Bleaching of bacteriorhodopsin by continuous light

Abstract

A new two step photobleaching process is observed under continuous illumination of bacteriorhodopsin. This photobleaching is considerable even at physiological temperatures and becomes large at 50^60³C. The photobleaching also increases with increasing pH from 7 to 10. We suggest that the bleaching at its final stage could be due to the dissociation of the retinal and a local thermal denaturation-like process. These facts may question the generally held belief that BR is a stable protein in vivo for a long period of time. Our results may have relevance also to practical applications of bacteriorhodopsin where the stability of bacteriorhodopsin is a key issue. In certain instances, the use of bacteriorhodopsin may require cooled conditions. Here, we defined the conditions under which bacteriorhodopsin is stable. The permanent photobleaching offers a new way of picture imaging and information input for bacteriorhodopsinbased optical devices.z 1999 Federation of European Biochemical Societies.

Directed evolution of bacteriorhodopsin for applications in bioelectronics

“…The P state contains a bound 9-cis chromophore, and was originally defined as having an absorption maximum (l max ) of approximately 490 nm [73]; however, further research shows that the P state is actually a mixture of two photostates, described as P 1 (l max ¼ 525 nm) and P 2 (l max ¼ 445 nm). These photostates undergo a dynamic equilibrium after the absorption of a photon [54], and because 9-cis retinal is unstable in the binding pocket, the chromophore thermally decays to form the Q state (l max ¼ 380 nm) [54,70,74,75]. The Q photoproduct contains a hydrolysed 9-cis retinal chromophore trapped inside the BR binding pocket (figure 1) [70].…”

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“…The heating, however, is not harmless. Using the approach of Dancshazy and coworkers [210], it is possible to surmise that a single UV quantum should locally heat a 100-unit RNA-like polymer by tens of degrees Celsius. Even if the absorbed energy can be promptly transmitted to a template, local heating of the template can eventually cause ablation of the molecule [211].…”

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“…The heating, however, is not harmless. Using the approach of Dancshazy and co-workers [210], it is possible to surmise that a single UV quantum should locally heat a 100-unit RNA-like polymer by tens of degrees Celsius. Even if the absorbed energy can be promptly transmitted to a template, local heating of the template can eventually cause ablation of the molecule [211].…”

“…Author's response : The estimate from [210], which I have used, was calculated for an ensemble of bacteriorhodopsin molecules, each being hit by one quantum of visible light. I would strongly encourage a corresponding quantitative analysis for RNA molecules and UV light quanta .…”