Thermal behavior of long wavelength absorption transitions in Spirulina platensis photosystem I trimers

In photosystem I trimers of Spirulina platensis a major long wavelength tra nsition is irreversibly bleached by illumination with high-intensity white light. The photobleaching hole, identified by both absorption and circular dichroism spectroscopies, is interpreted as the inhomogeneously broadened Q (y) transition of a chlorophyll form that absorbs maximally near 709 nm at room temperature. Analysis of the mean square deviation of the photobleachi ng hole between 80 and 300 K, in the linear electron-phonon frame, indicate s that the optical reorganization energy is 52 cm(-1), four times greater t han that for the bulk, short-wavelength-absorbing chlorophylls, and the inh omogenous site distribution bandwidth is close to 150 cm(-1). The room temp erature bandwidth, close to 18.5 nm, is dominated by thermal (homogeneous) broadening. Photobleaching induces correlated circular dichroism changes, o f opposite sign, at 709 and 670 nm, which suggests that the long wavelength transition may be a low energy excitonic band, in agreement with its high reorganization energy. Clear identification of the 709-nm spectral form was used in developing a Gaussian description of the long wavelength absorptio n tail by analyzing the changing band shape during photobleaching using a g lobal decomposition procedure. Additional absorption states near 720, 733, and 743 nm were thus identified. The lowest energy state at 743 nm is prese nt in substoichiometric levels at room temperature and its presence was con firmed by fluorescence spectroscopy. This state displays an unusual increas e in intensity upon lowering the temperature, which is successfully describ ed by assuming the presence of low-lying, thermally populated states.