Spectroscopic investigations of metalloporphyrin-oligopeptide systems: Evidence for peptide aggregation

Anionic pentapeptides consisting of a string of four glutamic acid residues terminated by either tyrosine (Glu4Tyr) or tryptophan (Glu4Trp) were synthesized, and their aggregation properties in buffered (pH = 7.0) aqueous solutions were investigated using two different approaches. In the first approach, the effects of the concentration of peptide used as its own probe (intrinsic probe) on its fluorescence emission, circular dichroism, surface tension, and solution pH yielded similar critical peptide concentrations of around 175 μM. This particular concentration was taken as evidence for peptide aggregation. In the second approach, peptide aggregation was investigated using cationic metalloporphyrins, tetrakis(N-methyl-4-pyridyl)-porphyrin (Pd ^IITMPyP ⁴⁺ and Zn ^IITMPyP ⁴⁺), as extrinsic probes. The effect of peptide concentration on porphyrin ground-state absorption confirmed peptide aggregation, but at a lower critical peptide concentration near 125 μM. This difference was attributed to the possible distortion introduced by the association of one (or more) large metalloporphyrin molecule with the peptide aggregates. Evidence for peptide aggregation was also demonstrated from the effect of peptide concentration on Pd ^IITMPyP ⁴⁺ triplet-state decay. The fast component (k _f, associated with electron transfer from the target Tyr and Trp residues to the porphyrin triplet state) was found to be independent of peptide concentration, implying no noticeable effect of peptide aggregation on the electron-transfer event. This was attributed to the fact that species formed by excitation of porphyrin associated with ion-pair complexes or bound to peptide aggregates and the diffusion together of the separate T ₁ and peptide entities in the bulk phase are kinetically similar. On the other hand, the slower component (k _s) of the decay, which is associated with the diffuse formation of an encounter complex between the free peptide and T ₁ porphyrin (bulk phase), was peptide-dependent and displayed a critical peptide concentration near 125 μM, above which it became practically independent of peptide concentration. This invariance of k _s was taken as an indication that the free peptide concentration in the bulk phase remains constant above 125 μM, the concentration at which peptide molecules prefer to associate as aggregates.