Modulating dipoles for structure-function correlations in the gramicidin Achannel

Dipoles of the tryptophan indole side chains have a direct impact on ion co nductance in the gramicidin channel. Here, fluorination of the indoles (bot h 5- and 6-fluoro) is used to manipulate both the orientations and the magn itudes of the dipoles. The orientations and positions with respect to the c hannel axis were determined using H-2 solid State NMR of uniformly aligned lipid bilayer preparations. By exchange of the remaining four protons in th e indole ring for deuterium, comparison could be made to d(5)-indole spectr a that have previously been recorded for each of the four indoles of gramic idin A. After making the assignments which were aided by the observation of F-19-H-2 dipolar interactions, we found that fluorination caused only mino r changes in side chain conformation. With the high-resolution structural c haracterization of the fluorinated indoles in position 11, 13, and 15, the electrostatic interactions with a cation at the channel and bilayer center can be predicted and the influence of the modified dipoles on ion conductan ce estimated. The importance of the long-range electrostatic interaction wa s recently documented with the observation of alpha-helical dipoles oriente d toward the bilayer center on the ion conductance pathway for the Streptom yces K+ channel. We present direct measurements of the orientation of grami cidin channel F-Trp positions for use in analysis of dipole effects on chan nel permeation.