Wavelength-selective fluorescence in ion channels formed by gramicidin A in membranes

Gramicidins are linear peptides that form ion channels that are specific for monovalent cations in membranes. The tryptophan residues in the gramicidin channel play a crucial role in the organization and function of the channel. The natural mixture of gramicidins, denoted as gramicidin A′, consists of mostly gramicidin A, but also contains gramicidins B, C and D as minor components. We have previously shown that the tryptophan residues in ion channels formed by the naturally occurring peptide, gramicidin A′, display wavelength-dependent fluorescence characteristics due to the motionally restricted environment in which they are localized. In order to check the influence of ground-state heterogeneity in the observed wavelength-selective fluorescence of gramicidin A′ in membranes, we performed similar experiments with pure gramicidin A in model membranes. Our results show that the observed wavelength- selective fluorescence characteristics of naturally occurring gramicidin A′ are not due to groundstate heterogeneity.