床次 俊郎

Photoactivated adenylate cyclases (PACs) are multidomain BLUF proteins that regulate the cellular levels of cyclic adenosine 3',5'-monophosphate (cAMP) in a light-dependent manner. The signaling route and dynamics of PAC from Oscillatoria acuminata (OaPAC), which consists of a light sensor BLUF domain, an adenylate cyclase domain, and a connector helix (α3-helix), were studied by detecting conformational changes in the protein moiety. Although circular dichroism and small-angle X-ray scattering measurements did not show significant changes upon light illumination, the transient grating method successfully detected light-induced changes in the diffusion coefficient (diffusion-sensitive conformational change (DSCC)) of full-length OaPAC (FL-PAC) and the BLUF domain with the α3-helix. DSCC of FL-PAC was observed only when both protomers in a dimer were photoconverted. This light intensity dependence suggests that OaPAC is a cyclase with a nonlinear light intensity response. The enzymatic activity indeed nonlinearly depends on light intensity, that is, OaPAC is activated under strong light conditions. It was also found that both DSCC and enzymatic activity were suppressed by a mutation in the W90 residue, indicating the importance of the highly conserved Trp in many BLUF domains for the function. Based on these findings, a reaction scheme was proposed together with the reaction dynamics.

We carried out optical pump-terahertz (THz) probe measurements to investigate the charge carrier dynamics of diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) thin films. In contrast to tetrabenzoporphyrin, DPP-BP exhibits broad and intense absorption, ranging from the visible and near infrared region. After the photoexcitation at 400 nm, we observed an instantaneous appearance of transient THz signals which are due to formation of free charge carriers. Transient signals decay with the time constants of 0.3 ps and 4 ps, meaning that charge carrier recombination and/or trapping take place on this time scale. For DPP-BP thin films, we found that near-infrared photoexcitation produces mobile charge carriers efficiently even without significant excess excitation energy. This will help to improve the power conversion efficiency for DPP-BP based solar cells.