Mutual orientation of absorbing chromophores and long wavelength pigments in photosystem I particles

The fluorescence anisotropy of photosystem I particles, isolated from spinach chloroplasts and containing approximately 200 chlorophyll molecules per reaction center, has been investigated at low temperatures. Fluorescence anisotropy has been measured upon excitation with laser lines at 476.5 and 632.8 nm. Using our data for the fluorescence anisotropy at these conditions and the new 'practical' formula for the degree of polarization of a triple-chromophore complex under steady-state excitation, derived recently by Demidov, we estimate the mutual orientation of absorbing chromophores and long wavelength pigments-chlorophyll a molecules, that absorb at wavelengths longer than the corresponding reaction center, in Photosystem I particles. The angle between the transition dipole moments of chlorophyll a, belonging to the light-harvesting complex of PS I and absorbing the excitation at 632.8 nm, and the emitting long wavelength pigment at 735 nm is estimated to be 40 degrees, whereas the angle between the transition dipole moments of chlorophyll b, belonging to the light-harvesting complex of PS I and absorbing the excitation at 476.5 nm, and the emitting long wavelength pigment at 735 nm-60 degrees. (C) 1998 Elsevier Science B.V. All rights reserved.