# Timescale of Protonation and Deprotonation across micelle layers

I am curious about the timescales of protonation and deprotonation of solvent systems. As a followup, how is this affected when the proton source is separated by an organic phase? For instance, it is known that the pH inside a liposome can be maintained from the outside. What are the timescales of protonation when the protons need to be transported across various micelles thickness.

• If anyone does weigh in, I'd be interested in some details regarding the self-ionization of water. How quickly do these proton transfers occur. What order of magnitude are the energy barriers associated with these? More importantly, does anyone have a good citation that deals with this, particularly for reaction mechanisms? If not I may just make a new submission. – LordStryker Apr 26 '12 at 14:49
• The timescale of protonation is very fast, to the point that is makes almost no sense to ask the question. More importantly, you are asking about the timescale of diffusion of hydronium ion through the micelle. – Lighthart Feb 25 '13 at 5:10
• @Lighthart, I agree with your insight. I am still looking for a quantifiable value for both. – bobthejoe Feb 25 '13 at 10:15

Reported values for proton permeability coefficients vary over a wide range, from $10^{-2} \, \mathrm{cm/s}$ to $10^{-7} \, \mathrm{cm/s}$ (Nichols and Deamer, 1980; Nichols et al., 1980; Biegel and Gould, 1981; Elamrani and Blume, 1983; Grzesiek and Dencher, 1986; Perkins and Cafiso, 1986; Norris and Powell, 1990). This variation relates to the use of different techniques, lipids, and bilayer systems (liposomes and planar bilayers) or different pH ranges (Perkins and Cafiso, 1986). The proton data of this study agree with the higher reported values.
As revealed by Fig. 4, proton permeability coefficients strongly depend on membrane thickness, starting at a maximum value of $1.3 \cdot 10^{-2} \, \mathrm{cm/s}$ for the shortest lipid and steadily decreasing to a value of $4.9 \cdot 10^{-5} \mathrm{cm/s}$ for the longest lipid. It follows that proton permeability decreases by a factor of approximately 250 as the thickness of the hydrophobic region is increased from $20 \, \mathring{\mathrm{A}}$ to $37 \, \mathring{\mathrm{A}}$.