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1 September 2001 Thermodynamics of the Early Steps in the Photocycle of Natronobacterium pharaonis Halorhodopsin. Influence of Medium and of Anion Substitution
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Abstract

The enthalpy (ΔH) and structural volume changes (ΔV) associated with the formation and decay of the early intermediate K600 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 K600 formation, its value depending on the medium and on the anion (Cl, NO3, Br, I). A smaller expansion is associated with K600 decay to L520. 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 L520 determine the direction and sign of charge translocation. The linear correlation between ΔH and ΔV 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, ΔGK = (97 ± 16) kJ/mol and ΔGKL = −(2 ± 2) kJ/mol. Different from other systems, ΔS correlates negatively with ΔV 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.

Aba Losi, Ansgar A. Wegener, Martin Engelhard, and Silvia E. Braslavsky "Thermodynamics of the Early Steps in the Photocycle of Natronobacterium pharaonis Halorhodopsin. Influence of Medium and of Anion Substitution," Photochemistry and Photobiology 74(3), 495-503, (1 September 2001). https://doi.org/10.1562/0031-8655(2001)074<0495:TOTESI>2.0.CO;2
Received: 29 January 2001; Accepted: 1 May 2001; Published: 1 September 2001
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