Excited State Dynamics of Physiologically Active Molecules : Picosecond and Nanosecond Time Resolved Vibrational Spectroscopy

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The main results of this research are as follows :(1) 8-Methoxypsoralen (8-MOP), a photo-activated medicine for skin diseases, exhibited completely different behavior in the photochemical reaction compared to psoralen (Ps) and 5-methoxypsoralen (5-MOP). Picosecond time-resolved absorption spectra revealed that the wavelengths and lifetimes of the lowest excited singlet state S, and lowest excited triplet state T_1 of 8-MOP were quite different from those of Ps and 5-MOP. Ab initio DFT calculations showed that only 8-MOP took an open-ring structure in the T_1 state. This is the reason why the lifetime of T_1 of 8-MOP is extremely short compared to other psoralens. The fact that the adverse side effect of 8-MOP is much less compared to other psoralens may be understood if the cleavage of the psoralen-ring inhibits the interaction of 8-MOP with pyrimidine bases of DNA.(2) Pyridoxal-5-phosphoric acid (PLP) Shiff base is an important coenzyme related to the transformation of methyl groups in amino acids. In order to understand the mechanism of this reaction, picoseconds time-resolved Raman and absorption spectra of pyridoxal 5-phosphoric acid methylamine Shiff base, a model compound of PLP, were measured. Ab initio DFT calculations were carried out for this compound to obtain the most probable structure and normal frequencies.(3) Nicotinamide adenine dimucleotide in the reduced form (NADH) is a coenzyme which plays important roles in the oxidation-reduction reaction in vivo. Time-resolved Raman and UV-visible absorption spectra reveal that three transients and solvated electrons were involved in the photo-oxidation reaction of NADH. The transient species exhibiting the absorption bands at 370nm and 560nm and the Raman band at 1627cm^<-1> can be assigned to the cation radical NADH^<+*> which is converted into the neutral radical NAD^* with time This suggests that three-step process is operative in the oxidation -reduction reaction of NADH.