Metastability and polymorphism in the gel phase of 1,2-dipalmitoyl-3-SN-phosphatidylcholine. A Fourier transform infrared study of the subtransition.

Fourier transform infrared spectroscopy was used to study the metastability of 1,2-dipalmitoyl-3-sn-phosphatidylcholine (DPPC) at temperatures near 0 degrees C. It was found that when DPPC is incubated at 2 degrees C for three days the two-dimensional acyl chain packing changes from one resulting in spectra typical of an orthorhombic subcell to one resembling that found in triclinically packed acyl systems. This transition proceeds in two stages. The first step, requiring less than one day, approximates first-order kinetics; the second stage proceeds with second- or higher-order kinetics. Comparison of spectra recorded at -36 degrees C with and without prior incubation at 2 degrees C shows that there are two stable low temperature forms of DPPC; that is, DPPC is metastable only within a narrow temperature range. A study of the thermotropic behavior in the range 0-45 degrees C shows that the subtransition near 15 degrees C is a transition from the alternate form to one with orthorhombic characteristics. Spectral changes at the pretransition and the main phase transition demonstrate that there are differences in behavior that are related to the thermal history of the sample.