Phototransformation of the Red-light-absorbing Form of Undegraded Pea Phytochrome by Laser Flash Excitation

The phototransformation of undegraded pea phytochrome, which was estimated to be about 30 to 40% pure, from its red-light-absorbing form to its far-red-light-absorbing form was examined at 2-4°C after laser flash excitation at 655 nm using a custom-designed multichannel transient spactra analyzer. A difference spectrum measured 10 μsec after the flash showed an absorbance increase at 692 nm, a decrease at 660 nm and a slight decrease at 604 nm. With time in darkness, the peak at 692 nm gradually decreased in magnitude and shifted to 695 nm. A decay curve of absorbance at 692 nm between 10 and 500 μsec after the flash could be resolved into two reaction components, one proceeding with a rate constant of 46,000 sec(-1) and the other with one of 2,500 sec(-1). The faster component has not been reported previously. Difference spectra also indicated that a small but significant increase in absorbance between 400 and 410 nm and decrease around 360 nm took place 10-260 μsec after the flash.