The retinal protonated Schiff base of bacteriorhodopsin is photoreactive to reducing agents such as NaBH4. In the present work we have studied the effect of different protein hydration levels on the photoreductive reaction, as well as the consequences of preventing isomerization around the critical C13=C14 retinal double bond. It was revealed that the rate of light-induced NaBH4 reaction can be fitted to three phases, between 100 and 87%, from 87 to 35% and below 35% relative humidities (r.h.). The three phases are attributed to three protein regions characterized by different water affinities. Furthermore, it is shown that the PSB reduction reaction is light catalyzed even in artificial pigments derived from retinal analogs, in which isomerization around the C13=C14 double bond is prevented. It is suggested that the protein experiences light-induced conformational alterations that are not associated with C13=C14 double bond isomerization. In the 13-cis locked pigment the rate of reduction reaction is affected by r.h. levels only below 35%. The relatively low r.h. required for withdrawing water from the protein is attributed to the increased protein-water affinity in this specific pigment.
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