title: Photodichroic studies of the photoreaction center from Rhodospirillum rubrum. I. Attribution of P870 to two non parallel dipoles.
abstract: A randomly oriented sample of photoreaction center prepared from Rhodospirillum rubrum was excited at 77 degrees K by an actinic linearly polarized light of 870 nm. Under such conditions, only those chromophores with components of their absorption dipoles oriented parallel to the polarization of the actinic light are bleached. The change in absorbance at 900 nm of this photoselected sample was observed while varying the angle of polarization of a weak measuring light. The polarization of the absorbance change was thus evaluated as 0.25. This value is interpreted to mean that P870 is attributable to two absorption dipoles forming an angle included between 35.75 degrees and 90 degrees. Comparison with the p value of 0.5 obtained on a similar preparation by polarization of fluorescence (Ebrey, T. G. and Clayton, R.K. (1969) Photochem. Photobiol. 10, 109-117) leads to the conclusion that either these two dipoles emit fluorescence without being coupled by singlet-singlet energy transfer or that only one of them is a fluorescence emitter in the absence of reversible singlet-singlet energy transfer.
mesh_terms: D003580:Cytochromes / Q000378:metabolism*; D008027:Light; D008433:Mathematics; D011487:Protein Conformation; D012247:Rhodospirillum rubrum / Q000378:metabolism* / Q000648:ultrastructure; D013055:Spectrophotometry, Infrared; D013057:Spectrum Analysis
chemical_list: D003580:Cytochromes