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* Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto, Japan; and Research Institute for Advanced Science and Technology, Department of Biological Science, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka, Japan
Correspondence: Address reprint requests to Masahide Terazima, Dept. of Chemistry, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan. E-mail: mterazima{at}kuchem.kyoto-u.ac.jp.
Photochemical reaction of a plant blue-light photoreceptor, Arabidopsis phototropin 1-LOV (light-oxygen-voltage sensing) domain 2, was studied with a view to the diffusion coefficients (D) using the pulsed-laser-induced transient grating method. Although the reaction dynamics completes at a rate of several microseconds as long as it is monitored by the absorption change, the diffusion coefficient was found to be time-dependent in a time range of submilliseconds to seconds. The observed signal can be analyzed by the two-state model, which includes the D-value decrease from D of the reactant (9.8 ± 0.4) x 10–11 m2/s to D of the product (8.0 ± 0.4) x 10–11 m2/s. The D-value of the reactant implies that the dominant form in the ground state of phototropin 1 LOV2 is the monomeric form in a concentration range of 50–200 µM. According to the Stokes-Einstein relationship, the D-change can be explained by a volume increase of 1.8 times. Furthermore, the rate of the D-change was roughly proportional to the concentration of the sample. These two observations indicate that the LOV2 domain transiently forms a dimer upon photoexcitation. When the sample concentration is increased (>180 µM), a new signal component appears within a few milliseconds. This signal represents a D increase from 8.0 x 10–11 m2/s to 9.8 x 10–11 m2/s with a time constant of 300 µs. The completely opposite D-change from that observed in a lower concentration, as well as the concentration dependence, implies that a dimer is formed in the ground state in a higher concentration range, even though the fraction of the dimer is still minor in this range. This dimer is photodissociated, with a time constant of 300 µs. This research clearly shows that the time-resolved diffusion measurement is a very powerful tool for detecting spectrally silent association/dissociation processes during chemical reactions. The photoreaction of the LOV2 domain is discussed.
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