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Escherichia coli DNA photolyase, which photorepairs cyclobutane pyrimidine dimers, contains two chromophore cofactors, 1,5-dihydroflavin adenine dinucleotide (FADH2) and 5,10-methenyltetrahydrofolate (MTHF). Previous work has shown that MTHF is the primary photoreceptor which transfers energy to the FADH2 cofactor; the FADH2 singlet excited state then repairs the photodimer by electron transfer. In this study, we have determined the rate constants for these photophysical processes by time-resolved fluorescence and absorption spectroscopy. From time-resolved fluorescence, we find that energy transfer from MTHF to FADH2 and FADH degrees occurs at rates of 4.6 x 10(9) and 3.0 x 10(10) s-1, respectively, and electron transfer from FADH2 to a pyrimidine dimer occurs at a rate of 5.5 x 10(9) s-1. Using Förster theory for long-range energy transfer and assuming K2 = 2/3, the interchromophore distances were estimated to be 22 A in the case of the MTHF-FADH2 pair and 21 A for the MTHF-FADH degrees pair. Picosecond absorption spectroscopy identified an MTHF single state which decays to yield the first excited singlet state of FADH2. The lifetimes of MTHF and FADH2 singlets and the rates of interchromophore energy transfer, as well as the rate of electron transfer from FADH2 to DNA measured by time-resolved fluorescence, were in excellent agreement with the values obtained by picosecond laser flash photolysis. Similarly, fluorescence or absorption lifetime studies of the folate-depleted enzyme with and without photodimer suggest that FADH2, in its singlet excited state, transfers an electron to the dimer with 89% efficiency. The distance between FADH2 and the photodimer was calculated to be ca. 14 A.

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Magnetoencephalographic (MEG) studies have revealed enhancement of neural activity of the N1m response of auditory evoked fields in long-term trained musicians, reflecting neuroplastic modification of the representation of the auditory cortex. In contrast, the amplitude of the P2 response of auditory evoked potentials is modified by musical experience, with no alteration of N1. Here, we performed a comprehensive MEG study using stimulation of successive musical-instrument tones to examine how the neural activities of different MEG responses are modified in long-term experienced musicians who commenced musical lessons at ages of ϳ5 years and had continued to practice. The dipole moment of the P2m response occurring at 160 -180 ms was significantly enlarged in musicians compared with that in individuals who had not received musical lessons. The enlargement was found for the dipole moment of N1m occurring at 100 -120 ms in a restricted condition but not for the moment of P1m at 50 -60 ms. Furthermore, the dipole moment of P2m for successive stimuli, normalized by the moment for the first stimulus, was significantly larger for chord tones than single tones and was significantly larger in the musicians than controls. These results suggest that the P2m response is susceptible to be modified by musical training in a period of neural maturation, with a short refractory period of neural activity for the auditory input of composite tones. The P2m activity may be specialized to the processing of multifrequency sounds, such as musical timbre consisting of abundant harmonics.

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