ANALYSIS OF CIRCULAR-DICHROISM SPECTRA OF ORIENTED PROTEIN-LIPID COMPLEXES - TOWARD A GENERAL APPLICATION

A new application of circular dichroism on oriented films of protein-l ipid complexes is presented in this work, which provides quantitative information on the orientation of alpha-helices with respect to the in cident light beam. We used literature reference circular dichroism spe ctra for the various secondary structures to develop a new set of spec tra, where the different directions of absorption within the molecular axis frame for each secondary structure type are taken into account. Using this new set of spectra, we could determine the orientation of t he helical part of melittin in oriented films composed of various phos pholipids. The orientation of the helix axis is found to be perpendicu lar to the membrane normal of films of dioleoylphosphatidylcholine, di oleoylphosphatidylglycerol, and dioleoylphosphatidylserine, whereas th e helix was aligned preferentially parallel to the membrane normal for membranes composed of dimyristoylphosphatidylcholine and dimyristoylp hosphatidylglycerol at a hydration of approximately 2-4 water molecule s per lipid. The orientations found by circular dichroism at this low hydration for the various systems agreed very well with those obtained by Fourier transform infrared measurements on these samples. Upon inc reasing the hydration of the film (to approximately 20 water molecules per lipid), it is shown by circular dichroism that the orientation of the helix of melittin changes most in films of dioleoylphosphatidylse rine, where it adopted, under these conditions, a preferred parallel o rientation with respect to the membrane normal. Complications in the a nalysis of circular dichroism spectra of oriented samples are discusse d and illustrated using patches of native purple membrane containing b acteriorhodopsin and films of alamethicin in dioleoylphosphatidylserin e membranes.