Calibration of the electrochromic absorbance changes in chromatophores of Rhodopseudomonas capsulata

Abstract

Carotenoid absorbance changes induced by addition of poorly permeating ions in the presence of CCCP were studied. The reported experimental results support the idea that such absorbance changes reflect the screening of surface charges, thus changing the surface potentials. A value of −101±5mV for the surface potential was determined. Evidence is presented that the blue shift observed upon screening of surface charges indeed is an electrochromic response to the surface potential charges. Surface potentials affect measurements of the absorbance change induced by diffusion potentials of Rb + or K + in the presence of valinomycin. As shown in a previous publication [6] a correction can be applied by adding complementary concentrations of an impermeant ion ( e.g. Na + ) when measuring the calibration curve, thus keeping the total ion concentration constant for each addition. In the present work calibration curves were determined by generating diffusion potentials of Rb + and K + at different, but for each curve constant total ion concentrations. The results could be explained fully by the screening effect of the ions. Other factors influencing the calibration, with diffusion potentials, of light-induced membrane potentials were shown to give rise to overestimated values. Upon application of the proper correction a steady state value of 80±8 mV for the membrane potential induced by continuous illumination was determined. This value agrees very well with values ranging from 70 to 140 mV obtained by flow-dialysis measurements.