PHOTOPROCESSES IN SELF-ASSEMBLED COMPLEXES OF OLIGOPEPTIDES WITH METALLOPORPHYRINS

Ion-pair complexes between the cationic metalloporphyrins, tetrakis(N- methyl-4-pyridyl)porphyrin (Pd(II)TMPyP4+ and Zn(II)TMPyP4+), and anio nic pentapeptides consisting of a string of four glutamic acid residue s terminated either by tyrosine (Glu4Tyr) or by tryptophan moieties (G lu4Trp) have been assembled and studied by steady-state and time-resol ved spectroscopy. Evidence for ion association between porphyrin and p eptide was provided by the effect of peptide concentration on the grou nd state absorption spectra of the porphyrin. Flash photolysis experim ents showed that, in the presence of peptide, Pd(II)TMPyP4+ triplet de cay was described by the sum of two exponential terms. The fast decay component (6.6 +/- 0.2 10(6) s(-1) for the Tyr variant and 1.4 +/- 0.1 5 10(7) s(-1) for the Trp analog) was found to be independent of pepti de concentration. The slow decay component showed a linear increase in value with peptide concentration, and a bimolecular rate constant of 6.2 x 10(9) M-1 s(-1) was extracted for both peptides. The fast contri bution to the T-1 decay was associated with an intracomplex electron t ransfer, whereas the slow contribution was associated with the diffusi ve formation of an encounter complex between free peptide and porphyri n molecules (bulk phase) followed by electron transfer. Evidence for t his intracomplex electron transfer reaction was derived from the study of the effect of pH-induced alteration of the fast rate component. An increase of the rate constant resulted from the pH-governed increases in Delta E-0 for oxidation. However, at pH values >8.5, although the driving force continued to increase, the rate constant reached a limit ing value and became pH-independent (10(7) s(-1) for the Tyr residue a nd 2.4 x 10(7) s(-1) for Trp). To explain this, a mechanism was invoke d in which segmental diffusion within the porphyrin-peptide complex is supposed to precede the electron transfer step, this putative diffusi on requirement becoming rate-determining at high pH (high driving forc e). Additional evidence for electron transfer within the ion-pair comp lex was obtained when Zn(II)TMPyP4+ was used as the redox partner.