Protein-coated beta-ferric hydrous oxide particles. An electrokinetic and electrooptic study.

Using microelectrophoresis and electric light scattering techniques, we investigated the adsorption characteristics, surface coverage and surface electric parameters of superstructures from two isoforms of plastocyanin, PCa and PCb, in an oxidized state adsorbed on beta-ferric hydrous oxide particles. The surface electric charge and electric dipole moments of the composite particles and the thickness of the protein adsorption layer are determined in a wide pH range, at different ionic strengths and concentration ratios of PC to beta-FeOOH. The adsorption of the two proteins was found to shift the particles' isoelectric point and to alter the total electric charge and the electric dipole moments of the oxide particles to different extent. A "reversal" in the direction of the permanent dipole moment is observed at lower pH for PCb- than for PCa-coated oxide particles. Strict correlation is found between the changes in the electrokinetic charge of the composite particles and the variation in their "permanent" dipole moments. Data suggest that the adsorption of the proteins is driven by electrostatic and/or hydrophobic interactions with the oxide surfaces dependent on pH. The adsorption behaviour is consistent with the involvement of the "eastern" and "northern" patches of the plastocyanin molecules in their adsorption on the oxide surfaces that are differently charged depending on pH.

Irreversible temperature-induced changes in purple membranes studied by electrooptics.

The thermal stability of purple membranes is studied by electric light scattering. Information on the polarization mechanisms is obtained. Incomplete recovery of the initial electric state (i.e., of permanent dipole moment, p( perpendicular), and electric polarizability, gamma(fast, ||)) after the membranes are heated at temperatures above 60 degrees C is revealed. Additional slow polarizability components, gamma(slow, perpendicular) and gamma(slow, ||), relaxing at different characteristic frequencies than the fast longitudinal polarizability gamma(fast, ||) appear in the temperature range where the order-disorder transition takes place. The slow polarizability gamma(slow, perpendicular) is probably connected with counterion displacement in the electrical double layer perpendicular to the disk plane. The results are important for understanding the polarization mechanisms and the origin of slow orienting moments.

Contribution of LHC II complex to the electric properties of thylakoid membranes: an electric light scattering study of Chl b-less barley mutant.

Electric light scattering measurements demonstrate a strong decline in the permanent electric dipole moment and electric polarizability of both thylakoid membranes and photosystem II-enriched particles of the Chlorina f2 mutant which has severely reduced levels of light-harvesting chlorophyll a/b-binding proteins compared to the wild type barley chloroplasts. The shift in the electric polarizability relaxation to higher frequencies in thylakoids and photosystem II particles from Chlorina f2 reflects higher mobility of the interfacial charges of the mutant than that of the wild type membranes. The experimental data strongly suggest that the major light-harvesting complex of photosystem II directly contribute to the electric properties of thylakoid membranes.

Electrokinetic charge of the anesthetic-induced bR480 and bR380 spectral forms of bacteriorhodopsin.

The translational and rotational electrokinetics of the anesthetic-induced spectral transitions bR568-->bR480-->bR380 of bacteriorhodopsin have been investigated. Formation of the bR480 form is associated with an increase of the purple membrane negative electrokinetic charge, while the transformation of bR480 into bR380 is accompanied by a decrease of the membrane negative charge as compared to that of the 480 nm-absorbing form. Removal of anesthetics leads to the back transitions bR480-->bR568 and (in part) bR380-->bR568; however, the electrokinetic charge of the native membranes is not restored. A strong decrease in the electric polarizability and the appearance of a slow polarizability component are also observed in anesthetic-treated membranes. Comparison with the electrokinetic behaviour of partially delipidated membranes and with that of liposomes composed of purple membrane total lipids suggests that: (i) anesthetic molecules partition mainly at the protein/lipid interface inducing irreversible rearrangement of the boundary lipid layer, and (ii) different mode(s) or site(s) of interaction are responsible for the spectral and surface charge effects. The data are compatible with the hypothesis of anesthetics acting through partial dehydration of the membrane surface.

Influence of cation binding on the electro-optically determined electric moments of purple membranes.

Analysis of the electro-optically determined permanent dipole moment and electric polarizability of purple membrane fragments reveals the complex nature of the membrane electric moments. The problem to distinguish between the contribution of the membrane structural charges (charged groups of the polypeptide chain and polar lipid headgroups), bound cations and the electric double layer structure deserves particular attention not only because of its importance for electro-optics but also in respect to the relation of the membrane surface electric properties to the membrane transport function. The removal of divalent cations (Ca2+ and Mg2+) bound to purple membrane in the native state induces a cation-free species of purple membrane (deionized--blue membrane) with drastically changed spectroscopic properties and function. The present paper summarizes our study on the electric moments of blue membrane and their changes during the blue to purple transition. We intended to provide an insight into the possible regulation of this reversible transition (purple-to-blue and blue-to-purple) through changes of the asymmetric charge distribution and the importance of the asymmetric interfacial charge distribution for the proton transfer in purple membranes. The changes in the electric moments (permanent and induced dipole moments) of purple membrane fragments upon di- and trivalent cations binding to cation-depleted purple membranes were studied by electric light scattering (rotational electrokinetics) in d.c. and a.c. electric fields, and by electric pulses with reversing polarity. The results show a recovery of the membrane charge asymmetry (permanent dipole moment) though not of the induced dipole moment.