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To study the dynamics and mechanism of intramolecular photoinduced electron transfer (PET) reactions, a series of (Zn II -Fe III ) meso-tetraarylmetalloporphyrin dimers were synthesized and the kinetics of their PET reactivity was measured. Molecular building blocks were prepared by selective nucleophilic aromatic substitution of a para fluorine on tetraarylporphyrins containing a single pentafluorophenyl group. This synthetic approach allows a wide variety of systematic modifications such as type and length of spacer, metal center, and redox-potential difference between donor and acceptor. The edge-to-edge distance between the two porphyrins varies from 14.4 to 27.3 Å. Into a symmetric dimer, with two identical porphyrins covalently linked by a rigid partly saturated bridge, one zinc(II) and one iron(III) can be inserted. From measurements of fluorescence lifetimes the rate constants for PET from the electronically excited state of the zinc porphyrin to the bis(imidazole)iron porphyrin cation were evaluated. The electron-transfer rate decreases by a factor of only 165 when the distance increases by 13 Å. This small decrease is indicative of a surprisingly weak attenuation of the electronic coupling with distance.

The N-alkylhemin formation during iron(II1) porphyrin-catalyzed epoxidation of alkenes is studied in a homogeneous system using pentafluoroiodosobenzene (PFIB) as the oxidant. The system is a model for the enzyme cytochrome P-450. A second N-alkyl species other than the well-documented primary N-alkylhemin is observed during the oxidation of terminal alkenes. It is unstable and decomposes under the reaction conditions. It is also a catalyst for alkene epoxidation by PFIB. Its characteristics are consistent with a secondary N-alkylhemin. The kinetics were partially resolved, and rate constants were estimated for the formation of both N-alkylhemins, for the decomposition of the secondary N-alkylhemin, and for the epoxidation by the hemin and the N-alkylhemins. The partition ratios were measured by a kinetic method, which provides better insight for the mechanisms of both N-alkylation and epoxidation than the measurements by product analysis using heterogeneous systems. We suggest that alkenes reversibly inhibit cytochrome P-450 through the formation of the unstable N-alkylporphyrin species.Suicide inactivation of cytochromes P-450 during oxidation of alkenes and alkynes catalyzed by these enzymes involves the alkylation of the porphyrins at a pyrrolic nitrogen.' Most of the substrates found to cause suicide inactivation are terminal alkenes, and the isolated porphyrin product contains an Nsubstituent attached at the primary carbon (Figure 1).* Synthetic models of cytochromes P-450 also form N-alkylhemins with both terminal and nonterminal alkene^.^-^ Unlike the enzymes, however, the synthetic N-alkylhemins were found to catalyze the epoxidation of alkenes (Scheme l).5 R I lo N-Alkylhemin 2 O N-Alkylhemin The structure proposed for N-alkylhemins contains a fivemembered metallocycle as shown in Figure 2A. The existence *Please address reprint requests to Frater, Y.; Gibbs, A. H.; De Matteis, F.; Lamb, J. H.; Farmer, P. B.; Naylor, S. Biochem. J . 1991, 274, 843-848. (2) Kunze, K. L.; Mangold, B. L. K.; Wheeler, C.; Beilan, H. S.; Ortiz de Montellano, P. R. J . Biol. Chem. 1983, 258, 4202-4207. (3) (a) Mansuy, D.; Devocelle, L.; Artaud, I.; Battioni, J.-P. New J. Chem. 1985, 9, 711-716. (b) Artaud, I.; Devocelle, L.; Battioni, I.-P.; Girault, J. P.; Mansuy, D.

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