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 anionic pentapeptides consisting of a string of four glutamic acid residues terminated either by tyrosine (Glu4Tyr) or by tryptophan moieties (Glu4Trp) have been assembled and studied by steady-state and time-resolved spectroscopy. Evidence for ion association between porphyrin and peptide was provided by the effect of peptide concentration on the ground state absorption spectra of the porphyrin. Flash photolysis experiments showed that, in the presence of peptide, Pd(II)TMPyP⁴⁺ triplet decay was described by the sum of two exponential terms. The fast decay component (6.6 ± 0.2 10⁶ s^-1 for the Tyr variant and 1.4 ± 0.15 10⁷ s^-1 for the Trp analog) was found to be independent of peptide concentration. The slow decay component showed a linear increase in value with peptide concentration, and a bimolecular rate constant of 6.2 x 10⁹ M^-1 s^-1 was extracted for both peptides. The fast contribution to the T₁ decay was associated with an intracomplex electron transfer, whereas the slow contribution was associated with the diffusive formation of an encounter complex between free peptide and porphyrin molecules (bulk phase) followed by electron transfer. Evidence for this 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 ΔE⁰ for oxidation. However, at pH values >8.5, although the driving force continued to increase, the rate constant reached a limiting value and became pH- independent (10⁷ s^-1 for the Tyr residue and 2.4 x 10⁷ s^-1 for Trp). To explain this, a mechanism was invoked in which segmental diffusion within the porphyrin-peptide complex is supposed to precede the electron transfer step, this putative diffusion requirement becoming rate-determining at high pH (high driving force). Additional evidence for electron transfer within the ion-pair complex was obtained when Zn(II)TMPyP⁴⁺ was used as the redox partner.