TIME-RESOLVED MEASUREMENTS OF PROTON DIFFUSION IN HEXII WATER-PHOSPHOLIPIDS MIXTURE

Under well defined conditions, water and phospholipids are spontaneous ly assembled into a well-ordered structure, where the lipids form long tubes, of known diameter, filled with water. This structure is referr ed to as HexII. In our study, we dissolved Pyranine (8-hydroxy 1,3,6 t risulfonate) in the aqueous phase of HexII, and by monitoring the dyna mics of proton dissociation, gauged the physical-chemical properties o f the aqueous matrix, organized as long tubes with diameter varying be tween 16 Angstrom < 2r(w) < 50 Angstrom. The measurements, based on re al time detection of proton dissociation, indicate that the lipid-wate r interface is characterized by diminished capacity of water to bind p rotons and intensified electrostatic interactions. Both properties var ied with the diameter of the tubes. These observations are explained b y considering the molecular properties of water at the hydration layer and by the effect of dielectric discontinuity at the water-lipid junc tion.