ORIENTATIONAL CHANGES OF THE ABSORBING DIPOLE OF RETINAL UPON THE CONVERSION OF RHODOPSIN TO BATHORHODOPSIN, LUMIRHODOPSIN, AND ISORHODOPSIN

  • 1 January 1982
    • journal article
    • research article
    • Vol. 37  (3) , 603-616
Abstract
The orientational change of the absorbing dipole of the retinal chromophore in vertebrate rhodopsin (rhodo) upon photo-excitation to bathorhodopsin (batho), lumirhodopsin (lumi) and isorhodopsin (iso), has been studied by polarized absorption and linear dichroism measurements on magnetically oriented frog [Rana esculenta] rod suspensions that were blocked at liquid N2 temperature. Both the azimuthal component .DELTA..theta. and the polar component .DELTA..vphi. of the total angular change were studied in separate experiments. .DELTA..theta. was estimated from polarized absorption measurements on rods oriented transversally with respect to the analyzing beam. The data show unequivocally that upon the rhodo .fwdarw. batho transition, the dipole shifts out of the membrane plane by only a few degrees: .DELTA..theta. .simeq. -3.degree.. This azimuthal shift was nearly exactly reversed upon the batho .fwdarw. lumi decay. A very small shift (.DELTA..theta. .ltoreq. 1.degree.) toward the membrane plane was observed upon a rhodo .fwdarw. iso conversion. The polar component .DELTA..vphi. of the angular shift was estimated by studying the photoreversion of linear dichroism induced by photo-excitation with polarized light in rods oriented parallel to the analyzing beam. Upon the rhodo .fwdarw. batho transition, there was a shift .DELTA..vphi. = 11 .+-. 3.degree.. The overall angular shift upon this 1st photo-exciting step, which corresponded to the isomerization of retinal, was only .DELTA..OMEGA. = 11 .+-. 3.degree.. This is smaller than what may be expected for a cis-trans isomerization of a retinal molecule with 1 end fixed, and differs from other reports. These discrepancies are discussed.