Formation of the early photoproduct Lumi-R of cyanobacterial phytochrome Cph1 observed by ultrafast mid-infrared spectroscopy

The photoreactions of the Pr ground state of cyanobacterial phytochrome Cph1 from Synechocystis PCC 6803 have been investigated by picosecond time-resolved mid-infrared spectroscopy at ambient temperature. With femtosecond excitation of the Pr state at 640 nm, the photoisomerized Lumi-R product state is generated with kinetics and associated difference spectra indicative of vibrational cooling with tau(1) = 3 ps time constant and excited state decay with tau(1) = 3 ps, tau(2) = 14 ps, and tau(3) = 134 ps time constants. The Lumi-R state is characterized by downshifted absorption of three C=C modes assigned to C-15=C-16, C-4=C-5, and a delocalized C=C mode, in addition to the downshifted C-19=O mode. The Lumi-R minus Pr difference spectrum is indicative of global restructuring of the chromophore on the ultrafast timescale, which is discussed in light of C-15 Z/E photoisomerization in addition to changes near C-5, which could be low bond order torsional angle changes.