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A MODEL FOR THE MOLECULAR STRUCTURE AND ORIENTATION OF THE CHROMOPHORE IN A DIMERIC PHYTOCHROME MOLECULE *
Author(s) -
Tokutomi Satoru,
Sugimoto Tohru,
Mimuro Mamoru
Publication year - 1992
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.1751-1097.1992.tb02199.x
Subject(s) - chromophore , phytochrome , chemistry , molecule , crystallography , dipole , transition dipole moment , photochemistry , stereochemistry , red light , botany , organic chemistry , biology
Abstract— A model for the molecular structure and orientation of red‐light absorbing form of phytochrome (P,) chromophores in a dimeric molecular model of P r is proposed. A chromophore model with probable molecular structures was generated to reproduce the absorption spectrum produced by its π‐electron conjugating system. The model has C 5 ‐Z, syn , C 10 ‐E, anti and C 15 ‐Z, syn configurations and a protonation at a C‐ring nitrogen. Orientation of the chromophore model in the dimeric phytochrome molecular was analyzed by displaying the atoms of the chromophore, the coordinates of which were converted into those with respect to the molecular axes to the dimeric molecule, on a 3‐D graphic workstation. The conversions were performed by using the azimuthal angles between the Z axis of the dimeric molecule (axis of 2‐fold rotational symmetry) and the dipole moments of the electronic transition at the blue‐ (384 nm) and red‐ (667 nm) absorbing bands of the chromophore, which were calculated as 55.5° and 59.3°, respectively, based on linear dichroism of the oriented phytochrome molecules. The result demonstrates that the long axis of the P, chromophore lies almost parallel to the Y axis of the molecular model, and that the tetrapyrrolic chromophore is well contained within the flat chromophoric domain without protruding from it, a configuration that assures that the chromophore is protected against aqueous environments. The model may explain the rotation angle of the transition moment of the red‐absorbing band, induced by the phototransformation from P r to P rr which we measured as smaller than that measured in nonoriented preparations by a photoselection technique. The model also suggests a molecular basis for the polarotropic response of phytochrome.