Noncontact dipole effects on channel permeation. IV. Kinetic model of 5F-Trp(13) gramicidin A currents

Nonlinear least squares fitting was used to assign rate constants for the t hree-barrier, two-site, double-occupancy, single-filing kinetic model for p reviously reported current-voltage relations of (5F-Indole)Trp(13) gramicid in A and gramicidin A channels (Busath et al., Biophys. J., 1998, 75:2830-2 844). By judicious coupling of parameters, it was possible to reduce the pa rameter space from 64 parameters to 24, and a reasonable fit consistent wit h other experimental data was obtained. The main features of the fit were t hat fluorination increased the rate constant for translocation by a factor of 2.33, consistent with a free energy change in the translocation barrier of -0.50 kcal/mol, and increased first-ion binding affinity by a factor of 1.13, primarily by decreasing the first-ion exit rate constant. The translo cation rate constant was 5.62 times slower in diphytanoyl phosphatidylcholi ne (DPhPC) bilayers; than in monoolein (GMO) bilayers; (coupled for the fou r combinations of peptide and salt), suggesting a 44.2-mV difference in the projection of the interfacial dipole into the channel. Thus fluorination c aused increased currents in DPhPC bilayers, where a high interfacial dipole potential makes translocation more rate limiting because the translocation barrier was reduced, and decreased currents in GMO bilayers, where ion exi t or entry is rate limiting because these barriers were increased.