Interactions of Water Soluble Porphyrin Derivatives with a Zwitterionic Polypeptide

Abstract

Complex formation of water-soluble porphyrin derivatives with the β-sheet structure of zwitterionic poly(Glu-Val-Lys-Val) was studied by absorption spectra and circular dichroism measurements. Interaction of tetrakis(4-N-methylpyridyl)porphine (TMPyP) with the polypeptide is not observed, whereas tetraphenylporphinetetrasulfonic acid (TPPS) interacts when TPPS exists as a diacid species. Positive charges in porphyrin center for the diacid species may play an important role in complex formation. The monomeric TPPS diacid species aggregates by the addition of the polypeptide, and two types of aggregation species with S-chiral conformation are observed. From [P]/[D]=1 to 5, one aggregation species has absorption bands at 489 nm and 701 nm, which increases with [P]/[D] ratios or decreasing pH. Both the monomer and aggregation species can more effectively bind to the polypeptide with decreasing pH. The other aggregation species is present above [P]/[D]=10, it increases with increasing [P]/[D]. Interconversion between two types of aggregation species would take place from [P]/[D]=5 to 10. The β-sheet content of the polypeptide is kept constant above [P]/[D]=2, but increases slightly around [P]/[D]=1.