POLARIZED SITE-SELECTED FLUORESCENCE SPECTROSCOPY OF ISOLATED PHOTOSYSTEM-I PARTICLES

Polarized steady-state fluorescence spectra have been obtained from Photosystem I core complexes of the cyanobacterium Synechocystis PCC 6803 and from LHC-I containing Photosystem I (PS I-200) complexes of spinach by selective laser excitation, at 4 K. Excitation above 702 nm in Synechocystis and 720 nm in PS I-200 results in highly polarized emission, suggesting that pigments absorbing at these and longer wavelengths are not able to transfer excitation energy at 4 K. In both systems the peak wavelength of the emission (lambda(em)) depends strongly on the excitation wavelength (lambda(em)). This indicates that in both systems the long-wavelength bands responsible for the steady-state emission are inhomogeneously broadened. The width of the inhomogeneous distribution is estimated to be about 215 cm(-1) in Synechocystis and 400 cm(-1) in PS I-200. We conclude that the peaks of the total absorption spectra of the long-wavelength pigments of Synechocystis and PS I-200 are at 708 and 716 nm, respectively, and therefore designate these pigments as C-708 and C-716. The results further show that C-708 and C-716 are strongly homogeneously broadened, i.e. carry broad phonon side-bands. The width of these bands is estimated to be about 170 and 200 cm(-1) for C-708 and C-716, respectively. The Stokes' shifts appear to be large: about 200 cm(-1) (10 nm) for C-708 and about 325 cm(-1) (17 nm) for C-716. These values are much higher than usually observed for 'normal' antenna pigments, but are in the same order as found previously for a number of dimeric systems. Therefore, we propose that the long-wavelength pigments in Photosystem I are excitonically coupled dimers. Based on fitting with Gaussian bands the presence of one C-708 dimer per P700 is suggested in the core antenna of Synechocystis.