Polarity and packing at the carbonyl and phosphate regions of lipid bilayers

The packing and polarity of dipalmitoylphosphatidylcholine unilamellar vesi cles in the gel state were measured in the presence of trehalose and phlore tin by using fluorescent probes inserted at different levels of the bilayer interface. The molecular probes used were Laurdan (LA), Prodan, (PA), Dans yl derivatives (DA), and Merocyanine 540 (MC540). The binding of trehalose to the carbonyl groups and of phloretin to the phosphate groups affects the packing and the polarity of the glycerol backbone region. This effect is m ore pronounced when the bilayers are dehydrated and rehydrated in the prese nce of the sugar, thus indicating that the effect is due to water replaceme nt. The results can be interpreted in terms of the rearrangement of the wat er structure in the environment around the C=O and P=O groups.