Proton migration on membranes is a crucial step in the bioenergetics of the cell. It has typically been regarded as slow successive proton transfers between ionizable moieties within the membrane, but recent measurements suggest fast lateral diffusion in the membrane's hydration layer.
Figures at a glance
Figure 1: Time-resolved fluorescein-derivative signal for proton migration on three membranes: diphytanoyl phosphatidylcholine (PC, pKa ≈ 2.2), glycerolmonoleate (GMO, pKa ≈ 8.5), and diphytanoyl phosphatidylethanolamine (PE, pKa ≈ 9.6), in a solution containing 0.1 mM CAPSO buffer (pH = 9). 
The solution of the diffusion equation subject to the initial condition σ(x,0) = Aδ(x), a Dirac delta function of strength A, is a Gaussian multiplied by a decaying exponential, as shown in the inset. The observed fluorescence signal is F(t) = Fmax– σ(r,t), where r = 70 μm is the distance between the illuminated rectangles, and Fmax the maximal fluorescence intensity at pH 9. Our fits (black lines) give D = 6.0, 8.5 and 12.0 (10−5 cm2 s−1) and k = 0.43, 0.36 and 0.15 s−1, for PC, GMO and PE, respectively. In comparison, D = 9.3 × 10−5 cm2 s−1 for protons in bulk water. The dashed magenta line corresponds to two-dimensional diffusion (dividing the equation in the inset by t1/2) with k = 0 and D = 6.5 × 10−5 cm2 s−1. Data adapted from Fig. 2 of ref. 9 (PC and GMO were interchanged there), courtesy of P. Pohl.
Figure 2: A schematic model for front formation, where the spheres represent the protons, forming a proton front that can be compared to marbles spreading on a corrugated surface. 
