Spectroscopic Studies of Cryptophyte Light Harvesting Proteins: Vibrations and Coherent Oscillations

The first step of photosynthesis is the absorption of light by antenna complexes. Recent studies of light-harvesting complexes using two-dimensional electronic spectroscopy have revealed interesting coherent oscillations. Some contributions to those coherences are assigned to electronic coherence and therefore have implications for theories of energy transfer. To assign these femtosecond data and to gain insight into the interplay among electronic and vibrational resonances, we need detailed information on vibrations and coherences in the excited electronic state compared to the ground electronic state. Here, we used broad-band transient absorption and femtosecond stimulated Raman spectroscopies to record ground- and excited-state coherences in four related photosynthetic proteins: PC577 from Hemiselmis pacifica CCMP706, PC612 from Hemiselmis virescens CCAC 1635 B, PC630 from Chroomonas CCAC 1627 B (marine), and PC645 from Chroomonas mesostigmatica CCMP269. Two of those proteins (PC630 and PC645) have strong electronic coupling while the other two proteins (PC577 and PC612) have weak electronic coupling between the chromophores. We report vibrational spectra for the ground and excited electronic states of these complexes as well as an analysis of coherent oscillations observed in the broad-band transient absorption data.