MOTIONALLY RESTRICTED TRYPTOPHAN ENVIRONMENTS AT THE PEPTIDE-LIPID INTERFACE OF GRAMICIDIN CHANNELS

The tryptophans in the gramicidin channel play a crucial role in the o rganization and function of the channel. The localization and dynamics of these tryptophans have been studied using fluorescence spectroscop y, especially utilizing environment-induced effects on the rates of so lvent relaxation around these residues in membranes. When incorporated into model membranes of dioleoyl-sn-glycero-3-phosphocholine (DOPC), the tryptophans in the gramicidin channel exhibit a red edge excitatio n shift (REES) of 6 nm. In addition, fluorescence polarization shows b oth excitation and emission wavelength dependence. Fluorescence lifeti me analysis shows a biexponential decay, corresponding to a short- and a long-lifetime component. The mean lifetime was found to be dependen t on both excitation and emission wavelengths. Analysis of time-resolv ed emission spectra (TRES) shows a heterogeneous environment for the t ryptophans consistent with the lifetime information. Taken together, t hese observations point out the motional restriction experienced by th e tryptophans in the gramicidin channel. This is consistent with other studies in which such restrictions are thought to be imposed due to h ydrogen bonding between the indole rings of the tryptophans and the ne ighboring lipid carbonyls. The significance of such organization in te rms of functioning of the channel is brought out by the fact that subs titution, photodamage, or chemical modification of these tryptophans i s known to give rise to channels with altered conformation and reduced conductivity.