Tuning Energy Transfer in the Peridinin–chlorophyll Complex by Reconstitution with Different Chlorophylls

Abstract

In vitro studies of the carotenoid peridinin, which is the primary pigment from the peridinin chlorophyll-a protein (PCP) light harvesting complex, showed a strong dependence on the lifetime of the peridinin lowest singlet excited state on solvent polarity. This dependence was attributed to the presence of an intramolecular charge transfer (ICT) state in the peridinin excited state manifold. The ICT state was also suggested to be a crucial factor in efficient peridinin to Chl-a energy transfer in the PCP complex. Here we extend our studies of peridinin dynamics to reconstituted PCP complexes, in which Chl-a was replaced by different chlorophyll species (Chl-b, acetyl Chl-a, Chl-d and BChl-a). Reconstitution of PCP with different Chl species maintains the energy transfer pathways within the complex, but the efficiency depends on the chlorophyll species. In the native PCP complex, the peridinin S1/ICT state has a lifetime of 2.7 ps, whereas in reconstituted PCP complexes it is 5.9 ps (Chl-b) 2.9 ps (Chl-a), 2.2 ps (acetyl Chl-a), 1.9 ps (Chl-d), and 0.45 ps (BChl-a). Calculation of energy transfer rates using the Förster equation explains the differences in energy transfer efficiency in terms of changing spectral overlap between the peridinin emission and the absorption spectrum of the acceptor. It is proposed that the lowest excited state of peridinin is a strongly coupled S1/ICT state, which is the energy donor for the major energy transfer channel. The significant ICT character of the S1/ICT state in PCP enhances the transition dipole moment of the S1/ICT state, facilitating energy transfer to chlorophyll via the Förster mechanism. In addition to energy transfer via the S1/ICT, there is also energy transfer via the S2 and hot S1/ICT states to chlorophyll in all reconstituted PCP complexes.

Identification of a single peridinin sensing Chl- a excitation in reconstituted PCP by crystallography and spectroscopy

“…The electrostatic effect of the N89L mutation is predicted to shift the two lowest lying energy levels of Per-614 to higher energies (see Origin of the absorption shift in the N89L 10-K absorption spectrum in the SI Text and Table S4). Consistent with recent studies on PCP (16,24,25), we also observe a small bleaching signal (a downward dip in the line shape) in TA measurements on both RFPCP and the N89L mutant at delay times after Per has relaxed back to the ground state, but while Chl-a is still excited (see Fig. S4 and Fig.…”

“…This type of fitting gives spectral profiles, termed ''evolution associated difference spectra'' (EADS). The fitting results are consistent with recently published data reported for PCP (24,25) and are discussed in detail in the SI Text (Fig. S5).…”

“…It is important to ask what role Per-614 plays in energy transfer to Chl-a. Several studies, including the present work, have suggested that Per-614 has the strongest interaction with Chl-a (19,21,(24)(25)(26)(27)(28). In the N89L mutant, the large spectral shift of the Per-614 S 0 3S 2 transition (see Fig.…”

“…S3). This apparent discrepancy can be rationalized if the major energy transfer route is not via the Per S 2 3Chl-a Q x pathway, but by the Per S 1 /ICT3Chl-a Q y route (21,24,36). Although the shape of the broad TA transition associated with the S 1 /ICT state changes to some extent, the lifetime stays roughly the same (see Fig.…”

“…1), whose environment has been postulated to result in a dramatic blue shift of its absorption spectrum, leading to less efficient direct energy transfer to Chl-a, compared to that from the other bound Pers (19)(20)(21)(22)(23). Another prominent example is Per-614, which has been assigned the strongest electronic interaction with Chl-a (19,21,(24)(25)(26)(27)(28). This enhanced interaction was identified from transient absorption measurements of the onset of bleaching of a Per absorption band Author contributions: E.H. and H.A.F.…”