Electrochromic absorbance changes in spinach chloroplasts induced by an external electrical field

Abstract

Absorbance changes induced by electrical field pulses were studied in osmotically swollen spinach chloroplasts. The results and their interpretation on the basis of the geometry and electrical properties of the material may be summarized as follows:1. The spherical vesicles, ‘blebs’, formed upon dilution of a chloroplast suspension consist of only a single membrane, while part of the thylakoid system remains concentrated in a few patches on its surface.2. When an electrical field pulse is applied, an up to 3000-fold enhanced field is built up in the membrane, with a time constant of about 20 μs. From this the specific capacitance of the bleb wall was found to be 2 μF · cm−2.3. The electrical field in the membrane causes several absorbance changes of the photosynthetic pigments with different dependencies on the direction of polarization of the measuring light. Some of these are due to field-induced changes in orientation, in particular of chlorophyll a, and have a relaxation time of less than 100 μs. Most of the absorbance changes directly reflect the kinetics of the membrane potential and can be ascribed to electrochromic shifts of photosynthetic pigments, mainly of carotenoids.4. The carotenoid absorbance changes depend quadratically on the membrane potential; an apparent saturation at high applied field strengths is ascribed to dielectric breakdown at a membrane potential of about 1 V.5. All carotenoids in the membrane contribute to the absorbance changes induced by an externally applied field, whereas the well-known light-induced electrochromic absorbance change at 518 nm is mainly caused by a minor fraction of permanently polarized and spectrally red-shifted carotenoids. A computer simulation showed that this interpretation quantitatively explains the results and requires no unreasonable values of the various parameters involved.