Characterization of the sub-main-transition in distearoylphosphatidylcholine studied by simultaneous small- and wide-angle X-ray diffraction.

Simultaneous small- and wide-angle X-ray diffraction was used to investigate the structural conversions in the so-called sub-main-transition of fully hydrated multilamellar vesicles of distearoyl phosphatidylcholine (DSPC). The small-angle diffraction patterns show a modification in the supramolecular structure of the corrugated lipid bilayer and reveal that the sub-main-transition does not abolish the general features of the ripple phase (P(beta')). Concomitantly in the wide-angle region the diffraction patterns exhibit a rearrangement of the hydrocarbon chain packing and an onset of chain melting. The presence of KCl show an enhancement of the effects of the sub-main-transition. Moreover, in the ripple phase (P(beta')) below the sub-main-transition the hydrocarbon chains are positioned on an orthorhombic lattice in the presence of KCl.