Phase behaviour of membrane lipids containing polyenoic acyl chains

Abstract

The low-temperature thermal behaviour of di-18:2 phosphatidylethanolamine (di-18:2 PE) is shown to be characterised by similar broad low-enthalpy transitions to those previously reported for polyenoic samples of phosphatidylcholines (Keough and Kariel (1987) Biochim. Biophys. Acta 902, 11–18), and monogalactosyldiacylglycerol (Sanderson and Williams (1992) Biochim. Biophys. Acta. 1107, 77–85). Real-time X-ray diffraction measurements indicate that these transitions correspond to transitions between the gel (L β) and liquid-crystal (L α) phases of the lipids. The gel phase of these lipids is, however, much more loosely packed than the corresponding phases of membrane lipids containing monoenoic or fully-saturated acyl chains. The low enthalpy and reduced co-operativity of the L α a ̊ L β phase transitions of the polyenoic lipids is attributed to the reduced contribution of van der Waals interactions between their acyl chains in the gel-state of these lipids. Comparison with the earlier results obtained for MGDG suggest that the acyl chains of polyenoic lipids can form well-ordered lattices but require the additional energy input associated with the formation of a hydrogen bond network between the lipid headgroups in order to do so.