Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 2. Infrared and phosphorus-31 NMR spectroscopic studies

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Abstract
The polymorphic phase behavior of aqueous dispersions of a number of representative phosphatidylcholines with methyl iso-branched fatty acyl chains was investigated by Fourier transform IR (FT-IR) and 31P NMR spectroscopy. For the longer chain phosphatidylcholines, where 2 transitions are resolved on the temperature scale, the higher temperature event can unequivocally be assigned to the melting of the acyl chains (i.e., a gel/liquid-crystalline phase transition), whereas the lower temperature event is shown to involve a change in the packing mode of the methylene and carbonyl groups of the hydrocarbon chains in the gel state (i.e., a gel/gel transition). The IR spectroscopic data suggest that the methyl iso-branched phosphatidylcholines assume a partially dehydrated, highly ordered state at low temperatures, resembling the Lc phase recently described for the long-chain n-saturated phosphatidylcholines. At higher temperatures, some branched-chain phosphatidylcholines appear to assume a fully hydrated, loosely packed gel phase similar to, but not identical with, the P.beta. phase of their linear saturated analogs. Thus, the iso-branched phosphatidylcholine gel/gel transition corresponds, at least approximately, to a summation of the structural changes accompanying both the subtransition and the pretransition characteristic of the longer chain n-saturated phosphatidylcholines. In the low-temperature gel state, there are significant differences between the odd- and even-numbered isoacylphosphatidylcholines with respect to their hydrocarbon chain packing modes as well as to their head group and interfacial hydration states. The 3 phases resolved for the longer chain isoacylphosphatidylcholines were also characterized by different 31P NMR line shapes, which are indicative of differences in the mobilities of the P head group of 3 states. The motion of the phosphate head group is quite restricted in the low-temperature gel state but increases considerably at temperatures above the gel/gel transition, approaching the relatively fast motion characteristic of these iso-branched phosphatidylcholines in the liquid-crystalline state. In cases where only a single thermotropic event was resolved, both spectroscopic techniques showed that the structural changes characteristic of that event were equivalent to those of both the gel/gel transition and the gel/liquid-crystalline phase transition, which either occurred concomitantly or occurred in rapid succession within a narrow temperature range.

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