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 mole cules per reaction center, has been investigated at low temperatures. Fluorescence anisotropy has been measured upon excitation with laser l ines at 476.5 and 632.8 nm. Using our data for the fluorescence anisot ropy at these conditions and the new 'practical' formula for the degre e of polarization of a triple-chromophore complex under steady-state e xcitation, derived recently by Demidov, we estimate the mutual orienta tion of absorbing chromophores and long wavelength pigments-chlorophyl l a molecules, that absorb at wavelengths longer than the correspondin g reaction center, in Photosystem I particles. The angle between the t ransition dipole moments of chlorophyll a, belonging to the light-harv esting complex of PS I and absorbing the excitation at 632.8 nm, and t he emitting long wavelength pigment at 735 nm is estimated to be 40 de grees, whereas the angle between the transition dipole moments of chlo rophyll b, belonging to the light-harvesting complex of PS I and absor bing the excitation at 476.5 nm, and the emitting long wavelength pigm ent at 735 nm-60 degrees. (C) 1998 Elsevier Science B.V. All rights re served.