Excitation energy transfer in the Lhca1 subunit of LHC I-730 peripheral antenna of photosystem I

Femtosecond transient absorption spectroscopy at 77 K was used to study excitation energy transfer processes in the reconstituted (r-)Lhca1 subunit of the peripheral light-harvesting antenna complex LHCI-730 of Photosystem I from higher plants. Under selective excitation of chlorophyll (Chl) b at 645 nm with 200 fs laser pulses, two energy transfer processes from Chl b to spectral forms of Chl a with lifetimes of 500 fs and 2.8 ps are induced. The subpicosecond phase reflects the energy transfer between closely located Chl b and Chl a molecules. Excitation at 670 nm induces a similar ultrafast process of energy transfer between neighboring Chl a molecules with a lifetime of 300 fs. The kinetic processes with the lifetime of 3-4 ps indicate an excitation equilibration among pools of Chl b and Chl a in the r-Lhca1, which involves the longest-wavelength spectral form of Chl a at 684 nm. This is in contrast to the Lhca4 subunit of the LHCI-730 heterodimer, which binds the longest-wavelength (red) pigments absorbing at 700-705 nm and responsible for the red fluorescence at 735 nm in LHCI. The spectral pattern of transient absorption changes in the r-Lhca1 is similar to that of the minor light-harvesting complex CP29 of PSII with which the Lhca1 shares significant similarity in protein sequences. A tentative assignment of the observed Chl spectral forms to the pigments in the Lhca1 subunit is presented based on the derivative absorption spectroscopy, transient absorption spectra, and the suggestion that the pigment-protein interactions dominate the spectral heterogeneity of the Q_y transitions of Chl a and b.