Polarized absorption and fluorescence spectra of single crystals of C-phycocyanin

Abstract

Polarized optical spectra of thin transparent single crystals of C-phycocyanin from Mastigocladus laminosus have been recorded. The isotropic absorption spectra of single crystals differ only at the short-wavelength side of the absorption peak from the corresponding solution spectra. This is presumably due to additional coupling of the β155 chromophores in the crystal. The crystals show dichroism. The position of the absorption maximum shifts between 612 nm ( E parallel to the optical axis c) and 626 nm ( E perpendicular to c) by varying the polarization direction. The observed polarized absorption spectra are compared with theoretical spectra, calculated from the deconvoluted isotropic solution spectra of the individual chromophores (Mimuro, M., Füglistaller, P., Rümbeli, R. and Zuber, H. (1985) Biochim. Biophys. Acta 848, 155–166) and the orientations of the chromophores as determined by X-ray structure analysis (Schirmer, T., Bode, W. and Huber, R. (1987) J. Mol. Biol. in the press). Good agreement is found if the short- and long-wavelength component spectra of the β subunit (β s and β f, respectively) are assigned to chromophores β155 and β84, respectively. The fluorescence polarization spectra indicate that the fluorescence originates predominantly from chromophores α84 and β84, thus giving direct evidence that energy transfer is operative in the crystals and that β84 is the long-wavelength chromophore of the β subunit.