A novel method for measuring membrane conductance changes by a voltage-sensitive optical probe

Electroporation of the photosynthetic membrane: A study by intrinsic and external optical probes

The study examines the relationship between electric field-induced conductivity and permeability changes in a biological membrane (electroporation) and the amplitude-duration parameters of the externally applied electric field. These reversible changes were characterized in giant photosynthetic membrane vesicles by means of the calibrated response of an intrinsic voltage-sensitive optical probe (electrophotoluminescence) and by the uptake studies of dextran-FITC fluorescent probes of different molecular weights. We quantitatively monitored electric field-induced conductivity changes by translating the electrophotoluminescence changes into conductivity changes. This was carried out by measuring the attenuation of the electrophotoluminescent signal after the addition of known amounts of gramicidin. The results demonstrate that electroporation involves the reversible formation of discrete holes in the membrane having radii <5.8 nm. The total area of the electric field-induced holes was 0.075% of the total surface of the vesicle. The formation of the electropores was affected differently by the electric field strength than by its duration. Increase in electric field strength caused increase in the total area of the vesicle that undergoes electroporation. Increase in the duration of the electric field increases the area of single electropores. Each of the two electric parameters can be rate limiting for the dynamics of electropore formation. These results are in accordance with the model of electroporation based on electric field-induced expansion of transient aqueous holes.

Spatial and temporal electroselection patterns in electric field stimulation of polarized luminescence from photosynthetic membrane vesicles

Electroselection processes of charge recombination are manifested in the study of electric field induced polarized emission from photosynthetic membrane vesicles. The study explores the coupled spatial-temporal characteristics of electric field induced charge recombination by examining the dependence of the integrated polarized emission and the time dependent polarization on electric field strength. The experimental results were fitted to theoretical models by computer simulations employing empirical parameters. Simulation of the dependence of the integrated polarized components of emission on electric field strength, suggests field-dependent increased ratio between radiative and nonradiative rates of charge recombination. The observation that the initial polarization values are independent of electric field strength supports the assumption that electric field induced emission originates from the pole area and then spreads away from it towards the equator. The propagation rate of this electric field induced charge recombination from the pole area towards the equator is reflected by the decay of polarization which increases upon raising the electric field strength. Simulation of the polarization's decay, based on a calculated angle of 26.3 +/- 0.4 degrees between the transition moment of emission and the plane of the membrane, establishes coupled temporal spatial patterns of electroselection in intramembrane electron transfer invoked by exposing preilluminated photosynthetic vesicles to a homogeneous electric field.

Electroluminescence

An overview is presented of research based on the observation by Arnold and Azzi (1971) (Photochem Photobiol 14: 233-240), that an electric field induces charge-recombination luminescence in a suspension of photosynthetic membrane vesicles. The 'electroluminescence' signals from Photosystems I and II are discussed in relation to the shape of the vesicles and the membrane potentials generated by the externally applied electric field. The use of the electroluminescence amplitude as a probe to study the kinetics and energetics of charge separation, and of its kinetics to monitor the electric-field induced charge recombination process are reviewed. Currently unresolved issues regarding the emission yield of electroluminescence are briefly discussed and the properties are summarized of the unexplained Photosystem II luminescence which is not sensitive to the membrane potential.

Electroporation of the photosynthetic membrane: structural changes in protein and lipid-protein domains

A biological membrane undergoes a reversible permeability increase through structural changes in the lipid domain when exposed to high external electric fields. The present study shows the occurrence of electric field-induced changes in the conductance of the proton channel of the H(+)-ATPase as well as electric field-induced structural changes in the lipid-protein domain of photosystem (PS) II in the photosynthetic membrane. The study was carried out by analyzing the electric field-stimulated delayed luminescence (EPL), which originates from charge recombination in the protein complexes of PS I and II of photosynthetic vesicles. We established that a small fraction of the total electric field-induced conductance change was abolished by N,N'-dicyclohexylcarbodiimide (DCCD), an inhibitor of the H(+)-ATPase. This reversible electric field-induced conductance change has characteristics of a small channel and possesses a lifetime < or = 1 ms. To detect electric field-induced changes in the lipid-protein domains of PS II, we examined the effects of phospholipase A2 (PLA2) on EPL. Higher values of EPL were observed from vesicles that were exposed in the presence of PLA2 to an electroporating electric field than to a nonelectroporating electric field. The effect of the electroporating field was a long-lived one, lasting for a period > or = 2 min. This effect was attributed to long-lived electric field-induced structural changes in the lipid-protein domains of PS II.