Energy transfer pathways in the peridinin chlorophyll-a protein complex as revealed by the near-infrared time-resolved spectroscopy

Abstract

In vitro studies of the primary pigment peridinin from peridinin chlorophyll-a protein (PCP) light harvesting complex showed a strong dependence of the lifetime of peridinin lowest singlet excited state on solvent polarity as well as appearance of a stimulated emission (SE) band in the near-IR range of the transient absorption spectrum in the polar solvent methanol. Both the lifetime dependence and the SE band were attributed to the presence of a new state with charge transfer (CT) character. The aim of this study was to investigate dynamics within the PCP using near-IR femtosecond pump-probe spectroscopy. Kinetics obtained in our measurements reveal that the lowest singlet state of peridinin in the PCP complex has a lifetime of 2.1 ¿ 2.5 ps. Here, we used the transient absorption in near-IR region, which is almost free from contributions from other pigments and represents absorption signal from the lowest singlet excited state to the S2 state of peridinin. Measurements of the near-IR transient absorption spectra showed that the SE band observed earlier for peridinin in methanol is also clearly present for peridinin in PCP. Kinetics measured across the SE band exhibit the same dynamics as kinetics measured at spectral regions corresponding to absorption from the lowest singlet excited state of the peridinin as well as to the bleaching of the chlorophyll-a band. These results suggest that the CT state is directly involved in the energy transfer process within PCP light harvesting complex.