Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing dl-methyl anteisobranched fatty acids. 1. Differential scanning calorimetric and phosphorus-31 NMR spectroscopic studies

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Abstract
The thermotropic phase behavior of aqueous dispersions of nine dl-methyl branched anteisoacylphosphatidylcholines was studied by differential scanning calorimetry and 31P nuclear magnetic resonance spectroscopy. The calorimetric studies demonstrate that these compounds all exhibit a complex phase behavior, consisting of at least two minor, low-enthalpy, gel-state transitions which occur at temperatures just prior to the onset of the gel/liquid-crystalline phase transition. In addition, at still lower temperatures, anteisobranched phosphatidylcholines containing fatty acyl chains with an odd number of carbon atoms show a major, higher enthalpy, gel-state transition, which was assigned to a conversion from a condensed to a more loosely packed gel phase. No such transition was observed for the even-numbered compounds in aqueous dispersion, but when dispersed in aqueous ethylene glycol, a major gel-state transition is clearly discernible for two of the even-numbered phospholipids. The major gel-state transition exhibits heating and cooling hysteresis and is fairly sensitive to the composition of the bulk aqueous phase. 31P NMR spectroscopic studies indicate that the major gel-state transition is accompanied by a considerable change in the mobility of the phosphate head group and that, at temperatures just prior to the onset of the gel/liquid-crystalline phase transition, the mobility of the phosphate head group is comparable to that normally exhibited by the liquid-crystalline state of most other phospholipids. The temperatures at which the gel/liquid-crystalline phase transition occurs and the enthalpy change associated with this process are considerably lower than those of the saturated n-acyl-PC''s of comparable acyl chain length. The calorimetric and 31P nuclear magnetic resonance data thus indicate that, although the methyl anteisobranched phosphatidylcholines can form a fairly condensed gel state at sufficiently low temperatures, these phospholipids normally adopt a more loosely packed gel state, and possibly a more ordered liquid-crystalline state, than do phosphatidylcholines containing linear saturated fatty acids of comparable chain length.

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