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Synthetic models for active sites of reduced blue copper proteins: minimal geometric change between two oxidation states for fast self-exchange rate constants
Inorganic Chemistry Communications 7 (2004) 1188–1190 www.elsevier.com/locate/inoche
Synthetic models for active sites of reduced blue copper proteins: minimal geometric change between two oxidation states for fast self-exchange rate constants Kiyoshi Fujisawa a,*, Koyu Fujita b, Tatsuya Takahashi b, Nobumasa Kitajima b, Yoshihiko Moro-oka b, Yuki Matsunaga a, Yoshitaro Miyashita a, Ken-ichi Okamoto b
a
a Department of Chemistry, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8571, Japan Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received 5 May 2004; accepted 8 September 2004 Available online 1 October 2004
Abstract We have prepared two thiolato copper redox pairs ligated by HBð3; 5-i PrpzÞ 3 as models for blue copper proteins. The kex value of the coordination number invariant Cu(II/I) couples is 51 times faster than that of the coordination number variant Cu(II/I) couples at 20 C. We also discuss the electron-transfer process. 2004 Elsevier B.V. All rights reserved. Keywords: Copper(I); Hydrotris(pyrazolyl)borate; Blue copper protein; Crystal structure; Model complex
Blue copper proteins are electron carriers in biological systems [1]. Because of their very unusual spectral features, including EPR and UV–Vis spectra, the active site structures of this class of proteins have received considerable attention for quite some time [1]. Based on X-ray analysis, it has been shown that the blue copper active site has an unusual distorted tetrahedral coordination geometry in the oxidized form, with two imidazole N atoms from histidine residues, one thiolate S atom from cysteine residue, and one sulfide S atom of methionine residue [2]. This unique active site structure is associated with the unusual spectral features and is believed to be related to the function of blue copper proteins, specifically their high redox potentials and fast electron-transfer rates [1–3].
*
Corresponding author. Tel.: +81 29 853 6922; fax: +81 29 853 6503. E-mail address: [email protected] (K. Fujisawa). 1387-7003/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.09.004
In order to probe these hypotheses, many model compounds have been reported, which include coordination number invariant Cu(II/I) couples [1b,4], and coordination number variant Cu(II/I) couples [1b,5]. Contrary to generally accepted hypotheses, a fast electron-transfer rate is not always observed with coordination number invariance upon redox reaction [1b,4]. Therefore, we synthesized a set of thiolato copper complexes which show both small and large differences in the inner-coordination sphere of the copper atoms upon converting from Cu(II) to Cu(I) with the same ligand, HB(3,5-iPr2pz)3 (= hydrotris(3,5-diisopropyl-1-pyrazolyl)borate anion; referred to as L1). The copper atoms in both [CuII(SC6F5)(L1)] (1) [6] and K[CuI(SC6F5)(L1)] (2) (HSC6F5 = pentafluoro benzenethiol) display a distorted tetrahedral geometry. In contrast to this fixed coordination number system, we also prepared a system which changes coordination number upon redox reaction: the five-coordinate Cu(II) complex [CuII(SMeIm)(L1)] (3) [7] (square-pyramidal) most likely changes to the four-coordinate Cu(I) complex
K. Fujisawa et al. / Inorganic Chemistry Communications 7 (2004) 1188–1190
Fig. 1. Molecular structure of the anion of 2 (ORTEP, 50% proba˚) bility ellipsoids) with atomic numbering. Selected bond distances (A and angles () are as follows. Cu–S, 2.192(2); Cu–N11, 2.188(7); Cu– N21, 2.065(7); Cu–N31, 2.062(7) and S–Cu–N11, 126.9(2); S–Cu–N21, 119.7(2); S–Cu–N31, 131.0(2); N11–Cu–N21, 89.0(3); N11–Cu–N31, 90.3(3); N21–Cu–N31, 87.2(3).
K[CuI(SMeIm)(L1)] (4) (distorted tetrahedral) upon reduction (HSMeIm = 2-mercapto-1-methylimidazole). The electron self-exchange rate constants of both Cu(II/ I) redox pairs (1/2 and 3/4) and the activation parameters of Cu(II/I) redox pairs (1/2) were determined by the dynamic 1H NMR line-broadening techniques in (CD3)2CO. In a manner analogous to that applied to the preparation of thiolato complex [CuII(SC6F5)(L1)] (1) [6], [CuII(SMeIm)(L1)] (3) has been synthesized and structurally characterized [7]. The Cu(II) atom in 3 has a square pyramidal geometry with an N4S ligand donor set [8]. The oxidized complex 1 well reproduces some of the physicochemical properties of blue copper proteins [6]. The reduced forms of 2 and 4 were prepared by the reaction of the ligand KL1 with CuSR (R = C6F5, MeIm). 1 Only the molecular structure of 2 was determined by X-ray crystallography (Fig. 1). 2 Because 4 is very unstable toward dioxygen, its structure has not been 1
Experimental details are reported in the supporting information. Crystal data for 2 Æ 2((CH3)2C(OH)CH2COCH3): white plate 0.40 · 0.20 · 0.08 mm3, Formula: C45H70N6BO4F5SKCu, Fw: 999.59, at 213 K, monoclinic space group P21/a (#14), a = 23.490(6), ˚ , b = 109.52(2), V = 5348(2) A ˚ 3, Z = 4, b = 11.445(3), c = 21.109(4) A 3 1 Dcalc = 1.24 g cm , l(Mo Ka) = 5.86 cm . Of 6130 unique reflections measured, 3378 [I > 2r(I)] were used in refinement (R = 0.059, Rw = 0.063, GOF = 1.78). 5: white block, 0.20 · 0.20 · 0.10 mm3, Formula: C31H51N8BSZn, Fw: 644.05, at 300 K, monoclinic space group P21/n (#14), a = 15.334(9), b = 9.821(6), c = 25.165(12), ˚ 3, Z = 4, Dcalc = 1.17 g cm3, l(Mo b = 104.29(4), V = 3672(3) A 1 Ka) = 7.57 cm . Of 6161 unique reflections measured, 1961 [I > 3r(I)] were used in refinement (R = 0.071, Rw = 0.075, GOF = 1.83). The crystallographic data and collection details are summarized in Table S1 (supporting information). 2
1189
defined yet. When the structure of 2 is compared with that ˚ ; 1, of 1 [6], the Cu–S bond length is constant (2, 2.192 (2) A ˚ 2.176 (4) A) and the two Cu–N bond lengths are marginally increased. Small changes include a slight expansion of the copper site as measured with bond distances (parallel to the expected slight increase in the ionic radius of Cu(I) compared with that of Cu(II)) and movement of the copper ion mainly toward axial N21 (distance of copper from ˚ ; 1, 0.34 A ˚ ). The the plane (N11, N31, and S): 2, 0.41 A Cu(I)–S bond distance in 2 is the same as that in the reported copper (I) thiolato complex, K[Cu(SC6H4-p˚ ) (HB(3, NO2){HB(3,5-Me2pz)3}] Æ 2C3H6O (2.19(1) A 5-Me2pz)3 = hydrotris(3,5-dimethyl-1-pyrazolyl)borate anion) [9]. The small geometry change associated with the redox reaction involving 1 and 2 is consistent with the requirements for fast electron-transfer. Therefore, the reorganization energy associated with redox reaction should be small [1–3]. Similar small geometric changes have been found between the reduced and oxidized structures in blue copper proteins: azurin, plastocyanin, pseudoazurin, rusticyanin, amicyanin, and stellacyanin [2]. As mentioned above, the coordination structure of 4 was difficult to determine. However, the four-coordinate Zn(II) derivative [Zn(SMeIm)(L1)] (5) was characterized by X-ray crystallography (Figure S2) 1, 2. A comparison of the 1H and 13C NMR data for 4 and 5 suggests that 4 should be also four-coordinate geometry (the imidazole nitrogen does not coordinate the d10 metal ion in 4 or 5). Thus, a large geometric change must occur in the redox reaction involving 3 and 4. The electron self-exchange rate constants, kex (M1 s1), for [CuII/I(SR)(L1)]0/1 (1/2 and 3/4) were measured by using the line-width method for the determination of 1/T2 [4,5] (Eq. (1)). A plot of 1/T2obs versus added [CuII(SR)(L1)] (1 or 3) gives a straight line with a slope equal to the self-exchange rate constants, kex (Fig. 2). pDm1=2 ¼ 1=T 2obs ¼ 1=T 2n þ 1=T 2e ¼ k ex ½CuII ðSRÞðL1Þ þ 1=T 2n :
ð1Þ
The resulting experimental values for kex at 20 C in (CD3)2CO are 1.76(2) · 104 M1 s1 (three different experiments) for [CuII/I(SC6F5)(L1)]0/1 (1/2) and 3.47(17) · 102 M1 s1 (three different experiments) for [CuII/I(SMeIm)(L1)]0/1 (3/4). Thus, the kex of the redox pair 1/2 is about 51 times faster than that of the redox pair 3/4. Moreover, the data used to construct the Eyring plots, and least-squares analyses of these data gave DH = 35(2) kJ mol1, DS = 22(8) J mol1 K1 for the redox pair 1/2. 1 This result indicates an outersphere electron-transfer mechanism [1–3] and the kex of the redox pair 1/2 at 25 C is calculated as 3.14 · 105 M1 s1 obtained by extrapolation of the corresponding Eyring plot. These self-exchange rate constants and their activation parameters are consistent with the values for blue copper proteins [1–3].
1190
K. Fujisawa et al. / Inorganic Chemistry Communications 7 (2004) 1188–1190
displacement parameters, bond distances, bond angles, and torsion angles are available. Crystallographic data reported in this paper have been deposited with Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC-233829 (2) and 233830 (5). Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or [email protected]). Supplementary data associated with this article can be found, in the online version at doi:10.1016/j.inoche.2004.09.004. Fig. 2. Plots of 1/T2obs (s1) versus [CuI(SR)(L1)] (mmol dm3) for pyrazole rings proton resonance of [CuI(SR)(L1)] at 20 C in (CD3)2CO: (d) R = C6F5, (m) R = MeIm. Measurements were performed with [Cu(SR)(L1)] at 16.7 mmol dm3 for 2 and 18.4 mmol dm3 for 4.
Thus, in the case of the same supporting ligand, the coordination number invariant system, whose properties are very similar to blue copper proteins, gives faster self-exchange rate constants (51 times), and its reorganization energy should be low through an outer-sphere electron-transfer process. In contrast, the previously reported coordination number invariant system did not have fast electron-transfer rate constants [1b,4–5]. Solomon and co-workers have suggested that the changes in electronic structure (short Cu–S(Cys) bond distance and/or long axially coordinated Cu–S(Met) bond distance) are associated with its electron-transfer rate constants [1,10]. Therefore, the electron self-exchange rate constants are related to its electronic change as well as its geometric change upon redox reaction in model compounds and blue copper proteins. Moreover, the hydrophobic interactions of the electron self-exchange process are also important [1–3]. We are currently further investigating these electron transfer processes by using other ligand systems.
Acknowledgements This research was in part supported by Grant-in-Aid for Scientific Research (B) (13555257 and 14350471) and the 21st Century COE program from Japan Society for the Promotion of Science and Tokuyama Science Foundation.
Appendix A. Supplementary material Experimental details (2, 4, and 5) are provided. And tables of crystallographic details, atomic coordinates, positional and isotropic thermal parameters, anisotropic
References [1] (a) E.I. Solomon, R.K. Szilagyi, S. DeBeer George, L. Basumallick, Chem. Rev. 104 (2004) 419; (b) D.B. Rorabacher, Chem. Rev. 104 (2004) 651; (c) E.I. Solomon, D.W. Randall, T. Glaser, Coord. Chem. Rev. 200–202 (2000) 595; (d) D.W. Randall, D.R. Gamelin, L.B. LaCroix, E.I. Solomon, J. Biol. Inorg. Chem. 5 (2000) 16. [2] A. Messerschmidt, R. Huber, K. Wieghardt, T. Poulos, (Eds.), Handbook of Metalloproteins, Wiley, 2001. (Coordination atoms of typical blue copper proteins: plastocyanin, His 37, His 87, Cys 84, and Met92). [3] (a) H.B. Gray, B.G. Malmstro¨m, R.J.P. Williams, J. Biol. Inorg. Chem. 5 (2000) 551; (b) R.J.P. Williams, Eur. J. Biochem. 234 (1995) 363; (c) B.G. Malmstro¨m, Eur. J. Biochem. 223 (1994) 711. [4] (a) B. Xie, T. Elder, L.J. Wilson, D.M. Stanbury, Inorg. Chem. 38 (1999) 12; (b) S. Flangan, J. Dong, K. Haller, S. Wang, W.R. Scheidt, R.A. Scott, T.R. Webb, D.M. Stanbury, L.J. Wilson, J. Am. Chem. Soc. 119 (1997) 8857; (c) S. Knapp, T.P. Keenan, X. Zhang, R. Fikar, J.A. Potenza, H.J. Schugar, J. Am. Chem. Soc. 112 (1990) 3452. [5] (a) E.A. Amburndo, Q. Yu, L.A. Ochrymowycz, D.B. Rorabacher, Inorg. Chem. 42 (2003) 5267; (b) Q. Lu, C.A. Salhi, E.A. Ambundo, M.J. Heeg, L.A. Ochrymowycz, D.B. Rorabacher, J. Am. Chem. Soc. 123 (2001) 5720. [6] N. Kitajima, K. Fujisawa, M. Tanaka, Y. Moro-oka, J. Am. Chem. Soc. 114 (1992) 9232. [7] L. Basumallick, S. DeBeer George, D.W. Randall, B. Hedman, K.O. Hodgson, K. Fujisawa, E.I. Solomon, Inorg. Chim. Acta 337 (2002) 357. [8] (a) D.M. Arciero, B.S. Pierce, M.P. Hendrich, A.B. Hooper, Biochemistry 41 (2002) 1703; The oxidized complex 3 reproduced the spectral properties of red copper proteins, including the 360 nm band in UV–Vis spectrum and large Ai value (157 G) in the EPR spectrum. Unfortunately their self-exchange rate constants have not been reported yet. (b) R.L. Lieberman, D.M. Arciero, A.B. Hooper, A.C. Rose nzweig, Biochemistry 40 (2001) 5674. [9] J.S. Thompson, T.J. Marks, J.A. Ibers, J. Am. Chem. Soc. 101 (1979) 4180. [10] (a) L.B. LaCroix, S.E. Shadle, Y. Wang, B.A. Averill, B. Hedman, K.O. Hodgson, E.I. Solomon, J. Am. Chem. Soc. 118 (1996) 7755; (b) J.A. Guckert, M.D. Lowery, E.I. Solomon, J. Am. Chem. Soc. 117 (1995) 2817.
Synthetic models for active sites of reduced blue copper proteins: minimal geometric change between two oxidation states for fast self-exchange rate constants Kiyoshi Fujisawa a,*, Koyu Fujita b, Tatsuya Takahashi b, Nobumasa Kitajima b, Yoshihiko Moro-oka b, Yuki Matsunaga a, Yoshitaro Miyashita a, Ken-ichi Okamoto b
a
a Department of Chemistry, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8571, Japan Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received 5 May 2004; accepted 8 September 2004 Available online 1 October 2004
Abstract We have prepared two thiolato copper redox pairs ligated by HBð3; 5-i PrpzÞ 3 as models for blue copper proteins. The kex value of the coordination number invariant Cu(II/I) couples is 51 times faster than that of the coordination number variant Cu(II/I) couples at 20 C. We also discuss the electron-transfer process. 2004 Elsevier B.V. All rights reserved. Keywords: Copper(I); Hydrotris(pyrazolyl)borate; Blue copper protein; Crystal structure; Model complex
Blue copper proteins are electron carriers in biological systems [1]. Because of their very unusual spectral features, including EPR and UV–Vis spectra, the active site structures of this class of proteins have received considerable attention for quite some time [1]. Based on X-ray analysis, it has been shown that the blue copper active site has an unusual distorted tetrahedral coordination geometry in the oxidized form, with two imidazole N atoms from histidine residues, one thiolate S atom from cysteine residue, and one sulfide S atom of methionine residue [2]. This unique active site structure is associated with the unusual spectral features and is believed to be related to the function of blue copper proteins, specifically their high redox potentials and fast electron-transfer rates [1–3].
*
Corresponding author. Tel.: +81 29 853 6922; fax: +81 29 853 6503. E-mail address: [email protected] (K. Fujisawa). 1387-7003/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2004.09.004
In order to probe these hypotheses, many model compounds have been reported, which include coordination number invariant Cu(II/I) couples [1b,4], and coordination number variant Cu(II/I) couples [1b,5]. Contrary to generally accepted hypotheses, a fast electron-transfer rate is not always observed with coordination number invariance upon redox reaction [1b,4]. Therefore, we synthesized a set of thiolato copper complexes which show both small and large differences in the inner-coordination sphere of the copper atoms upon converting from Cu(II) to Cu(I) with the same ligand, HB(3,5-iPr2pz)3 (= hydrotris(3,5-diisopropyl-1-pyrazolyl)borate anion; referred to as L1). The copper atoms in both [CuII(SC6F5)(L1)] (1) [6] and K[CuI(SC6F5)(L1)] (2) (HSC6F5 = pentafluoro benzenethiol) display a distorted tetrahedral geometry. In contrast to this fixed coordination number system, we also prepared a system which changes coordination number upon redox reaction: the five-coordinate Cu(II) complex [CuII(SMeIm)(L1)] (3) [7] (square-pyramidal) most likely changes to the four-coordinate Cu(I) complex
K. Fujisawa et al. / Inorganic Chemistry Communications 7 (2004) 1188–1190
Fig. 1. Molecular structure of the anion of 2 (ORTEP, 50% proba˚) bility ellipsoids) with atomic numbering. Selected bond distances (A and angles () are as follows. Cu–S, 2.192(2); Cu–N11, 2.188(7); Cu– N21, 2.065(7); Cu–N31, 2.062(7) and S–Cu–N11, 126.9(2); S–Cu–N21, 119.7(2); S–Cu–N31, 131.0(2); N11–Cu–N21, 89.0(3); N11–Cu–N31, 90.3(3); N21–Cu–N31, 87.2(3).
K[CuI(SMeIm)(L1)] (4) (distorted tetrahedral) upon reduction (HSMeIm = 2-mercapto-1-methylimidazole). The electron self-exchange rate constants of both Cu(II/ I) redox pairs (1/2 and 3/4) and the activation parameters of Cu(II/I) redox pairs (1/2) were determined by the dynamic 1H NMR line-broadening techniques in (CD3)2CO. In a manner analogous to that applied to the preparation of thiolato complex [CuII(SC6F5)(L1)] (1) [6], [CuII(SMeIm)(L1)] (3) has been synthesized and structurally characterized [7]. The Cu(II) atom in 3 has a square pyramidal geometry with an N4S ligand donor set [8]. The oxidized complex 1 well reproduces some of the physicochemical properties of blue copper proteins [6]. The reduced forms of 2 and 4 were prepared by the reaction of the ligand KL1 with CuSR (R = C6F5, MeIm). 1 Only the molecular structure of 2 was determined by X-ray crystallography (Fig. 1). 2 Because 4 is very unstable toward dioxygen, its structure has not been 1
Experimental details are reported in the supporting information. Crystal data for 2 Æ 2((CH3)2C(OH)CH2COCH3): white plate 0.40 · 0.20 · 0.08 mm3, Formula: C45H70N6BO4F5SKCu, Fw: 999.59, at 213 K, monoclinic space group P21/a (#14), a = 23.490(6), ˚ , b = 109.52(2), V = 5348(2) A ˚ 3, Z = 4, b = 11.445(3), c = 21.109(4) A 3 1 Dcalc = 1.24 g cm , l(Mo Ka) = 5.86 cm . Of 6130 unique reflections measured, 3378 [I > 2r(I)] were used in refinement (R = 0.059, Rw = 0.063, GOF = 1.78). 5: white block, 0.20 · 0.20 · 0.10 mm3, Formula: C31H51N8BSZn, Fw: 644.05, at 300 K, monoclinic space group P21/n (#14), a = 15.334(9), b = 9.821(6), c = 25.165(12), ˚ 3, Z = 4, Dcalc = 1.17 g cm3, l(Mo b = 104.29(4), V = 3672(3) A 1 Ka) = 7.57 cm . Of 6161 unique reflections measured, 1961 [I > 3r(I)] were used in refinement (R = 0.071, Rw = 0.075, GOF = 1.83). The crystallographic data and collection details are summarized in Table S1 (supporting information). 2
1189
defined yet. When the structure of 2 is compared with that ˚ ; 1, of 1 [6], the Cu–S bond length is constant (2, 2.192 (2) A ˚ 2.176 (4) A) and the two Cu–N bond lengths are marginally increased. Small changes include a slight expansion of the copper site as measured with bond distances (parallel to the expected slight increase in the ionic radius of Cu(I) compared with that of Cu(II)) and movement of the copper ion mainly toward axial N21 (distance of copper from ˚ ; 1, 0.34 A ˚ ). The the plane (N11, N31, and S): 2, 0.41 A Cu(I)–S bond distance in 2 is the same as that in the reported copper (I) thiolato complex, K[Cu(SC6H4-p˚ ) (HB(3, NO2){HB(3,5-Me2pz)3}] Æ 2C3H6O (2.19(1) A 5-Me2pz)3 = hydrotris(3,5-dimethyl-1-pyrazolyl)borate anion) [9]. The small geometry change associated with the redox reaction involving 1 and 2 is consistent with the requirements for fast electron-transfer. Therefore, the reorganization energy associated with redox reaction should be small [1–3]. Similar small geometric changes have been found between the reduced and oxidized structures in blue copper proteins: azurin, plastocyanin, pseudoazurin, rusticyanin, amicyanin, and stellacyanin [2]. As mentioned above, the coordination structure of 4 was difficult to determine. However, the four-coordinate Zn(II) derivative [Zn(SMeIm)(L1)] (5) was characterized by X-ray crystallography (Figure S2) 1, 2. A comparison of the 1H and 13C NMR data for 4 and 5 suggests that 4 should be also four-coordinate geometry (the imidazole nitrogen does not coordinate the d10 metal ion in 4 or 5). Thus, a large geometric change must occur in the redox reaction involving 3 and 4. The electron self-exchange rate constants, kex (M1 s1), for [CuII/I(SR)(L1)]0/1 (1/2 and 3/4) were measured by using the line-width method for the determination of 1/T2 [4,5] (Eq. (1)). A plot of 1/T2obs versus added [CuII(SR)(L1)] (1 or 3) gives a straight line with a slope equal to the self-exchange rate constants, kex (Fig. 2). pDm1=2 ¼ 1=T 2obs ¼ 1=T 2n þ 1=T 2e ¼ k ex ½CuII ðSRÞðL1Þ þ 1=T 2n :
ð1Þ
The resulting experimental values for kex at 20 C in (CD3)2CO are 1.76(2) · 104 M1 s1 (three different experiments) for [CuII/I(SC6F5)(L1)]0/1 (1/2) and 3.47(17) · 102 M1 s1 (three different experiments) for [CuII/I(SMeIm)(L1)]0/1 (3/4). Thus, the kex of the redox pair 1/2 is about 51 times faster than that of the redox pair 3/4. Moreover, the data used to construct the Eyring plots, and least-squares analyses of these data gave DH = 35(2) kJ mol1, DS = 22(8) J mol1 K1 for the redox pair 1/2. 1 This result indicates an outersphere electron-transfer mechanism [1–3] and the kex of the redox pair 1/2 at 25 C is calculated as 3.14 · 105 M1 s1 obtained by extrapolation of the corresponding Eyring plot. These self-exchange rate constants and their activation parameters are consistent with the values for blue copper proteins [1–3].
1190
K. Fujisawa et al. / Inorganic Chemistry Communications 7 (2004) 1188–1190
displacement parameters, bond distances, bond angles, and torsion angles are available. Crystallographic data reported in this paper have been deposited with Cambridge Crystallographic Data Centre as Supplementary Publication No. CCDC-233829 (2) and 233830 (5). Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or [email protected]). Supplementary data associated with this article can be found, in the online version at doi:10.1016/j.inoche.2004.09.004. Fig. 2. Plots of 1/T2obs (s1) versus [CuI(SR)(L1)] (mmol dm3) for pyrazole rings proton resonance of [CuI(SR)(L1)] at 20 C in (CD3)2CO: (d) R = C6F5, (m) R = MeIm. Measurements were performed with [Cu(SR)(L1)] at 16.7 mmol dm3 for 2 and 18.4 mmol dm3 for 4.
Thus, in the case of the same supporting ligand, the coordination number invariant system, whose properties are very similar to blue copper proteins, gives faster self-exchange rate constants (51 times), and its reorganization energy should be low through an outer-sphere electron-transfer process. In contrast, the previously reported coordination number invariant system did not have fast electron-transfer rate constants [1b,4–5]. Solomon and co-workers have suggested that the changes in electronic structure (short Cu–S(Cys) bond distance and/or long axially coordinated Cu–S(Met) bond distance) are associated with its electron-transfer rate constants [1,10]. Therefore, the electron self-exchange rate constants are related to its electronic change as well as its geometric change upon redox reaction in model compounds and blue copper proteins. Moreover, the hydrophobic interactions of the electron self-exchange process are also important [1–3]. We are currently further investigating these electron transfer processes by using other ligand systems.
Acknowledgements This research was in part supported by Grant-in-Aid for Scientific Research (B) (13555257 and 14350471) and the 21st Century COE program from Japan Society for the Promotion of Science and Tokuyama Science Foundation.
Appendix A. Supplementary material Experimental details (2, 4, and 5) are provided. And tables of crystallographic details, atomic coordinates, positional and isotropic thermal parameters, anisotropic
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