Spectroscopic characterization of an assembled pair of free-base and zinc porphyrins linked by the cyclic ?-sheet peptide Gramicidin SElectronic supplementary information (ESI) available: complete synthetic details and characterization of the GS derivatives. See http://www.rsc.org/suppdata/nj/b4/b402541e/

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.