Kinetic isotope effects of proton transfer in aqueous and methanol containing solutions, and in gramicidin A channels Original Research Article

The electrochemical conductivities of HCl and DCl were measured in: H2O and D2O; in methanol and fully deuterated methanol; and in water–methanol solutions. The single channel conductances to H+ (gH) and D+ (gD) in various gramicidin A (gA) ion channels incorporated in glycerylmonooleate planar bilayers were also measured. Kinetic isotope effects (KIE) were estimated from the ratio of conductivity measurements. In 1 and 5 M HCl aqueous solutions and in 1 M HCl+3.7 M methanol, the KIE (≈1.35) is not different from values previously determined in dilute acid solutions. This suggests that the mobility of protons in those solutions is largely determined by proton transfer. In 10 M HCl, however, where the mobility of protons is likely to be determined by hydrodynamic diffusion, the measured KIE is considerably larger (1.47). Possible causes for this effect are discussed. The KIE of proton conductivities in 5 and 50 mM HCl in methanol and d-methanol is ≈1.15. This is considerably smaller than the ratio between conductivities of 5 mM KCl in methanol and d-methanol (1.24). The KIE values (1.22–1.37) for gH in gA channels in 1 M HCl are significantly larger than for other monovalent cations and consistent with H+ transfer. Methanol reduces gH in gA channels. The KIE of this effect is not different from the one measured in the absence of methanol. Possible mechanisms for the methanol-induced block of H+ conductivities in solution and gA channels are discussed.