Applications of Energy Transfer

• Non-Covalent Bioconjugation of Light-Harvesting Proteins

Natural photosynthetic light-harvesting proteins incorporate a collection of light absorbing pigments within a protein scaffolding, numbering anywhere from six to dozens of pigment molecules within one protein complex. The individual pigments are oriented and bound by their local protein environment in such a way that the aggregate of pigments as a whole functions as a highly-efficient light-capturing antenna. 

Importantly, simple light absorption is insufficient to stimulate the chemical reactions that power photosynthesis. The energy extracted from the photon is transiently stored in electronic excitation within a single or few pigments, but it needs to be shuttled toward the reaction center complex where it triggers electron transfer and kick starts the chemical reactions of photosynthesis. To accomplish that task requires sometimes dozens of energy transfer events between pigments, each of which has to beat the "clock" of natural de-excitation which takes place in ~1 ns. The light-harvesting proteins utilized in photosynthetic organisms have a common trait in that they are capable of funneling that photon energy through their own pigment network rapidly for efficient transport toward the reaction center. This is accomplished by careful design of the distances and orientations of individual pigments within the protein, which "wires" them together through an electronic coupling network. 

Our research aims to test the robustness of that electronic coupling network, and also the effects of non-covalent binding on the energy transfer mechanism. Specifically, aggregation of a synthetic dye which shows complementary spectral properties to the natural protein, provides a way to examine whether a separate energy transfer "donor" can non-selectively be incorporated into the network to improve the spectral capture of the whole aggregate. Alternatively, uncontrolled placement of the additional chromophore can also cause energy quenching, inhibiting the energy transfer efficiency of the antenna protein. We are investigating these effects in a variety of light-harvesting proteins to develop a broader picture of pigment-protein structure and its sensitivity to perturbations by other visible chromophores. These carefully crafted pigment aggregates have been improved upon for millions of years through the process of evolution, which makes them exemplar models for design strategies toward artificial organic light-harvesting materials, which is the basis of our research in this area.