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Quantum coherence in photosynthesis: towards single-molecule light-conversion devices

Periodic Reporting for period 1 - QuantumPhotosynth (Quantum coherence in photosynthesis: towards single-molecule light-conversion devices)

Reporting period: 2015-10-01 to 2017-09-30

Photosynthesis (PS) performed by plants, algae, and photosynthetic bacteria is the biological process that transforms solar energy into chemical energy with high quantum efficiency. A complete understanding of the factors that modulate the efficiency of this process constitutes a challenge centered at the nexus of fundamental physical chemistry and biology, and obtaining that knowledge could pave the way for the design of the next generation of cheap and highly-efficient light-conversion devices. Humanity progressively realizes that new energy sources are needed to guarantee a decent survival for our civilization and future generations to come. On the other hand, the energy from sun irradiation hitting our planet every hour, almost equals the global demand energy consumption per year. The complete understanding of the mechanisms behind photosynthesis could help to develop solutions for exploiting solar energy in a sustainable society.

The natural PS machinery consists of a series of membrane-associated pigment-protein complexes organized in photosystems that harvest energy and transfer it towards a reaction center, which converts the energy into a transmembrane charge separation. The Light-Harvesting Complex II (LHCII) is the most abundant protein in photosynthetic membranes, it organizes in trimers, and serves two functions; harvests excitation in moderate light, and in high-light conditions, LHCII can switch to a dissipative state and regulate the amount of excitation transferred to the rest of the system. Recent studies provided a detailed picture of the structural organization of this complex in higher plants. The LHCII structure contains several pigments; fourteen chlorophylls (Chl) and four carotenoids (Car) per monomer. Chls absorb light in the red and provide efficient transfer pathways towards the reaction centers, through delocalized excitonic clusters. Cars play fundamental roles to maintain the structural integrity of the protein, they absorb light in the blue-green (complementing Chls), going into the second excited state (S2, S1 is symmetry forbidden), and decaying to the S1. These excited states can transfer to Chls and play other roles since they also quench triplet states and excess of excitation.

Vibrational spectroscopies have allowed the detailed study of some of the molecular vibrations of the different pigments in PS. In particular, Raman spectroscopy has permitted establishing fingerprints for the Cars in PS samples. Also, energy transfer dynamics in PS have been extensively studied using a variety of techniques, including transient absorption spectroscopy (TAS). In combination with analysis schemes, TAS has allowed mapping the excited states of PS to a great extent. Unfortunately, some states (that may play roles in regulatory mechanisms) could be “dark” states, being not accessible to TAS, and vibrational spectroscopies could help fill this gap. A combination of these approaches (TAS and Raman), femtosecond stimulated Raman (fs-Raman) was developed. Fs-Raman is challenging since it requires separating the Raman signal from a transient absorption envelope with a good signal-to-noise ratio (SNR). To overcome these limitations and enable the study of complex samples with high resolution, in our lab we have developed a variation of the technique based on spectral watermarking that allows recovering the fs-Raman response with a high SNR. Briefly, the experiment is done using a shaped broadband pulse instead of a narrowband Raman pulse, allowing locking a watermark to the Raman signal.

Also, in PS there are other questions concerning how the behavior of individual molecules affects the performance of the ensemble. In that sense, there is a clear need for new techniques able to provide the electronic properties at the nanoscale, accessing the single-molecule behavior of proteins and other biomolecules essential for the PS and other crucial biological processes.

The primary aim of this project is to understand how molecular vibrations influence and modulate the efficiency of PS. Here we used femtosecond-stimulated Raman spectroscopy (fs-Raman) and other spectroscopies to demonstrate the specific vibrational modes that participate in the PS process and modulate its efficiency. Also, we developed a new Scanning Probe Microscopy (SPM) technique capable of providing a complete electronic characterization at the nanoscale with potential applications in PS, other biological processes, and nanoscale devices.
We have applied fs-Raman to the study of LHCII trimers in buffer solution and obtained time-resolved spectra of the dynamics of spinach LHCII trimers in buffer solution by exciting the sample in the carotenoid region. The results show the dynamics of the vibrational modes of carotenoids over time and, using global and target analysis we obtain spectral signatures for the different pigments in LHCII, providing details of the energy transfer pathways. These results are part of a manuscript in preparation that will be published soon in an international peer-reviewed journal.

Also, we have developed a new SPM mode able to provide a complete electronic characterization of biomolecules and surfaces with unprecedented resolution. By adding an AC modulation to the potential applied between the sample and probe electrodes in the Electrochemical Scanning Tunneling Microscope, we can deconvolute an electronic conductance signal from the topography signal, obtaining a new image simultaneously. We demonstrate the application of the technique on a metal/semiconductor system and a redox protein, obtaining sub-molecular resolution. These results have been recently published in a paper.

Also, the results from the different parts of the project have been communicated in multiple international conferences, workshops, and meetings.
These results constitute a step forward and beyond the state-of-the-art in PS research, as they open the doors for studies addressing the natural design behind photosynthesis and its regulatory mechanisms.
Beyond the scientific impact, a potential broader impact of these results is that they could pave the way for the design of the next-generation of highly-efficient light-harvesting devices that may help to solve the problem of the global energy demand in the context of a world affected by climate change, benefiting society at large.