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THE KINETICS OF THE EARLY STAGES OF THE PHYTOCHROME PHOTOTRANSFORMATION P r → P fr . A COMPARATIVE STUDY OF SMALL (60 kDalton) and NATIVE (124 kDalton) PHYTOCHROMES FROM OAT
Author(s) -
Ruzsicska Bela P.,
Braslavsky Silvia E.,
Schaffner Kurt
Publication year - 1985
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1985.tb03623.x
Subject(s) - phytochrome , kinetics , chemistry , arrhenius equation , absorbance , chromophore , photochemistry , activation energy , crystallography , stereochemistry , red light , physics , chromatography , botany , biology , quantum mechanics
— A comparative study of the decay kinetics of photogenerated transients from small (60 kDalton) and native (124 kDalton) oat phytochrome in the red‐absorbing form (P r ) in phosphate buffer containing 5 mM ethylenediamine tetraacetic acid, pH 7.8, (PB) and in PB containing 20% ethylene glycol, has been carried out in the temperature range 275–298 K. The analysis confirmed that at least two primary photoproducts, intermediates I i 700 s and I i 7oo are formed from P r . The kinetic parameters, as observed in PB at 695 nm and 275 K, are similar for the I 700 intermediates of both small and native phytochrome. Namely, the lifetimes are about 21 μs (component percentages 38%) for the I I i 700 s and about 200 μ.s (62%) for the I i 700 S‐ Arrhenius preexponential factors (A) of about 10 16 and 10 15 s‐ 1 and activation energies of about 61 and 56 kJ/mol were measured for the absorbance decays of the I 700 S of small and native phytochrome, respectively. The kinetic data favour parallel paths for the formation of the I i 700 s from P r , and the activation parameters indicate that the primary photoreactions of the transformation from P r to the far‐red‐absorbing form are restricted to the chromophore within the protein. Moreover, the relatively modest temperature dependence of the lifetimes of the I i 700 S from small and native P r supports the working hypothesis that the ground state reactions to the I i bl , intermediates–although somewhat influenced by the polypeptide fragment that is removed upon degradation of native to small P r –are localized to the chromophore, as is most probably the case also for the primary photoreactions. The effect of the addition of 20% ethylene glycol on the pre‐exponential factors of the time‐dependent decay functions is discussed in similar terms of the early stages of the phototransformation.