Lyotropic phase transitions in phospholipids as evidenced by small-angle synchrotron X-ray scattering

Hydration is an important factor in regulating the phase behaviour of lipids and besides affects their interactions with other compounds relevant for biological membranes. We present a reliable and fast method to detect and characterise hydration-induced phase transitions in phospholipids by means of small-angle synchrotron X-ray scattering. Films consisting of aggregations of representatives of the two important lipid classes lecithins (DPPC a, POPC and OPPC,a for abbreviations, see below) and cephalins (DPPE and DOPE) were investigated at room temperature in dependence on relative humidity. Qualitative changes in the sets of the diffraction patterns obtained in dynamic hydration/dehydration scans were taken as markers indicating the existence of lyotropic phase transitions. The efficiency of this methodology is demonstrated by illustrating the course of hydration-driven phase transitions between lamellar as well as nonlamellar phases. In detail, this was realised for chain melting in the mixed-chain lipids, POPC and OPPC, and for a novel nonlamellar-phase transition for DOPE between a disordered inverted ribbon phase designated as Palpha and the canonical H(II), phase, respectively.