A thermal broadening study of the antenna chlorophylls in PSI-200, LHCI, and PSI core

The intact photosystem I of maize containing its full antenna complement (PSI-200) has been purified and fractionated into the core and outer antenna (LHCI) components. It is demonstrated by absorption and fluorescence spectroscopy that at least 80% of the long wavelength absorbing antenna pigments (red forms) are located in LHCI. Absorption spectra in the Q(y) region of all three preparations were measured between 72 and 300 K and subjected to a thermal broadening analysis. Data are interpreted in the linear electron-phonon coupling assumption, and the average optical reorganization energy (Sv(m)) for the bulk pigment band and the red absorption tail determined. A marked asymmetry in Sv(m) values across the absorption band is demonstrated. The bulk pigments in all three preparations have rather low values, in the range of 15-25 cm(-1), suggesting that Stokes shifts for the absorption forms are in the 1.5-3 nm range. On the other hand the red forms have markedly greater reorganization energies. While a direct thermal analysis of the red tail indicates minimum Sv(m) values of around 60 cm(-1), when the contribution of the red tail of the bulk pigments is corrected for in LHCI, the more reliable value of 110 cm(-1) is obtained. These high Sv(m) values for the red pigment forms suggest that they have unusually wide homogeneously broadened absorption bands and large Stokes shifts (6-11 nm).