Transmembrane electron transfer as catalyzed by poly(ethylenimine)-linked manganese porphyrins.

Poly(ethy1enimine)-linked manganese porphyrins associate with phospholipid bilayers in a manner that allows the porphyrin to penetrate the membrane to a depth limited by the hydrocarbon spacer linking it to the extremely hydrophilic polymer. Control of the length of this spacer group and the sidedness of the incorporation of these complexes in liposome membranes permit systematic variation of the minimum distances separating two such derivatives incorporated from opposite sides of the membrane bilayer. Examination of transmembrane electron transport as catalyzed by these porphyrin complexes reveals that significant rates of electron transfer are observed only when the edge-to-edge distance separating two porphyrins is approximately 4 A or less. Comparison to theory and relevance to biological electron transfer are discussed. The study of electron-transfer reactions associated with biological membranes is hindered by our limited knowledge of the relative locations and orientations of the electrochemical prosthetic groups. '-4 The use of chemical models to provide insight into possible reactions in biological systems has been a valuable In an effort to better understand electron transport in bioenergetic membranes, our laboratory and others have reported model systems in which electrons are transported across lipid bilayers. Some of the membrane-soluble electron-transfer catalysts employed in these systems are ferrocene,lO*" quinones,'*'3 metall o p o r p h y r i n ~ , ' ~ ~ ~ ~ chlorophylls,16 viologens,I7 ruthenium complexes,18 and isolated cytochrome^. '^ The work of Hinkle'oJ1 and H a u ~ k a ' ~ . ~ ~ showed quinones to be capable of transporting both protons and electrons across a membrane, while ferrocene was 'This research was supported by research grants from the National Science Foundation (PCM-7816669) and the U S . Public Health Service (GM26098). Prefecture, Sakai, Osaka 591, Japan. Rochester. New York 14620. 'Current address: Department of Applied Chemistry, University of Osaka * Current address: Department of Chemistry, University of Rochester, 0002-7863/86/1508-5865$01.50/0 demonstrated to be an electrogenic catalyst, that is, capable of electron transport only.lo~" Our laboratory has shown that ( I ) Boyer, P. D. ; Chance, B. ; Ernster, L. ; Mitchell, P.; Racker, E. ; Slater, (2) Capaldi, R. A,; Malatesta, F. ; Darley-Usmar, V. M. Biochim. Biophys. (3) Hauska, G. ; Hurt, E. ; Gabellini, N. ; Lockau, W. Biochim. Biophys. (4) Trumpower, B. L. J . Bioenerg. Biomembr. 1981, 13, 1-24. (5) Ibers, J. A. ; Holm, R. H. Science (Washingran, DC) 1980, 209, (6) Traylor, T. G. Ace. Chem. Res. 1981, 14, 102-109. (7) Strothkamp, K. G. ; Lippard, S. J. Acc. Chem. Res. 1982, 15, 318-326. (8) Kong, K. L. Y. ; Spears, K. G. ; Loach, P. A. Phofochem. Phorobiol. (9) Fujita, I. ; Davis, M. T. ; Fajer, J. J . Am. Chem. SOC. 1980, 100, ( I O ) Hinkle, P. Biochem. Biophys. Res. Commun. 1970, 41, 1375-1381. (11) Hinkle, P. C. Fed. Proc., Fed. Am. SOC. Exp. B i d . 1973, 32, (12) Futami, A,; Hurt, E. ; Hauska, G. Biochim. Biophys. Acra 1979,547, (13) Futami, A. ; Hauska, G. Biochim. Biophys. Acta 1979,547, 597-608. E. C. Annu. Rev. Biochem. 1977, 46, 955-1026. Acta 1983, 726, 135-148. Acta 1983, 726, 97-133.