pH dependence of Anabaena sensory XXXX : retinal isomer composition, rate of dark adaptation, and photochemistry.

@entity8695 @entity7352 @entity10267
Microbial rhodopsins are photoactive proteins, and their binding site can accommodate either all-trans or 13-cis retinal chromophore. The pH dependence of isomeric composition, dark-adaptation rate, and primary events of Anabaena sensory @entity8695 (ASR), a microbial @entity8695 discovered a decade ago, are presented. The main findings are: (a) Two pKa values of 6.5 and 4.0 assigned to two different protein residues are observed using spectroscopic titration experiments for both ground-state retinal isomers: all-trans, @entity10267 ( @entity10267 ) and 13-cis, 15-syn ( @entity7352 ). The protonation states of these protein residues affect the absorption spectrum of the pigment and most probably the isomerization process of the retinal chromophore. An additional pKa value of 8.5 is observed only for @entity7352 -ASR. (b) The isomeric composition of ASR is determined over a wide pH range and found to be almost pH-independent in the dark (>96% @entity10267 isomer) but highly pH-dependent in the light-adapted form. (c) The kinetics of dark adaptation is recorded over a wide pH range, showing that the thermal isomerization from @entity7352 to @entity10267 retinal occurs much faster at high pH rather than under acidic conditions. (d) Primary photochemical events of ASR at pH 5 are recorded using VIS hyperspectral pump-probe spectroscopy with <100 fs resolution and compared with the previously recorded results at pH 7.5. For @entity10267 -ASR, these are shown to be almost pH-independent. However, photochemistry of @entity7352 -ASR is pH-dependent and slowed down in acidic environments.
