Structure and Dynamics of CO-Iron(II) Protoporphyrin IX in Dimethyl Sulfoxide † Randy W. Larsen,* ,‡ Jim Murphy, § and Eric W. Findsen § Departments of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, Hawaii 96822, and University of Toledo, Toledo, Ohio 43606 ReceiVed August 3, 1995 X In this report we examine the steady-state optical absorption, steady state and transient vibrational structure, and ligand rebinding kinetics of (CO)Fe II protoporphyrin IX ((CO)Fe II PPIX) in dimethyl sulfoxide (DMSO). Steady state optical absorption and resonance Raman spectra of this complex are characteristic of heme iron that is six-coordinate and low-spin. Absorption maxima are observed at 415 nm (Soret), 568 nm (R-band), and 535 nm (-band), and vibrational bands are observed at 1370 cm -1 (ν 4 ), 1496 cm -1 (ν 3 ), 1551 cm -1 (ν 38 ), 1584 cm -1 (ν 2 ), and 1626 cm -1 (ν CdC ). The transient absorption difference spectrum subsequent to photolysis displays an absorption maximum at 433 nm and a minimum at 414 nm that decays biphasically with pseudo-first-order rate constants of (2.11 ( 0.024) × 10 6 s -1 and (2.29 ( 0.036) × 10 2 s -1 . The corresponding transient resonance Raman spectrum displays vibrational bands at 1356 cm -1 (ν 4 ), 1470 cm -1 (ν 3 ), 1559 cm -1 (ν 2 ), 1584 cm -1 (ν 37 ), and 1618 cm -1 (ν CdC ). These results are consistent with the formation of a five-coordinate and high-spin transient species that is identical to the photolytic transient observed upon photolysis of the (DMSO) 2 Fe II PPIX complex (i.e., (DMSO)Fe II PPIX). The decay of this transient species is consistent with the binding of a DMSO to the five-coordinate heme iron followed by substitution of a bound DMSO with CO. Introduction Metalloporphyrins represent a diverse class of molecules capable of catalyzing a wide range of chemical reactions. The diverse reactivity of porphyrins is exemplified by the abundance of porphyrins and porphyrin-like molecules that are utilized as active sites in proteins and enzymes. The most common porphyrin employed in biological chemistry is iron protopor- phyrin IX (FePPIX) or heme. As an enzyme active site FePPIX can oxygenate organic substrates (monooxygenases), reduce dioxygen to water (oxidases), degrade hydrogen peroxide (peroxidases), reversibly transfer electrons (b- and c-type cytochromes), and reversibly transport and store oxygen (he- moglobin and myoglobin, respectively). To a large extent the nature of the coordination between the heme group and the protein modulates reactivity of the heme iron. For example, heme proteins responsible for reversible electron transfer contain heme iron that is six-coordinate and low-spin with axial ligand combinations derived from histidine, methionine, and/or lysine. 1-5 Proteins and enzymes that activate or transport oxygen, on the other hand, have heme iron that is five-coordinate and high- spin with axial ligands derived from histidine, cysteine or tyrosine. 6,7 Understanding iron coordination chemistry in heme contain- ing monooxygenases is of key interest in the design of biomimetic systems as well as in elucidating fundamental reaction mechanisms associated with this class of enzyme. Electronic and structural interactions associated with anionic oxygen-donating ligands coordinated to iron porphyrins are of specific importance since these ligands may provide insights into the nature of iron-oxygen interactions associated with intermediates formed during monooxygenase catalysis. 6,8 A vast majority of studies relating heme reactivity to iron axial ligation have focused on nitrogen-based ligands (e.g., imidazole, 2-meth- ylimidazole, etc.), small diatomic molecules (e.g., O 2 , CO, NO, etc.), and sulfur-based ligands. 2-8 In contrast, studies of heme reactivity with organic-based oxygen ligands have been limited. 9-11 A potentially useful anionic oxygen donating ligand to heme is dimethyl sulfoxide (DMSO). The binding of sulfoxide ligands to ferric iron porphyrins has been demonstrated to occur through coordination of the sulfoxide oxygen to the central iron of the heme macrocycle. 12 This coordination is accompanied by a shift in ν SdO of 110 cm -1 to lower frequency, relative to the unbound molecule. Both Mossbauer and EPR spectra demonstrate that formation of the bis-DMSO ferric heme complex results in a heme iron that is high-spin. In fact, ferric bis-DMSO heme complexes are commonly employed as model complexes for six-coordinate and high-spin heme proteins. 9-14 It has recently been shown that the (DMSO) 2 Fe II PPIX complex forms a photolabile six-coordinate low-spin com- plex. 15,16 We have previously found that photolysis of this * Author to whom correspondence should be sent. † R.W.L. would like to thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for supporting this work. ‡ University of Hawaii at Manoa. § University of Toledo. X Abstract published in AdVance ACS Abstracts, September 15, 1996. (1) Moore, G. R.; Pettigrew, G. W. Cytochromes c-EVolutionary, Struc- tural, and Physiochemical Aspects; Springer-Verlag: Berlin, 1990. (2) Gouterman, M. in The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. III. (3) Spiro, T. G. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds., Addison-Wiley: Reading, MA, 1983. (4) Smith, K. Porphyrins and Metalloporphyrins; Elsevier: Amsterdam, 1975. (5) Hoard, J. L. In Hemes and Heme Proteins; Chance, B., Estabrock, R. W., Yonetani, T., Eds.; Academic Press: New York, 1966. (6) Ortiz de Montellano, P. Cytochrome P-450: Structure, Mechanism, and Biochemistry; Plenum Press: New York, 1986. (7) Schonbaum, G. R., Chance, B. In The Enzymes, 3rd ed.; Boyer, P., Ed.; Academic Press: New York, 1976. (8) For reviews see: (a) Traylor, T. G. Pure Appl. Chem. 1991, 63, 265. (b) Jameson, G. B.; Ibers, J. A. Comments Inorg. Chem. 1983, 2, 97. (c) Morgan, B.; Dolphin, D. 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