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-lipid complexes is presented in this work, which provides quantitative information on the orientation of alpha-helices with respect to the incident light beam. We used literature reference circular dichroism spectra for the various secondary structures to develop a new set of spectra, 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 the helical part of melittin in oriented films composed of various phospholipids. The orientation of the helix axis is found to be perpendicular to the membrane normal of films of dioleoylphosphatidylcholine, dioleoylphosphatidylglycerol, and dioleoylphosphatidylserine, whereas the helix was aligned preferentially parallel to the membrane normal for membranes composed of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol at a hydration of approximately 2-4 water molecules 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 increasing 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 dioleoylphosphatidylserine, where it adopted, under these conditions, a preferred parallel orientation with respect to the membrane normal. Complications in the analysis of circular dichroism spectra of oriented samples are discussed and illustrated using patches of native purple membrane containing bacteriorhodopsin and films of alamethicin in dioleoylphosphatidylserine membranes.