MOLECULAR RESPONSE OF THE LIPID HEADGROUP TO BILAYER HYDRATION MONITORED BY H-2-NMR

The effect of hydration on the conformation and dynamics of the phosph atidylcholine headgroup has been investigated by H-2-NMR measurements of liquid crystalline dioleoylphosphatidylcholine in multilamellar lip osomes. Deuterium quadrupole splittings (Delta nu(Q)) and spin-lattice relaxation rates (1/T-1) were recorded for three selectively labeled headgroup segments (alpha, beta, and gamma) over the range of water/li pid mole ratios from 4 to 100. The smooth changes in Delta nu(Q) and 1 /T-1 are found to essentially parallel each other and can be described by a single exponential decay function. Progressive hydration thus in duces a concerted change in headgroup conformation together with an in crease in its rate of motion (detected by Delta nu(Q) and 1/T-1, respe ctively). The enhanced mobility is partially due to a shift in the lip id phase transition temperature (as monitored by differential scanning calorimetry) and is furthermore attributed to an entropic contributio n. It, is concluded that the choline dipole becomes slightly raised in its average orientation into the aqueous layer and that the rate is i ncreased at which the headgroup is fluctuating and protruding. The obs erved molecular changes can thus be accommodated within a model where the effective accessible headgroup volume expands with increasing hydr ation.