IR spectroscopic study of the structure and phase behavior of long-chain diacylphosphatidylcholines in the gel state

Fully hydrated dispersions of simple linear saturated diacylphosphatidylcholines with even-numbered acyl chains of lengths from 18 to 24 carbons can exist in a low-temperature, highly ordered, orthorhombic phase (G(o)) that differs from the L(beta') phase (G(d)) normally found for shorter chains. The temperature behavior of these dispersions has been studied by infrared spectroscopy. Chain packing in the G(o) phase was found to be nearly identical to that of the orthorhombic phase of crystalline n-alkanes. With increasing temperature, the G(o) phase undergoes a transition to G(d) at similar to 45 degrees C below T-m. This transition occurs at a higher temperature and becomes sharper with increasing chain length. Chain packing in the G(d) phase was found to be disordered in a way that can be expressed in terms of a distribution of subcell setting angles. The G(d) phase converts to a phase (G(h)) with hexagonal-like chain packing at temperatures below T-m. The results support and extend those of a recent x-ray diffraction study of the 24-carbon diacyclphosphatidylcholine gel.