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

Polarized steady-state fluorescence spectra have been obtained from Ph otosystem I core complexes of the cyanobacterium Synechocystis PCC 680 3 and from LHC-I containing Photosystem I (PS I-200) complexes of spin ach by selective laser excitation, at 4 K. Excitation above 702 nm in Synechocystis and 720 nm in PS I-200 results in highly polarized emiss ion, suggesting that pigments absorbing at these and longer wavelength s are not able to transfer excitation energy at 4 K. In both systems t he peak wavelength of the emission (lambda(em)) depends strongly on th e excitation wavelength (lambda(em)). This indicates that in both syst ems the long-wavelength bands responsible for the steady-state emissio n are inhomogeneously broadened. The width of the inhomogeneous distri bution 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 s pectra of the long-wavelength pigments of Synechocystis and PS I-200 a re at 708 and 716 nm, respectively, and therefore designate these pigm ents 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-ba nds. 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 'n ormal' 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.