Spectroscopic characterization of an assembled pair of free-base and zinc porphyrins linked by the cyclic β-sheet peptide Gramicidin S

Abstract

A pair of porphyrins was linked to the Orn side chains of Gramicidin S [cyclo(–D-Phe–Pro–Val–Orn–Leu–)₂] and its derivatives via the amide bonds; the assorted porphyrins were characterized by various spectroscopic methods. The high-field shifts of the porphyrin signals in the ¹H NMR spectrum and the exciton-coupled Cotton effects in the CD spectrum of cyclo[–D-Phe–Pro–Val–Orn(Por)–Leu–]₂ are both intense compared to those of cyclo[–D-Phe–Pro–Val–Dab(Por)–Leu–]₂ or cyclo[–D-Phe–Pro–Val–Lys(Por)–Leu–]₂ (Dab, diaminobutyric acid; Por, the side-chain linked porphyrin). Some ¹H NMR signals of the tolyl protons of the porphyrins coalesce at 353 K in CD₂Cl₂, which reveals dynamic processes of the porphyrins. The intensities of the Cotton effect are: Gramicidin S with a pair of free-base porphyrins ≈ Gramicidin S with a free-base porphyrin and a zinc porphyrin < Gramicidin S with a pair of zinc porphyrins. The ¹H NMR, UV-vis, CD and fluorescence spectra show that the solvent substantially influences the assembly of the porphyrins. These spectroscopic studies suggest that the strength of the intramolecular interactions between a pair of porphyrins is in the order of toluene > CH₂Cl₂ > DMF.