| Résumé | The thermotropic phase behavior of an odd- and an even-numbered member of the homologous series of 1,2-di-ω-cyclohexylphosphatidylcholines was studied using Fourier transform infrared spectroscopy. The results obtained indicate that the pronounced discontinuities in the behavior of the odd- and even-numbered homologoues observed by differential scanning calorimetry can be attributed to differences in the organization of their respective gel states. The single phase transition exhibited by the odd-numbered compounds upon heating is shown by infrared spectroscopy to be a direct transition from α condensed, subgel-like phase (Lc phase) to the liquid-crystalline state (Lα phase). In contrast, the multiple transitions exhibited by the even-numbered homologues are shown to be due to the initial conversion of an Lβ-like phase to a more loosely packed gel phase, followed by the acyl chain-melting transition. Moreover, the major changes in the interaction between the acyl chains, and in the organization of the interfacial region of the bilayers formed by the even-numbered homologous, occur at temperatures below that of the chain onset-melting phase transition. The infrared spectroscopic changes observed also suggest that above the chain-melting transition, the odd- and even-numbered homologues form similar liquid-crystalline phases that are more ‘ordered’ than those of normal saturated straight-chain phosphatidylcholines. Most likely this is because the large size and the intrinsic rigidity of the ω-cyclohexyl group reduces the conformational disorder of the liquid-crystalline state by ‘damping’ all acyl chain motions. The formation of a relatively ordered liquid-crystalline state may be the critical property exploited by the thermoacidophylic organisms in which ω-cyclohexyl fatty acids naturally occur. |
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