For Aim I, after successfully constructing several chromophore-protein assemblies via a novel protocol we have investigated their thermodynamic and electronic properties by a collection of steady-state spectroscopic techniques: Absorption, Fluorescence, Circular Dichroism, Resonance Raman, and Stark; by which we have reached a high level of understanding about our systems. Among other, the main results for our four-helix protein bundles are: i) several protein designs can accommodate four chromophores, ii) these chromophores interact forming two excitonically coupled dimers, iii) the thermal stability of the proteins increase upon chromophore binding, iv) upon protein denaturalization the chromophore remains bound to the protein and upon refolding the assembly folds back to its initial conformation, v) the lowest exciton state has charge-transfer character. These results demonstrate the robustness of our platform and our capacity to implement the design principles of photosynthesis (i and iii) in our quest to design and construct bio-inspired chromophore-protein assemblies for their future implementation into devices for solar-to-electricity and solar-to-fuel conversion. To improve the charge-transfer character, and thus the charge-separation capacity of our systems, we have designed new protein sequences with charged amino acids and with extra helices (designs currently under study). To investigate the photoinduced energy and electron transfer dynamics and the presence of coherence in our systems we have performed ultrafast Broad-Band Transient Absorption (BBTAS) and Two-Dimensional Electronic Spectroscopy (2DES) experiments. The results show that in the excitonically coupled dimers a short-lived (150 picoseconds) charge-transfer state is formed, which possibly yields a charge-separated state. Even though this possibility requires further investigation, it is worth noting that this observation indicates that we have managed to produce a photoinduced charge separation, which is the main objective of Aim I. This work has been presented in international conferences, and we are working on five manuscripts to be submitted for publication in the following months.
In parallel, we have performed a combined experimental/theoretical work to optimize the chromophore binding capacity of a protein via molecular dynamics simulations, which opens new avenues for the rational design of chromophore–protein complexes with advanced functionalities (higher chromophore loading, enhanced coupling, strong charge-separation efficiency). This work was presented in conferences and was published in Protein Science in 2023).
Aim II which focuses on the implementation of our assemblies in devices for solar-energy conversion, has not been started yet since it requires Aim I to be completed. However, in light of the new insights obtained by ultrafast spectroscopy, we consider that we are ready to start building such devices.
Aim III intends to advance the understanding of the role of coherence in enhancing function in photosynthesis. We have performed BBTAS and 2DES on light-harvesting and in core complexes (containing both light-harvesting and reaction centers). In isolated light-harvesting complexes, we have demonstrated coherent energy transfer, and we have studied the role of coherence in the photoprotection mechanism active in plants. In the more intact core complexes, we have investigated the understudied role of coherence in the energy transfer process from complex to complex, and the coherent effects in electron transfer in their reaction centers. Some of these outcomes have been presented in conferences, and four manuscripts are currently being prepared based on these results.
