Electroselection in the photosynthetic membrane: polarized luminescence induced by an external electric field

Abstract

Delayed luminescence emission (a general property of photosynthetic systems) is closely related to primary electron flow and energetic events in photosynthesis [ 1,2]. In particular, electrical phenomena at the membrane level have a marked influence on the emission; thus, for example, an artificially induced diffusion electric potential (positive inside) [3] or a change in the dielectric constant of the membrane [2] have been shown to stimulate delayed luminescence. An efficient and kinetically unique method to induce transmembrane potentials in a suspension of membrane-bound vesicles is the application of an external macroscopic electric field [4]. In this way, delayed luminescence can be significantly enhanced (by l-3 orders of magnitude) [5,6], especially in hypotonically extensively swollen particles originating from the chloroplasts (blebs [7]). This phenomenon, termed electrophotoluminescence (EPL) [6] has interesting kinetic features, not yet understood. The role of the electric field is explained [6,8] generally in terms of the primary photosynthetic charge separation in photosystem II, its vectorial nature in the membrane [9] and an additional term in the activation energy for its back reaction introduced by the transmembrane field [8]. The resulting charge recombination produces the chlorophyll excited singlet state, giving rise to luminescence. In order to obtain additional information on the mechanism of EPL production and its relation to membrane topology, one can make use of the directional nature of the external electric field as a triggering agent. Since the electric field induced within the membrane by the external field has a strong angular dependence on the external field direction [6], one