Two mechanisms of H+/OH- transport across phospholipid vesicular membrane facilitated by gramicidin A

Two rate-limiting mechanisms have been proposed to explain the gramicidin channel facilitated decay of the pH difference across vesicular membrane (Delta pH) in the pH region 6-8 and salt (MCl, M(+) = K+, Na+) concentration range 50-300 mM. 1) At low pH conditions (similar to 6), H+ transport through the gramicidin channel predominantly limits the Delta pH decay rate. 2) At higher pH conditions (similar to 7.5), transport of a deprotonated species (but not through the channel) predominantly limits the rate. The second mechanism has been suggested to be the hydroxyl ion propogation through water chains across the bilayer by hydrogen bond exchange. In both mechanisms alkali metal ion transport providing the compensating flux takes place through the gramicidin channels. Such an identification has been made from a detailed study of the Delta pH decay rate as a function of 1) gramicidin concentration, 2) alkali metal ion concentration, 3) pH, 4) temperature, and 5) changes in the membrane order (by adding small amounts of chloroform to vesicle solutions). The apparent activation energy associated with the second mechanism (similar to 3.2 kcal/mol) is smaller than that associated with the first mechanism (similar to 12 kcal/mol). In these experiments, Delta pH was created by temperature jump, and vesicles were prepared using soybean phospholipid or a mixture of 94% egg phosphatidylcholine and 6% phosphatidic acid.