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

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 H-1 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 H-1 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 approximate to Gramicidin S with a free-base porphyrin and a zinc porphyrin < Gramicidin S with a pair of zinc porphyrins. The H-1 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.