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Gramicidin conformational changes during riboflavin photosensitized oxidation in solution and the effect of N-methylation of tryptophan residues.

In the present work, we evaluated the role of gramicidin conformation in its photosensitized oxidation in organic solvents when irradiated in the presence of riboflavin. Gramicidin conformation has been described as monomeric in trifluoroethanol and as an intertwined dimer in methanol. Gramicidin showed extensive photo-oxidation upon irradiation in the presence of riboflavin in both solvents, and tryptophan residues were identified to be involved. We synthesized a gramicidin derivative methylated at position 1 of the indole ring of tryptophan to assess its effect on gramicidin conformation and photo-oxidation. Methylated gramicidin showed very similar absorption and emission spectra to gramicidin, but different conformations were identified by circular dichroism spectra. Upon irradiation, N-methylated tryptophan residues in the gramicidin derivative were not easily photo-oxidized by riboflavin compared to gramicidin. Circular dichroism spectra for gramicidin in methanol changed significantly upon irradiation in the presence of riboflavin indicating a change in conformation, while in trifluoroethanol no such changes were observed. Time-resolved fluorescence and anisotropy studies showed that oxidized gramicidin in methanol had shorter fluorescence lifetimes and a shorter rotational correlation time compared to non-irradiated gramicidin. Additionally, SDS-PAGE analysis showed a marked change in the electrophoretic pattern, whereas the high-molecular-weight bands disappeared upon irradiation. We interpret all these results in terms of a riboflavin photosensitized shift in gramicidin conformation from intertwined to monomeric.