IR SPECTROSCOPIC STUDY OF THE STRUCTURE AND PHASE-BEHAVIOR OF LONG-CHAIN DIACYLPHOSPHATIDYLCHOLINES IN THE GEL STATE

Fully hydrated dispersions of simple linear saturated diacylphosphatid ylcholines with even-numbered acyl chains of lengths from 18 to 24 car bons can exist in a low-temperature, highly ordered, orthorhombic phas e (G(o)) that differs from the L(beta') phase (G(d)) normally found fo r shorter chains. The temperature behavior of these dispersions has be en studied by infrared spectroscopy. Chain packing in the G(o) phase w as found to be nearly identical to that of the orthorhombic phase of c rystalline n-alkanes. With increasing temperature, the G(o) phase unde rgoes 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 inc reasing 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 o f subcell setting angles. The G(d) phase converts to a phase (G(h)) wi th hexagonal-like chain packing at temperatures below T-m. The results support and extend those of a recent x-ray diffraction study of the 2 4-carbon diacyclphosphatidylcholine gel.