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Time‐resolved Thermodynamic Analysis of the Oat Phytochrome A Phototransformation. A Photothermal Beam Deflection Study † ¶
Author(s) -
Michler Ingolf,
Braslavsky Silvia E.
Publication year - 2001
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 - 10.1562/0031-8655(2001)0740624trtaot2.0.co2
Subject(s) - photothermal therapy , phytochrome , deflection (physics) , chemistry , beam (structure) , optics , biophysics , physics , botany , biology , red light
The time‐resolved enthalpy and the structural volume changes after excitation of native oat phytochrome A were studied in the micro‐ to milliseconds range by photothermal beam deflection (PBD), a technique that follows the time‐resolved refractive index changes upon decay of the excited species. The first set of intermediates, I 700 1 and I 700 2 , stores ca 83% of the energy of the first excited state, in agreement with previous optoacoustic data, whereas the second set stores only ca 18%. The temperature dependence of the amplitudes ratio for the optical absorbances of the (I 700 1 + I 700 2 ) intermediates set is explained on the basis of the thermochromic equilibrium between P r,657 and P r,672 , which also is in line with the present PBD data. These data were best fitted with a parallel mechanism (with equal yield in each branch) for the production of the first set of intermediates, I 700 1 and I 700 2 , as well as the second set of intermediates, I bl 1 and I bl 2 . Thus, the final steps toward P fr should be largely driven by positive entropic changes brought about by protein movements, in line with previous resonance Raman data. For the production of the first set of intermediates (I 700 1 and I 700 2 ) an expansion of 18 ± 13 mL mol −1 was determined, and a further expansion ≥7 mL mol −1 was estimated for the decay from I 700 i to the set of I bl intermediates, indicating that the far red–absorbing form of phytochrome (P fr ) has a larger volume than the red‐absorbing form of phytochrome. This is in agreement with previous chromatographic and circular dichroism data according to which P fr shows a larger volume and the chromophore shows a higher accessibility, respectively, in the P fr state.