title: Comparison of lipid/gramicidin dispersions and cocrystals by Raman scattering.
abstract: Gramicidin crystals, dimyristoylphosphatidylcholine (DMPC)/gramicidin dispersions, and DMPC/gramicidin cocrystals were examined by Raman scattering to determine lipid/gramicidin stoichiometries and lipid organization. Calibrations of the choline (716-cm-1) and tryptophan (756-cm-1) peaks indicate that the cocrystals contain two lipids for each gramicidin monomer, a result confirmed by chemical analyses of washed crystals. In dispersions with high lipid/gramicidin ratios (e.g., 25:1), the lipid is ordered but becomes increasingly disordered as the gramicidin content is increased. Paradoxically, the DMPC/gramicidin cocrystals have highly ordered lipids that possibly contain no gauche bonds at all, despite their low lipid/gramicidin ratio. In addition, the polypeptide amide I peak position near 1670 cm-1 is found to be independent of the lipid/gramicidin ratio in the complexes and may indicate a beta-helix-type secondary structure at all ratios. However, the amide I peak broadens significantly at low lipid/gramicidin ratios and broadens still further in the cocrystals, suggesting that protein-protein interactions may induce band-broadening distortions of the polypeptide structure.
mesh_terms: D002794:Choline; D003460:Crystallization; D004134:Dimyristoylphosphatidylcholine; D006096:Gramicidin; D007700:Kinetics; D008081:Liposomes; D008954:Models, Biological; D008968:Molecular Conformation; D011487:Protein Conformation; D013059:Spectrum Analysis, Raman / Q000379:methods; D014364:Tryptophan
chemical_list: D008081:Liposomes; D006096:Gramicidin; D014364:Tryptophan; D002794:Choline; D004134:Dimyristoylphosphatidylcholine