Time-resolved detection of conformational changes in oat phytochrome A: time-dependent diffusion.

Conformational changes in oat phytochrome A (phy) in solution after photoexcitation of the red-absorbing form (Pr) were studied in time-domain by the pulsed laser-induced transient grating technique. It was found that the diffusion coefficient (D) of far-red-absorbing form (Pfr) of large phy (1.3 x 10(-11) m(2) s(-1)) is markedly reduced compared with that of Pr (5.8 x 10(-11) m(2) s(-1)). This large reduction indicates that the conformation of Pfr is significantly changed from that of Pr, so that the intermolecular interaction with water molecules increases. This change completes within 1 ms after the photoexcitation. On the other hand, D of Pr of intact phy (4.1 x 10(-11) m(2) s(-1)) first decreases upon photoexcitation to 0.89 x 10(-11) m(2) s(-1) within 1 ms and then gradually increases with a time constant of 100 ms to the value of Pfr, 1.7 x 10(-11) m(2) s(-1). This slower phase suggests that the conformation of the N-terminal region changes with 100 ms to decrease the intermolecular interaction with water after a global change in the large phy region. The increase of D was interpreted in terms of alpha-helix formation in the Pfr form from the random coil structure in the Pr form.