The Hydrogen-Bond Switch Reaction of the Blrb Bluf Domain of Rhodobacter sphaeroides

The BlrB protein from Rhodobacter sphaeroides is a small 136 amino acid photoreceptor belonging to the BLUF family of blue light receptors. It contains merely the conserved BLUF fold responsible for binding the flavin pigment and a short C-terminal extension of unknown function. We investigated the primary photoreactions of BlrB by picosecond fluorescence and transient absorption spectroscopy. After excitation of the flavin the fluorescence decays in an H/D isotope independent manner with time constants of 21 and 390 ps, indicating a BLUF characteristic heterogeneous excited state quenched by electron transfer. By transient absorption spectroscopy, we observed a rapid relaxation of a vibrationally hot excited state within 6 ps upon excitation at 400 nm. The relaxed excited state evolves biexponentially with 18 ps (27%) and 216 ps (73%) into the signaling state spectrum indicated by a growing absorptive feature at 492 nm. Additionally, a broad triplet feature is observed as residual absorbance at a delay of 5 ns, which we attribute to derive from a significant fraction of free flavin in the sample. The photochemistry of BlrB is similar to other small BLUF proteins in respect to the fast formation of the photoproduct but does not resolve any further intermediates. We compare the photoreaction with other BLUF proteins on the basis of available spectroscopic data and crystal structures. An arginine close to the C2=0 carbonyl of the flavin is likely to be a key determinant for the fast electron transfer in BlrB. Additionally, the orientation of the electron-donating tyrosine in respect to the flavin might play a role in the so far unique kinetic separation of the semiquinonic intermediates in Slr1694.