Evaluation of the electrical capacitance in biological membranes at different phospholipid to protein ratios

Photosynthetic chromatophores of Rhodobacter capsulatus were differently enriched in phospholipid content by freezing, thawing and sonicating in the presence of phospholipid vesicles. Closed vesicles, characterized by different phospholipid to protein molar ratios and increasing average radius at increasing phospholipid enrichment, were collected after sucrose density gradient sedimentation. The electrical capacitance of these systems was evaluated from the ratio of reaction center content, photooxidized by single turnover flash in the presence of antimycin, to the corresponding membrane potential difference, measured from the electrochromic red shift of the endogenous carotenoid band. The values obtained, normalized per protein content, increased at increasing phospholipid enrichment, and correlated linearly with the increasing phospholipid to protein molar ratios. The charging capacitance of chromatophores was evaluated to be 3–6×10^-17 F and was found to increase at increasing average radius of the phospholipid enriched vesicles, as predicted by the equation of the spherical shell dielectric. The carotenoid signal, elicited in the dark by imposing diffusion potentials of known extent with K⁺-valinomycin pulses, significantly decreased at high phospholipid enrichment, indicating that in the presence of large phospholipid excess, a partial displacement of the carotenoid molecules sensing the induced electric field is produced. Concomitantly, the energy transfer efficiency from carotenoids to core light harvesting complexes (B-875) was also partially affected, particularly at high phospholipid to protein molar ratio. All together, these results suggest that the reaction center complexes are dispersed within the lipid bilayer upon fusion and that carotenoids sense a delocalized light-induced transmembrane field.Abbreviations BChl bacteriochlorophyll - BChl]₂ reaction center - PL phospholipid - cyt cytochrome - transmembrane electrical potential difference - TES 2-2-Hydroxy-1,1-bis-(hydroxymethyl)ethyl-amino-ethanosulfonic acid - mg_p mg protein