ON THE PRIMARY PHOTOPROCESS OF 124‐kdalton PHYTOCHROME

The photoreaction between P τ and the first detectable intermediate, lumi‐R, of 124‐kdalton oat phytochrome has been investigated at low temperatures. The temperature dependence of the quantum yields of the photoreactions, P τ to lumi‐R and lumi‐R to P τ , has been determined. From measurements over a temperature range from 119 to 155 K, an activation barrier of 3.6 ± 0.5 kJ mol 1 is found for the photoreaction of P τ with 661‐nm actinic light. A higher value (5.7 ± 0.7 kJ mol ‐1 ) is found for the photoreaction of lumi‐R to P τ . with 698‐nm actinic light. Increased quantum yields are found in deuterated buffer solutions at low temperatures. The activation energies for deuterated phytochrome (3.2 ± 0.7 kJ mol –1 for P τ with 661‐nm irradiation and 6.2 ± 1.2 kJ mol ‐1 for lumi‐R at 698‐nm irradiation) are identical within the limits of error with those of protonated phytochrome. The lack of a deuterium effect for the activation energies favors the Z,E‐isomerization rather than proton transfer or tautomerization for the chromophore photochemistry during P τ ⇄lumi‐R conversion.