Proton dissociation dynamics in the aqueous layer of multilamellar phospholipid vesicles

The water layers interspacing between the phospholipid membranes of a multilamellar vesicle are 3-10 water layers across and their width is adjusted by osmotic pressure (Parsegian, V.A., et al., 1986. Methods Enzymol.127:400-416). In these thin water layers we dissolved pyranine (8 hydroxypyrene 1,3,6 trisulfonate), a compound which, upon photo excitation, ejects it hydroxy proton with time constant of 100 psec. (Gutman, M. 1986. Methods Enzymol.127:522-538). In the present study we investigated how the width of the aqueous layer, the density of phosphomoieties on the membrane's surface and the activity of water in the layer affect the capacity of protons to diffuse out from the electrostatic cage of the excited anion before it decays to the ground state. Using a combination of steady-state and subnanosecond time-resolved fluorescence measurements we determined the average number of proton excited-anion recombinations before the proton escapes from the Coulomb cage. The probability of recombination in thin water layer is significantly higher than in bulk. The factor contributing most to enhancement of recombination is the diminished water activity of the thin aqueous layer. The time frame for proton escape from an electrostatic trap as big as a membrane-bound protein is 3 orders of magnitude shorter than turnover time of membrane-bound enzymes. Thus the effects of local forces on proton diffusion, at the time scale of physiological processes, is negligible.