Akira TAKAKADO

DNA sequence-dependent thymine dimerization was evaluated using steady-state and transient absorption measurements, which may indicate UV-induced DNA self-repair.

Photoactive yellow protein (PYP) from Halorhodospira halophila is one of typical light sensor proteins. Although its photoreaction has been extensively studied, no downstream partner protein has been identified to date. In this study, the intermolecular interaction dynamics observed between PYP from Rhodobacter capsulatus (Rc-PYP) and a possible downstream protein, PYP-binding protein (PBP), were studied. It was found that UV light-induced a long-lived product (pUV*), which interacts with PBP to form a stable hetero-hexamer (Complex-Ⅱ). The reaction scheme for this interaction was revealed using transient absorption and transient grating methods. Time-resolved diffusion detection showed that a hetero-trimer (Complex-Ⅰ) is formed transiently, which produced Complex-II via a second-order reaction. Any other intermediates, including those from pBL do not interact with PBP. The reaction scheme and kinetics are determined. Interestingly, long-lived Complex-II dissociates upon excitation with blue light. These results demonstrate that Rc-PYP is a photochromic and new type of UV sensor, of which signaling process is similar to that of other light sensor proteins in the visible light region. The photochromic heterogeneous intermolecular interactions formed between PYP and PBP can be used as a novel and useful tool in optogenetics.

PYPs (photoactive yellow proteins) are blue light sensor proteins found in more than 100 species. Compared with the extensive and intensive studies of the reactions of PYP from Halorhodospira halophila (Hh-PYP), studies of the reactions of other PYPs are scarce. Here, the photoreaction of PYP from Rhodobacter capsulatus (Rc-PYP) was studied in detail using ultraviolet-visible absorption and transient grating methods. Rc-PYP exhibits two absorption peaks at 375 and 438 nm. By using the transient absorption and the temperature-dependent absorption spectrum, the absorption spectra of two forms, pUV and pBL, were determined. Upon photoexcitation of pBL, two intermediates are observed before returning back to the dark state, with a time constant of 1.2 ms, which is 3 orders of magnitude faster than the dark recovery of Hh-PYP. Upon photoexcitation of pUV, two intermediates are observed to produce a long-lived final product, although one of the processes is spectrally silent. The diffusion coefficients decreased transiently for both pBL and pUV reactions, suggesting a relatively large conformational change during the reactions. It is particularly interesting to observe that the blue light irradiation of the long-lived product of pUV returns the product to the dark state. This result suggests different opposing responses of the biological function due to photoexcitation by ultraviolet and blue lights.

Organic photoredox catalysis has become a useful tool for the development of metal-free radical reactions. Recently, we have reported that 1,4-bis(diphenylamino)naphthalene N serves as an efficient photoredox catalyst for radical monofluoromethylation with N-tosyl-S-monofluoromethyl-S-phenylsulfoximine 2. In this paper, we report the preparation and photo- and electrochemical properties of (diarylamino)naphthalene derivatives, 1,4-bis(di(p-tert-butylphenyl)amino)naphthalene 1a, 1,5-bis(di(p-tert-butylphenyl)amino)naphthalene 1b, and 1-(di(p-tert-butylphenyl)amino)naphthalene) 1c, as supported by density functional theory (DFT) and time-dependent-DFT calculations. In addition, their performance of photocatalysis has been evaluated by means of methoxy-monofluoromethylation of aromatic alkenes. Laser flash photolysis shows that the fluorescence of 1a in the excited state is efficiently quenched by 2 (quenching rate constant kq = ca. 2 × 109 M-1 s-1). Transient absorption spectroscopic analyses reveal that the excited species of 1a in the presence of 2 starts decreasing in ca. 100 ps, suggesting the occurrence of fast electron-transfer processes. These results lead to the unconventional concept for the catalyst design, that is, long lifetime of the excited state is not always a requisite for efficient photoredox catalysts.