Photoprocesses in AOT reverse micelles containing metalloporphyrins and oligopeptides

Steady-state and time-resolved spectroscopy of two cationic metalloporphyrins-tetrakis(N-methyl-4-pyridyl)porphyrin (Pd(II)TMPyP4+ and Zn(II)TMPy4+)-were studied in reverse micelles of AOT-heptane-H2O in the absence, and presence, of oligopeptides containing tyrosine and tryptophan residues (K2YL2 and K2WL2). Ground-state absorption spectra for both porphyrins in reverse micelles at different water-to-surfactant molar ratio (w(0)) values revealed a critical micellar water composition of w(0) approximate to 10. Above this critical value, a w(0)-independent red shift (Deltalambda = 3.1 nm) was observed, suggesting a similar aqueous porphyrin environment. Below w(0) approximate to 10, a w(0)-dependent blue shift was observed, indicating a porphyrin aqueous environment that is strongly related to the micellar size. At high micelle-water composition, the ground-state spectra never approached those observed in aqueous solution, strongly indicating that the porphyrin is preferentially solubilized in close vicinity of the H2O-AOT molecular interface. Fluorescence emission from ZnTMPyP4+ in reverse micelles showed characteristics that were similar to those observed from ground-state absorption. Fluorescence emission from the single tryptophan residues of K2WL2 in aqueous solution and in reverse micelles suggested a solubilization site of the peptide near the micellar oil-water interface. This solubilization site was found to be dependent on w(0) below w(0) values of similar to10 (reflecting a different peptide microenvironment) and independent of w(0) above similar to10 (reflecting a similar peptide microenvironment). Flash photolysis experiments showed that, in the presence of a peptide in reverse micelles, Pd(II)TMPy4+ triplet decay was described by the sum of two exponential terms. The fast decay component for both oligopeptides was found to be independent of the peptide concentration and dependent on w(0). The slow decay component showed a linearly dependent increase in value, relative to increases in the peptide concentration. The fast contribution to the porphyrin triplet state T-1 decay was associated with electron transfer in a discrete micelle between the target moiety (tyrosine or tryptophan) and the photoexcited porphyrin molecule. The slow contribution of the decay was associated to molecular quenching during the micellar exchange process. The fast decay component was found to be independent of pH, suggesting a solubilization of the peptide molecule in a pH-independent environment near the oil-AOT molecular interface, where a portion of the heptane chain may extend into the water phase.