Langmuir, Vol.15, No.20, 7031-7039, 1999
Chronoamperometric study of Tl(I) reduction at gramicidin-modified phospholipid-coated mercury electrodes
An investigation into the effect of gramicidin on the kinetics of the Tl(I)/Tl(Hg) process at dioleoyl phosphatidylcholine (DOPC)- and modified-DOPC-coated electrodes has been performed using chronoamperometry and cyclic voltammetry. In this system, the Tl+ reduction mechanism resembles a CrEr type mechanism. The thermodynamic (K) and kinetic (k) parameters were measured and conform to a process whereby the Tl+ ion is complexed by the gramicidin in the monolayer K = k(1)/k(-1) is a partition coefficient representing the concentration of Tl+ in the monolayer relative to the concentration of Tl+ in solution. k = k(1) + k(-1) relates to the rate at which partition preceding the charge transfer is attained. The values of the rate constants are consistent with the published values of the ion entry and exit rate constants of the bimolecular gramicidin channel. Approximate coverage values of gramicidin available for Tl+ binding and transport can be calculated which are low. K decreases with increase in negative potential. K increases by a factor of 5 in layers of DOPC with PS and by a factor of between 2 and 3 in layers of DOPC with retinol. k(1) increases in layers of DOPC with PS by a factor of 10.
Keywords:NUCLEAR MAGNETIC-RESONANCE;EQUILIBRIUM BINDING CONSTANTS;APPLIED VOLTAGE PROFILES;CHANNEL FUNCTION;ENERGY BARRIERS;PORE STRUCTURE;ION MOVEMENTS;MONOLAYERS;TRANSPORT;SPECTROSCOPY