Effect of structural transition of the host assembly on dynamics of an ion channel peptide: A fluorescence approach

Structural transition can be induced in charged micelles by increasing the ionic strength of the medium. We have monitored the organization and dynamics of the functionally important tryptophan residues of gramicidin in spherical and rod-shaped sodium dodecyl sulfate micelles utilizing a combination of wavelength-selective fluorescence and related fluorescence approaches. Our results show that tryptophans in gramicidin, present in the single-stranded beta(6.3) conformation, experience slow solvent relaxation giving rise to red edge excitation shift in spherical and rod-shaped micelles. In addition, changes in fluorescence polarization with increasing excitation or emission wavelength reinforce that the gramicidin tryptophans are localized in motionally restricted regions of these micelles. Fluorescence quenching experiments using acrylamide as a quencher of tryptophan fluorescence show that there is reduced water penetration in rod-shaped micelles. Taken together, we show that gramicidin conformation and dynamics is sensitive to the salt-induced structural transition in charged micelles. In addition, these results demonstrate that deformation of the host assembly could modulate protein conformation and dynamics.