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Thermodynamics of the Early Steps in the Photocycle of Natronobacterium pharaonis Halorhodopsin. Influence of Medium and of Anion Substitution † ¶
Author(s) -
Losi Aba,
Wegener Ansgar A.,
Engelhard Martin,
Braslavsky Silvia E.
Publication year - 2001
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2001)0740495totesi2.0.co2
Subject(s) - halorhodopsin , chemistry , enthalpy , ion , flash photolysis , chloride , spectroscopy , azide , photochemistry , kinetics , thermodynamics , bacteriorhodopsin , reaction rate constant , organic chemistry , biochemistry , physics , quantum mechanics , membrane
The enthalpy (Δ H ) and structural volume changes (Δ V ) associated with the formation and decay of the early intermediate K 600 in the photocycle of Natronobacterium pharaonis halorhodopsin (pHR), an inward‐directed anion pump, were obtained by laser‐induced optoacoustic spectroscopy. A large expansion is associated with K 600 formation, its value depending on the medium and on the anion (Cl − , NO 3 − , Br − , I − ). A smaller expansion is associated with K 600 decay to L 520 . A contraction is found for the same step in the case of the azide‐loaded pHR which is an efficient outward‐directed proton pump. Thus, the conformational changes in L 520 determine the direction and sign of charge translocation. The linear correlation between Δ H and Δ V for chloride‐loaded pHR observed upon mild medium variations is attributed to enthalpy–entropy compensation effects and allows the calculation of the free‐energy changes, Δ G K = (97 ± 16) kJ/mol and Δ G KL =−(2 ± 2) kJ/mol. Different from other systems, Δ S correlates negatively with Δ V in the first steps of the pHR photocycle. Thus, the space around the anion becomes larger and more rigid during each of these two steps. The photocycle quantum yield was 0.52 for chloride‐pHR as measured by laser flash photolysis.