Femtosecond absorption spectroscopy was applied to study for the first time excitation dynamics in isolated photosystem I trimers from Arthrospira platensis, which display extremely long-wavelength absorption peaks. Pump-probe spectra observed at 77 K in the timescale of dozens of picoseconds upon 70-fs excitation revealed two maxima near 710 and 730 nm, which correspond to red chlorophyll forms. Bleaching at 680 nm developed in ∼200 fs, whereas the bleaching kinetics at 710 and 730 nm exhibited two components with time constants of 1 and 5.5 ps. Comparison of the kinetics of bleaching development at 710 nm and 730 nm with that of bleaching decay at 680 nm indicated that both long-wavelength forms of trimers are populated mainly via direct energy transfer from bulk chlorophyll.
Kompanets, V., Shubin, V., Terekhova, I., Kotova, E., Kozlovsky, V., Novoderezhkin, V., … Razjivin, A. (2014). Red chlorophyll excitation dynamics in Arthrospira platensis photosystem i trimeric complexes as studied by femtosecond transient absorption spectroscopy. FEBS Letters, 588(18), 3441–3444. https://doi.org/10.1016/j.febslet.2014.07.038