Spin dynamics of reactive oxygen species as studied by probing electron spin polarization

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Electron spin polarization created in various systems of O_2(^1Δ_g) and radicals were observed by means of the time-resolved ESR (TR-ESR) spectroscopy. Electron spin polarization of TEMPO and PTIO radicals are 340 and 140 while those of galvinoxyl, DPPH and verdazyl radicals are 0.7, 0 and 0 in the unit of Boltzmann polarization. This result indicate that electron spin polarization is very efficiently created in TEMPO and PTIO while is not in galvinoxyl, DPPH and verdazyl radicals. According to the theoretical analysis of electron spin polarization, it was found that the efficiency of spin polarization creation is due to the magnitude of electron exchange interaction between O_2(^1Δ_g) and radical which is largely controlled by the size of SOMO orbital of radical.Electron spin polarization creation was also examined on the surface of α-lactalbumin as an representative of protein molecules. Selective photoexcitation of tryptophan residue in α-lactalbumin followed by the deactivation due to TEMPO radical dissolved in the sample solution results in strong electron spin polarization of TEMPO which was monitored by pulsed ESR method. This experiments clearly demonstrated that the spin polarization created on the surface of protein molecules can be monitored by TR-ESR method and promised that O_2(^1Δ_g) diffusion and deactivation process on protein surface can be monitored by spin probing method in the future.