Solvent Isotope Effect on the Dark Adaptation of Bacteriorhodopsin in Purple Membrane: Viewpoints of Kinetics and Thermodynamics

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• 作者：Han-Kuei Chiang ; Li-Kang Chu
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：March 13, 2014
• 年：2014
• 卷：118
• 期：10
• 页码：2662-2669
• 全文大小：361K
• 年卷期：v.118,no.10(March 13, 2014)
• ISSN：1520-5207

文摘

The thermal retinal isomerization from all-trans, 15-anti to 13-cis, 15-syn of bacteriorhodopsin in purple membrane in H₂O and D₂O during dark adaptation was investigated at 30鈥?5 掳C at neutral pH. In this temperature range, phase transition of purple membrane and destruction of the tertiary structure of bacteriorhodopsin did not take place. We found that the solvent isotope effect is inverted below about 45 掳C; i.e., k_f(D₂O)/k_f(H₂O) > 1. Applying the transition state theory, the changes in enthalpy from the initial state to the transition state along the thermal trans-to-cis forward reaction coordinate, 螖H_f^*, were determined to be 24.7 卤 1.2 and 20.1 卤 0.4 kcal mol^鈥? in H₂O and D₂O, respectively. The relative entropic change of the transition state in H₂O and D₂O, 螖螖S_f^* = 螖S_f^*(D₂O) 鈥?螖S_f^*(H₂O), was 鈭?4.4 卤 3.9 cal mol^鈥? K^鈥?. In addition, the Gibbs free energy of trans-to-cis thermal isomerization reaction in D₂O is 0.4鈥?.7 kcal mol^鈥? lower than that in H₂O. It is the first time the entropy and enthalpy of the transition state have been quantified to elucidate the solvent isotope effect in the retinal thermal isomerization of bacteriorhodopsin during dark adaptation. The solvent isotope effect on the thermodynamics properties and kinetics implied that the hydrogen bonding in the transition state during the dark adaptation of bR is stronger than that in the initial state.