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Lipids with phytanyl or phytanoyl chains are of basic interest because of their natural occurrence in the membranes of extremophilic archaebacteria and frequent use in biochemical studies and applications as a model for stable bilayers. Since data from physico-chemical studies of phytanoyl lipids in terms of structural and phase properties are rare and partly discrepant, we have studied films of diphytanoylphosphatidylcholine (DPhPC) and its ethanolamine analogue (DPhPE) in dependence on water activity using Fourier-transform infrared spectroscopy. DPhPC imbibes much more water than DPhPE as substantiated by Karl–Fischer-titration and gravimetric data. Marker bands due to the methylene stretching vibrations indicate an extremely high disorder of the acyl chains of both lipids over the whole hydration range. The spectral features of DPhPE are different from those of DPhPC in many respects but similar to those of DPhPE. This similarity encourages us to conclude a predominance of the inverted hexagonal H II phase in DPhPE. Hydration-induced structural changes of DPhPC are indicated by discontinuous wavenumber shifts at water activities of 0.6–0.8, and towards very low water activities for DPhPE, suggesting nonlamellar–lamellar and nonlamellar–nonlamellar phase transitions, respectively, to proceed. Thus, nonlamellar phases may play an important role also in DPhPC. |
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