Two phases of gramicidin photoinactivation in bilayer lipid membranes in the presence of a photosensitizer

It is shown that the kinetics of the light-induced decrease in the gramicid in-mediated current across a bilayer lipid membrane in the presence of a ph otosensitizer includes two phases: a slow one with a characteristic time of the order of 1 s and a fast one. It is concluded from the dependence of th e slow phase relative amplitude and characteristic time on the gramicidin-m ediated stationary conductance that this phase reflects the process of esta blishment of the equilibrium between gramicidin monomers and dimers in the membrane after the distortion of this equilibrium resulting from modificati on of a portion of gramicidin molecules by reactive oxygen species generate d upon excitation of the photosensitizer. The dependence of the fast phase contribution to the overall kinetics on the stationary conductance allowed us to conclude that the fast phase is associated with transition of gramici din dimers into a nonconducting state. The characteristic time of the fast phase measured with nanosecond laser exciting pulses is 1.5 ms. It is revealed that the slow phase of the decrease in the gramicidin-mediat ed current is considerably decelerated in the presence of Rose Bengal. The results obtained indicate that adsorption of Rose Bengal on the bilayer int erface leads to a reduction of the dipole potential drop at the membrane-so lution boundary, similarly to the action of phloretin.