Synthesis and Physical Characterization of Novel Heme-Based Model Systems for Photoinitiated Electron Transfer. 1. Complexation of a RuProHis Bifunctional Peptide and Microperoxidase-11 ² B. Fan, ‡ D. L. Fontenot, § R. W. Larsen, | M. C. Simpson, ⊥ J. A. Shelnutt, ⊥ R. Falcon, ‡ L. Martinez, ‡ S. Niu, | S. Zhang, ‡ T. Niemczyk, ‡ and M. R. Ondrias* ,‡ Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, Life Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822, and Fuel Sciences Division, Sandia National Laboratory, Albuquerque, New Mexico 87185 ReceiVed January 15, 1997 X This paper describes the synthesis and initial physical characterization of a new type of model system for the investigation of photoinitiated electron transfer (ET) in proteins and polypeptides. This system is based upon a derivative of a heme-containing digestion product of cytochrome c, microperoxidase-11 (MP-11). The vacant axial position of the five-coordinate heme of MP-11 is coordinated to the terminal histidine of a dipeptide having a photoactive ruthenium tris(bipyridine) moiety at its other end (RuProHis). Photoexcitation of the ruthenium group results in rapid (k ET > 10 7 s -1 ), reversible ET to the ferric heme. The equilibrium properties of MP-11 and the MP-11/Ru(peptide) complex were characterized with optical absorption, luminescence, and resonance Raman (RR) spectroscopies. Molecular modeling was also employed to examine the structure and energetics of the equilibrium species. Clear evidence for reversible photoinduced ET was observed in time-resolved luminescence and transient resonance Raman studies of the MP-11/RuProHis samples. Introduction Physical studies of biological systems aim at revealing the exact molecular bases for their function. Because of the complexity of most biological systems, it is usually more practical to examine the behavior of simpler yet relevant model systems. In addition to contributing to the understanding of basic mechanisms of biological function, the study of model systems has potential importance in the manufacture of artificial biomimetic devices. One such area in which model system design has found wide application is the study of biological electron transfer. 1 Most current model systems are based on the use of a conventional chemical species to mimic only the active sites of native proteins. To date, few efforts have been made to use protein fragments as integral parts of model systems. 2 Here we describe the synthesis of a new class of model electron transfer systems based upon the coordination of a photoactive ruthenated peptide to heme-containing frag- ments of cytochrome c. Initial spectroscopic and photophysical studies of these model systems show that, while bearing most of the desirable features of inorganic model systems (solubility, stability, etc.), they offer some unique opportunities for probing some of the less well-understood aspects of biological electron transfer (ET) processes. Microperoxidases are products of proteolytic digestion of cytochrome c. Using different combinations of pepsin and trypsin, three different species containing the heme active site and eight, nine, or eleven amino acids can be prepared. 3 In all three digestion fragments, the short peptides remain covalently linked to the heme with the two thioether bonds between cys- 14 and cys-17, and histidine-18 is still coordinated to the heme through its imidazole nitrogen. This unique structure preserves many of the basic heme-protein bonding interactions present in cytochrome c while offering a much simpler system that can be systematically modified by covalent or coordinative binding of photoactive species. Derivatives of ruthenium polypyridyl complexes are widely used to photoinitiate ET reactions in biological systems because of their useful photoelectrochemical properties. 4 Chemical derivatization of the bipyridine ligand allows the attachment of different reactive groups to the complex. These, in turn, can be used to covalently modify specific amino acid residues of native proteins 5 or functionalized peptides. ET processes can be initiated in these types of systems in a well-defined manner * To whom correspondence should be addressed. ² Abbreviations used: ET, electron transfer; MLCT, metal to ligand charge transfer; CLS, classical least-squares progression; RR, resonance Raman; MP-11, microperoxidase-11 (heme undecapeptide); acMP-11, microperoxidase-11 acetylated at both amino groups as described in the text; RuProHis, (bpy) 2Ru(4-methyl-4′-(histidylprolyl)bpy), where bpy ) 2,2′-bipyridine (see text for details). ‡ University of New Mexico. § Los Alamos National Laboratory. | University of Hawaii. ⊥ Sandia National Laboratory. X Abstract published in AdVance ACS Abstracts, August 1, 1997. (1) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (b) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (c) Orman, L. K.; Anderson, D. R.; Yabe, T.; Hopkins, J. B. In Electron Transfer in Biology and the Solid State; Johnson, M. 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