Ultrafast energy transfer dynamics resolved in isolated spinach light-harvesting complex I and the LHC I-730 subpopulation

Abstract

The energy migration in isolated bulk LHC I and its subpopulations LHC I-680 and LHC I-730 have been studied by fluorescence spectroscopy and femtosecond absorption spectroscopy. LHC I-680 and LHC I-730 display significant differences in both their steady-state and time-resolved fluorescence properties. The steady-state emission maximum is located at 680 nm for LHC I-680 and at 730 nm for LHC I-730. It was also found that different kinetic models were required to fit the time-resolved fluorescence data of the two subpopulations, two components of 374 ps and 3.9 ns were required for LHC I-680 and three components of 157 ps, 510 ps and 2.8 ns for LHC I-730, respectively. Using femtosecond absorption recovery spectroscopy we were able to resolve a fast 200–400 fs depolarization in bulk LHC I at 655, 665 and 670 nm. The results indicate a fastest hopping time between individual chlorophylls of 200–400 fs in bulk LHC I. In the subpopulation LHC I-730, on the other hand, both the isotropic absorption recovery decay and the depolarization process occurred with a 15 ps lifetime, while in the subpopulation LHC I-680 we could not resolve any ultrafast relaxation process. The 15 ps phase is assigned to a transfer of excitation energy to the pigment giving rise to far-red emission component F735 of PS I.