How does the structure of photosynthetic pigments contribute to their function?

Photosynthetic proteins lock pigments into certain geometries, contributing to their high quantum efficiency by tuning the pigment energies to allow maximum energy flow between them. We are seeking to identify the geometries of carotenoids and chlorophylls in major light harvesting complexes in order to determine the design principles behind the protein’s structure.

Carotenoids are photosynthetic pigments involved in light harvesting, photoprotection, and structural support within many light harvesting complexes. As they are composed of long polyene chains, their symmetry has traditionally prevented thorough investigation of their dark lowest-energy excited states. The specificity of each structural peak in infrared spectra allows us to identify the roles of each of these states in light harvesting and photoprotection.

The lowest energy states of many carotenoids, especially those containing carbonyl functional groups, display different behavior in polar and nonpolar solvents. This indicates that there are two states, with and without charge transfer character, in this energy region. The design principles evinced by these states remains unclear within protein environments. Our work seeks to isolate the contributions of each state.

Currently we are studying pigment-protein complexes containing carotenoids known to have intramolecular charge transfer (ICT) states (1,2,3). The role of these states in photosynthetic complexes is unclear. They may allow for stronger variance in the electronic energies of carotenoids, so that energy transfer may be fine-tuned across multiple pigments. The fine- tuning of pigment energies has been observed in major light harvesting complexes in plants (4). There is also evidence that intermolecular charge transfer is a photoprotective mechanism in plants (5). Such a mechanism may be more efficient if a participating carotenoid has an ICT state.

Determining the effects of the ICT and S1 states in carotenoids is complicated by the symmetry of the carotenoid backbone. Our apparatus allows us to directly access these “dark” states by two-photon absorption. Then they can be characterized individually using a mid- IR probe. Since many carotenoids with ICT states contain carbonyl groups in conjugation with their backbone polyene structure, they are ideal for studying with visible pump, infrared probe spectroscopy. The strong absorptions of carbonyl groups in the infrared, as well as their strong sensitivity to local environment, allow separation of multiple pigments within the same complex. The narrow spectral width of these peaks further isolates each pigment. Since the carbonyl group is in conjugation with the polyene backbone of the carotenoids we study, we can predict the changes in energy levels induced by structural conditions imposed by proteins. Moreover, the kinetics determined for each peak allow us to determine the direction and rate of energy flow within the protein complexes, so that we can distinguish between light harvesting and photoprotective processes. This process is helped by the presence of carbonyl groups on chlorophylls, which can also be identified through visible visible and visible-infrared pump-probe spectroscopy.