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• Articles by H.I. Mostafa
• Articles by L. Keszthelyi

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• Photoreactions of bacteriorhodopsin at acid pH

  Photoreactions of bacteriorhodopsin at acid pH Biophysical Journal, Volume 56, Issue 6, 1 December 1989, Pages 1143-1151 G. Váró and J.K. Lanyi Abstract It has been known that bacteriorhodopsin, the retinal protein in purple membrane which functions as a light-driven proton pump, undergoes reversible spectroscopic changes at acid pH. The absorption spectra of various bacteriorhodopsin species were estimated from measured spectra of the mixtures that form at low pH, in the presence of sulfate and chloride. The dependency of these on pH and the concentration of Cl- fit a model in which progressive protonation of purple membrane produces "blue membrane", which will bind, with increasing affinity as the pH is lowered, chloride ions to produce "acid purple membrane." Transient spectroscopy with a multichannel analyzer identified the intermediates of the photocycles of these altered pigments, and described their kinetics. Blue membrane produced red-shifted KL-like and L-like products, but no other photointermediates, consistent with earlier suggestions. Unlike others, however, we found that acid purple membrane exhibited a very different photocycle: its first detected intermediate was not like KL in that it was much more red-shifted, and the only other intermediate detectable resembled the O species of the bacteriorhodopsin photocycle. An M-like intermediate, with a deprotonated Schiff base, was not found in either of these photocycles. There are remarkable similarities between the photoreactions of the acid forms of bacteriorhodopsin and the chloride transport system halorhodopsin, where the Schiff base deprotonation seems to be prevented by lack of suitable aspartate residues, rather than by low pH. Abstract | PDF (949 kb)

• Purple-to-blue transition of bacteriorhodopsin in a neutral ...

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Abstract

Electrooptical measurements on purple membrane containing the wild-type and 10 different bacteriorhodopsin mutants have shown that the direction of the permanent electric dipole moment of all these membranes reverses at different pH values in the range 3.2–6.4. The induced dipole moment and the retinal angle exhibit an increased value at these pHs. The results demonstrate that the bacteriorhodopsin protein makes an important contribution to the electrooptical properties of the purple membrane.