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| Home > Publications database > 13-CIS and trans bacteriorhodopsin : Photochemistry and dark equilibrium |
| Home > Publications database > 13-CIS and trans bacteriorhodopsin : Photochemistry and dark equilibrium |
| Book/Report | FZJ-2018-00937 |
; ;
1977
Kernforschungsanlage Jülich, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/16919
Report No.: Juel-1374
Abstract: Flash photometric experiments were performed which help to clarify the photochemistry and interconversion of 13-cis bacteriorhodopsin (BR$^{548}_{13-cis}$) and trans bacteriorhodopsin (BR$^{568}_{trans}$). The two bacteriorhodopsin isomers were seperately prepared by incubating the retinal-free protein with 13-cis and trans retinal, respectively. Our results reveal that BR$^{548}_{13-cis}$ and B$^{568}_{trans}$ form different intermediates upon light excitation: BR$^{568}_{trans}$ undergoes a reaction cycle identical to that previously described for light adapted purple membrane. The intermediate $^{411}$T of this cycle was found to photoreact to BR$^{568}_{trans}$. BR$^{548}_{13-cis}$, in contrast, yields a photoproduct $^{\chi}$C, which is completely present in less than 50 nsec at -90$^{\circ}$ C and decays with a half-lifetime of 1.5 $\mu$sec to a further intermediat, $^{610}$C. $^{610}$C decays with a half-lifetime of 37 msea (20$^{\circ}$ C) to both B$^{548}_{13-cis}$ and BR$^{568}_{trans}$. In addition to the thermal pathway, $^{610}$C is photoconvertible to BR$^{548}_{13-cis}$.$^{\chi}$C and $^{610}$C are intermediates in the photoisomerization pathway of BR$^{548}_{13-cis}$ to BR$^{568}_{trans}$. An analytic method based on the distinct photoreactions of BR$^{548}_{13-cis}$and BR$^{568}_{trans}$ was developed to determine the ratio in a given sample. It was found that in the dark adapted state the purple membrane of living bacteria, isolated purple membrane and regenerated bacteriorhodopsin all contain a mixture of BR$^{548}_{13-cis}$ and BR$^{568}_{trans}$ in a ratio of about 1:1. The equilibrium constant K is only slightly dependent on the temperature between 0$^{\circ}$C and 60$^{\circ}$ (50% BR$^{568}_{trans}$ at 0$^{\circ}$C, 53% BR$^{568}_{trans}$ at 60$^{\circ}$C). $\Delta$H$^{o}$ for the reaction BR$^{548}_{13-cis} \rightarrow$ $BR^{568}_{trans}$ was calculated to be about 350 cal mole$^{-1}$. Under conditions of light adaptation, only BR$^{568}_{trans}$ is present.
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