11. Recently, copper(I1) dipeptide and its imidazolebridged complexes have been suggested as models of active sites of copper enzymes such as galactose oxidase and fully copper(H) substituted superoxide dismutase[2-4]. In addition, the complexes of copper(H) dipeptide with imidazoles can act as models of the bonding of copper(H) and imidazoles from histidine residues of blue and nonblue copper proteins [S]. l,lO-Phenanthroline and 2,2’-bipyridine inhibit the enzyme activities of the metalloenzymes by forming protein-metal inhibitor ternary complexes and, thus, block the biochemical reactions [6]. The copper(I1) dipeptide complexes of 2,2’-bipyridine and l,lO-phenanthroline have recently been reported as models of the above mentioned ternary complexes [7]. In view of the importance of copper(H) dipeptide complexes with the above mentioned nitrogen donor ligands, we have made an attempt to study the properties of these complexes using the X-ray absorption spectroscopic (XAS) technique. The results of this study are discussed in this paper.

SAuthor to whom correspondence should be addressed.

The compounds were prepared by the methods described in the literature [S, 73. The experimental set up used in the present work to record the Cu K-absorption spectra has been described elsewhere[S, 93. Essentially, a sealed Machlett Xray tube with tungsten target was employed as a source of continuous radiation. The X-ray tube was operated at 20 kV and lOmA. A 4Ocm Cauchois type X-ray spectrometer was used to record the spectra on the photographic plate. A @Ol) plane of mica, diffracting X-rays in the first order, gave a dispersion of about 12 X.U.mn-‘. Measurements were made on the microphotometer records. W&, , WLaI, WL~4and Wt,, emission lines appearing in the spectra were used as the reference lines. The wavelengths of these lines were taken from BEARDEN’S table [lo]. The error in the measurement of energies does not exceed f 0.5 eV.

RESULTSAND DISCUSSION The compounds studied in the present work have copper in the +2 oxidation state with square pyramidal geometry. This has been inferred from the single crystal X-ray diffraction studies [11-131 as well as from the electronic and magnetic properties [S, 71. The shapes of the K-absorption curves of copper in these copper(H) complexes are shown in Figs 1 and 2. The measured values of energy of the principal absorption maximum A are given in Table 1. The peak A on the high energy side of the absorption edge corresponds to an electron transition from 1s core level to the first unoccupied 4p level. As the complexes used in the present study have approximate D4,, symmetry [S, 71, the absorption maximum A can be assigned to the molecular orbital transition, ls-+a,,(n*)[14]. Normally, the position of the peak A shifts toward higher energy side with an increase in the oxidation state [15]. However, in the present work, copper in all the copper complexes is in the +2 oxidation state. Therefore, the shift in the position of peak A can be explained on the basis of the overlap of ligand and metal orbitals. More overlap between ligand and metal orbitals (more covalent character) is expected to form more stable bonding molecular orbital(s) and cor-

495

SUJATA PATIL et al.

496

I 9ooO

8980

I 9020

I 9040

I 9060

Energy (eV I 8980

I 9000

I 9020

Energy

Fig. 1. K-absorption

I 9040

(eV

I 9060

I 9080

3

1

edge and EXAFS of Cu in glycylglycine complexes of Cu(II).

respondingly less stable antibonding orbital(s). The shifts of peak A to the higher energy side may therefore be attributed to the overlap between metal orbitals and ligand orbitals which, in turn, implies an increase in the percentage covalency of the bond. A comparison of peak A position in structurally similar complexes can be made with the help of the above criterion. The complex Id has square pyramidal geometry about the copper [l l] ion with three donor

Table

1. Energies

(in eV) of the maxima

and minima

Compound

atoms of glycylglycinate anion, one donor nitrogen atom of imidazole and one water molecule at the apical positions. A similar geometry is expected for 2c and 3s (Fig. 3~). The peak A energy values of these copper complexes decrease in the order 3a > 2c > Id and, thus, reflect the decrease of covalency in the same order. The order of covalency in these complexes is opposite to the order of electron withdrawing capacity of groups R, of dipeptides (see Fig. 3~). The electron withdrawing

order of the R, group is: XH,

in EXAFS associated complexes

with K-absorption

edge of Cu in copper(U)

B

Radii of the first coordination sphere r* (in A)

9068.9

3.02

9067.0 9088.0

3.06

A

a

Id le

Cu(GlyGly).3H,O Cu(Gly.Gly)(bipy).3H,O Cu(Gly.Gly)(phen).3HZ0 Cu(Gly.Gly)(imH). 2H,O Cu(Gly.Gly)(2-meimH).2H,O

9003.9 9005.0 9003.7 9002.0 9006.7

9032.8 9029.2 9030.6 9029.8 9032.7

9052.4 905 1.6 9050.9 9051.7 9054.4

Za 2b

Cu(Gly.Tyr)(bipy).4H20 Cu(Gly.Tyr)(phen)‘3H,O

9005.6 9004.6

9030.0 9028.0

9055.7

9071.6 -

3.08

2c 2d

Cu(Gly.Tyr)(imH).H,O Cu(Gly.Tyr)(Z-meimH).HzO

9002.6 9003.7

9029.7 9030.6

9050.8 9050.9

9062.6 9067.0

3.57 2.23

3a

Cu(Gly.Phe)(imH).

9004.5

9036.1

9057.2

9076.8

2.77

la lb IC

HZ0

10

)

Fig. 2. K-absorption edge and EXAFS of Cu in glycyl-ttyrosine and glycyl-L-phenylalanine complexes of Cu(II).

EXAFS No.

I

9080

B

Gly.Gly = glycyl glycine, Gly.Tyr = glycyl-L-tyrosine, Gly.Phe = glycyl-L-phenylalanine, methylimidazole, bipy = 2,2’-bipyridine, phen = l,lO-phenanthroline. *Determined by LEVY’S method [31).

imH = imidazole.

2.1 I

2-meimH

= 2-

Copper complexes model metallobiomolecules

OH2 cH2-NT2c~_o” I o-c\/\o

2

N>c

1 2

--CNo

30. Structure of Cu (Gly. G(y).

3H20

TN? /“-I \ /A-”

o--c

N\

9

;7°-\o

3b. Structure of ternary complexes of Cu (II 1 dipeptider with l,lO- phenanthroline or 2,2’- bipyridine

1

I, IO-phenanthroline or 2,2-bipyridine

R-H for glycylglycine for

R-MeOH

glycyl -L- tyrosine

3c. Structure of odducts of imidozole ond 2- methylimidozole with

copper

(II 1dipeptides

R,=H

for glycylglcine

=CH,

-u

0 0

(342

OH for glycyl -I_- tyrosine

for gtycyt

-L-phenylolonine

-0

R,= R,= H for imidozole R,= CH3, R,- H for 2-methytimidozole

Fig. 3. Structure of ternary complexes of Cu(II) dipeptides with l,lO-phenanthroline, 2,2’-bipyridine, imidazole, and 2methylimidazole.

<
0 OH < H [16]. This causes the deucrease of covalency in these complexes in the order depicted by our experimental data. The more electron withdrawing order of -CH,

0 OH compared with H can also be seen -0 in the comparison of peak A in the compounds lb and 2a, lc and 2b and ld and 2c. A larger overlapping of ligand orbitals and metal orbitals in glycyl-L-tyrosine (Gly.Tyr) compounds, 2a, 2b and 2c, than in corresponding glycyl-glycine (Gly.Gly) compounds, lb, lc and ld, shifts the peak A to the higher energy side in

491

Gly.Tyr compounds compared with the corresponding Gly.Gly compounds. However, le and 2d are an exception to this where le shows a larger shift than 2d. On comparison of A values of lb and lc complexes (see Table l), the 2,2’-bipyridine derivative has higher value of A than the l,lO-phenanthroline derivative. A similar trend of A values is also found in 2a and 2b complexes. This suggests that 2,2’-bipyridine is a better donor than l,lO-phenanthroline in these complexes. This is in contrast with their proton affinity [ 171, and suggests that the steric hindrance of bulky l,lOphenanthroline [18] in its complexes lc and 2b could be attributed to the reversal of the above trend. A similar experimental observation was also found by other workers in case of [Cu(bipy)Cl,] and [Cu(Phen)Cl,] [19]. Lastly, a similar comparison can also be made for Id and lc, and for 2c and 2d. The -CH, group attached to imidazole will increase the electron donating nature of the ligand, and will therefore be more covalent. Our experimental data show a higher shift of peak A for le than for Id, and also for 2d than for 2c which corroborate with the above explanation. It has been noted by others that the principal absorption peak A in some compounds becomes very prominent, and it is called “white line” or “resonance absorption” [20,21]. The origin of the white line has been qualitatively understood in terms of the high density of final states or exciton effects [22-241. The ionic&y of the bonding is known to enhance the white line while the covalent character of the bond suppresses it [25-271. It appears that the chemical bond has appreciable covalent character in these copper complexes and this has suppressed the white line. This is a plausible explanation for the absence of the white line in the copper complexes studied in the present work. Alternatively, the bonding in these complexes can be understood in terms of molecular orbital theory. Considering the Dbk symmetry of these square pyramidal copper(I1) complexes, the bonding parameters have been calculated [5,7] with the help of EPR spectral data using a modified semiempirical LCASMO method of KEVILSON and NEIMAN[28-301. The values of the n-bonding parameters j?: (in-plane) and /?’ (out-of-plane) and cbonding parameters a2 (in-plane) and a” (out-ofplane) suggest appreciable covalency in these copper(I1) complexes. This result is consistent with the inference drawn from the X-ray absorption study. The extended fine structure (EXAFS) associated with the K-absorption edge of Cu is shown in Figs 1 and 2. The measured energies of the absorption maxima (A, B, . . .) and minima (a, fi, . . .) in EXAFS are included in Table 1. It should be mentioned here that the above notations LZ,j3, . . . (EXAFS minima) have a different meaning from those used earlier to represent the bonding parameters (a2, a’2, p2, fi:). The radii of the first coordination sphere calculated for some of these compounds using LEVY’Smethod [31] are also given in Table 1. The EXAFS data provide

SUJATA PATIL et al.

498

average bond lengths in the copper(H) studied in the present work.

complexes

Acknowledgements-One of the authors (A.K.) wishes to thank the Council of Scientific and Industrial Research, New Delhi, for financial support. Thanks are also due to Professor T. S SRIVASTAVAfor his interest and valuable suggestions. REFERENCES J. A. ALBERS and R. H. HOLM, Science 209,223 (1980). T. S. SR~VASTAVAand S. V. DESHPANDE, Inorg. chim. Acta 55, L39 (1981). Ii31 Y. NAKAO, W. MORI, T. SAKLJRAIand A. NAKAHARA, Inorg. chim. Actc 55, 103 (1981). [41 S. V. DESHPANDE and T. S. SRIVASTAVA, Polyhedror! 3, 462 (1984). [51 S. V. DESHPANDE and T. S. SRIVASTAVA, Polyhedron, 2, 761 (1983). [61 B. L. VALLEE and W. E. C. WACKER,~~I~ Protein, Vol. V, pp. 12943. Academic Press, New York (1970). [‘I S. V. DESHPANDE and T. S. SRIVASTAVA, Inorg. chim. Acta 78, 75 (1983). PI B. D. PADALIA, S. N. GUPTA and V. KRISHNAN, J. them. Phys. 58, 2084 (1973). II91P. N. KOUL, B. D. PADALIA and M. N. GHATIKAR, J. Phys. Sot. Japan 50, 246 (1981). J. A. BEARDEN, Rec. mod. Phys. 39, 78 (1967). Et; B. STRAUNDBERG, I. LINDQUIST and R. RESENTEIN, Z. Krisrullogr. 116, 266 (1961). [121 R. J. SUNDEBERGand R. B. MARTIN, Chem. Rec. 74,471 (1974).

[II

PI

M. C. LIM, E. SINN and R. B. MARTIN, Inorg. Chem. 15, 807 (1976). [I41 H. B. GRAY and C. J. BALLHAUSEN,J. Am. them. Sot. 85, 260 (1963). [151 G. L. GLEN and C. G. DODD, J. appl. Phys. 39, 5372 (1968). [161 J. D. ROBERTS and M. C. CASERIO, Modern Organic Chemistry, p. 369. Benjamin, New York (1967). [I71 A. E. MARTELL and R. M. SMITH, Critical Stability Constants, Vol. 5, pp. 248, 254. Plenum Press, New York (1982). E. D. MCKENZIE, Coord. Chem. Reo. 6, 187 (1971). I:;] A. KUMAR, A. N. NIGAM and B. D. SRIVASTAVA,J. Phps. Chem. 9, 3523 (1980). W. SEKA and H. P. HANSON, 500(l), 344 (1969). I;:] H. HANSON and W. W. BEEMAN, Phys. Rec. 76(l), 118 (1949). N. F. MO=, Proc. R. Sot. 62, 416 (1949). [Ej Y. CAUCHOIS and N. F. Morr, Phil. Msg. 40, 1260 (1949). [241 M. BROWN, R. E. PEIERLS and E. A. STERN, Phys. Rec. B15, 738 (1977). N. N. SEXANA, J. Phys. C8, 1450 (1975). 1::; U. C. SRIVASTAVAand H. L. NIGAM, Coord. Chem. Rec. 9, 275 (1973). [271 L. P. VERMA and B. K. AGARWAL, J. Phys. Cl, 1658 (1968). WI D. KEVILSON and R. NEIMAN, J. them. Phys. 35, 149 (1961). v91 L. CASELLA, M. GULLOTTI, A. PASINI and A. ROCKENBAUER, Inorg. Chem. 18, 2825 (1979). A. ROCKENBAUER, J. Mag. Res. 35,429 (1979). R. M. LEVY, J. them. Phys. 43, 1846 (1965).

[I31