Lipid headgroup hydration studied by 2H-NMR: A link between spectroscopy and thermodynamics
Ulrich AS., Watts A.
The conformational and dynamic response of the phosphatidylcholine headgroup to hydration has been characterized by 2 H-NMR measurements of selectively deuterated DOPC at positions α, β, and γ, and to complement the structural results from diffraction studies. Quadrupole splittings (Δν Q ) and spin-lattice relaxation rates (1/T 1 ) were monitored over the range of water/lipid mole ratios from 4 to 100. Their hydration dependence reveals a concerted change in headgroup conformation together with an increase in mobility. Both spectroscopic parameters (Δν Q and 1/T 1 ) are found to be linearly correlated with the activity of the interbilayer water, and it is thus concluded that the 2 H-NMR hydration curves are equivalent to lipid sorption isotherms. Using this thermodynamic correlation, the repulsive hydration force between apposing bilayers can be calculated from the 2 H-NMR data and a decay constant is evaluated for DOPC of around 3.5 water molecules per lipid. The characterization of the headgroup properties by 2 H-NMR is complemented by recent investigations on interbilayer 2 H 2 O, which demonstrates that the lipid-water interface constitutes a single thermodynamic entity. © 1994.