| 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 infrared (FT-IR) and phosphorus-3 1 nuclear magnetic resonance (31P NMR) spectroscopy. For the longer chain phosphatidylcholines, where two 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 (Le., a gel/gel transition). The infrared 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 PB phase of their linear saturated analogues. 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. The infrared spectroscopic data also show that, 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 three 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 phosphorus head group in the three states. In particular, 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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