Chapter Six - Hexagonal Phase Formation in Oriented DPPC–Melittin Samples: A Small-Angle X-ray Diffraction Study

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We investigated the influence of melittin on the organization of macroscopically oriented dipalmitoylphosphatidylcholine multilayers at 100% relative humidity as a function of temperature and peptide content by small-angle X-ray diffraction. Experiments were done under conditions known to lead to disk formation, a long-lived metastable state, below the transition temperature ( T m ), of the pure lipid and in excess water. For T > T m the system stays in a lamellar organization up to a lipid-to-peptide molar ratio, R i , of 5, that is, the lowest R i investigated herein. It was found that the macroscopically oriented system shows a rather complex behavior only below T m . For T < T m and for low peptide concentrations ( R i = 200) formation of the rippled phase was found to be abolished. At R i = 100 melittin induces the formation of a rippled phase at relative low temperature (29 °C). At higher peptide content and T < T m melittin induces the formation of a hexagonal phase, presumably metastable, in coexistence with a lamellar gel phase. A parallel is made with the well-known disk formation. An interpretation in terms of mismatch between the length of the peptide helix and the bilayer thickness is proposed.