Discovery of Multiple Light-Harvesting States of the Photosynthetic Protein PE545.

Cryptophytes are photosynthetic microalga that flourish in a remarkable diversity of natural environments by using pigment-containing proteins with absorption maxima tuned to each ecological niche. While this diversity in the absorption has been well established, the subsequent photophysics is highly sensitive to the local protein environment and so may exhibit similar variation. Thermal fluctuations of the protein conformation are expected to introduce photophysical heterogeneity of the pigments that may have evolved important functional properties in a manner similar to that of the absorption.

Sculpting photoproducts with DNA origami.

Natural light-harvesting systems spatially organize densely packed dyes in different configurations to either transport excitons or convert them into charge photoproducts, with high efficiency. In contrast, artificial photosystems like organic solar cells and light-emitting diodes lack this fine structural control, limiting their efficiency. Thus, biomimetic multi-dye systems are needed to organize dyes with the sub-nanometer spatial control required to sculpt resulting photoproducts.

Analysis of Poly(ethylene terephthalate) degradation kinetics of evolved IsPETase variants using a surface crowding model.

Poly(ethylene terephthalate) (PET) is a major plastic polymer utilized in the single-use and textile industries. The discovery of PET-degrading enzymes (PETases) has led to an increased interest in the biological recycling of PET in addition to mechanical recycling. IsPETase from Ideonella sakaiensis is a candidate catalyst, but little is understood about its structure-function relationships with regards to PET degradation.

Energetic driving force for LHCII clustering in plant membranes.

Plants capture and convert solar energy in a complex network of membrane proteins. Under high light, the luminal pH drops and induces a reorganization of the protein network, particularly clustering of the major light-harvesting complex (LHCII). While the structures of the network have been resolved in exquisite detail, the thermodynamics that control the assembly and reorganization had not been determined, largely because the interaction energies of membrane proteins have been inaccessible.

Observation of parallel intersystem crossing and charge transfer-state dynamics in [Fe(bpy)] from ultrafast 2D electronic spectroscopy.

Transition metal-based charge-transfer complexes represent a broad class of inorganic compounds with diverse photochemical applications. Charge-transfer complexes based on earth-abundant elements have been of increasing interest, particularly the canonical [Fe(bpy)]. Photoexcitation into the singlet metal-ligand charge transfer (MLCT) state is followed by relaxation first to the ligand-field manifold and then to the ground state.

Energetic robustness to large scale structural fluctuations in a photosynthetic supercomplex.

Photosynthetic organisms transport and convert solar energy with near-unity quantum efficiency using large protein supercomplexes held in flexible membranes. The individual proteins position chlorophylls to tight tolerances considered critical for fast and efficient energy transfer. The variability in protein organization within the supercomplexes, and how efficiency is maintained despite variability, had been unresolved.

Elucidating interprotein energy transfer dynamics within the antenna network from purple bacteria.

In photosynthesis, absorbed light energy transfers through a network of antenna proteins with near-unity quantum efficiency to reach the reaction center, which initiates the downstream biochemical reactions. While the energy transfer dynamics within individual antenna proteins have been extensively studied over the past decades, the dynamics between the proteins are poorly understood due to the heterogeneous organization of the network. Previously reported timescales averaged over such heterogeneity, obscuring individual interprotein energy transfer steps.

Engineering Exciton Dynamics with Synthetic DNA Scaffolds.

Excitons are the molecular-scale currency of electronic energy. Control over excitons enables energy to be directed and harnessed for light harvesting, electronics, and sensing. Excitonic circuits achieve such control by arranging electronically active molecules to prescribe desired spatiotemporal dynamics.

Single-photon absorption and emission from a natural photosynthetic complex.

Photosynthesis is generally assumed to be initiated by a single photon from the Sun, which, as a weak light source, delivers at most a few tens of photons per nanometre squared per second within a chlorophyll absorption band. Yet much experimental and theoretical work over the past 40 years has explored the events during photosynthesis subsequent to absorption of light from intense, ultrashort laser pulses. Here, we use single photons to excite under ambient conditions the light-harvesting 2 (LH2) complex of the purple bacterium Rhodobacter sphaeroides, comprising B800 and B850 rings that contain 9 and 18 bacteriochlorophyll molecules, respectively.

Ultrafast Dynamics of Photosynthetic Light Harvesting: Strategies for Acclimation Across Organisms.

Photosynthetic light harvesting exhibits near-unity quantum efficiency. The high efficiency is achieved through a series of energy and charge transfer steps within a network of pigment-containing proteins. Remarkably, high efficiency is conserved across many organisms despite differences in the protein structures and organization that allow each organism to respond to its own biological niche and the stressors within.

Activating charge-transfer state formation in strongly-coupled dimers using DNA scaffolds.

Strongly-coupled multichromophoric assemblies orchestrate the absorption, transport, and conversion of photonic energy in natural and synthetic systems. Programming these functionalities involves the production of materials in which chromophore placement is precisely controlled. DNA nanomaterials have emerged as a programmable scaffold that introduces the control necessary to select desired excitonic properties.

Photoenzymatic Catalysis in a New Light: "Ene"-Reductase Conjugates Possessing High-Energy Reactivity with Tunable Low-Energy Excitation.

Non-natural light-driven enzymatic reactivity was recently developed to perform the highly stereoselective reactions required for pharmaceutical synthesis. However, photoenzymes require high-intensity light to function because of the poor absorption properties of their photoactive intermediates. Inspired by the modular architecture of photosynthesis, we designed a conjugate composed of a covalently linked photoenzyme and a light antenna to separate light capture from catalysis.

Nanoscale 3D spatial addressing and valence control of quantum dots using wireframe DNA origami.

Control over the copy number and nanoscale positioning of quantum dots (QDs) is critical to their application to functional nanomaterials design. However, the multiple non-specific binding sites intrinsic to the surface of QDs have prevented their fabrication into multi-QD assemblies with programmed spatial positions. To overcome this challenge, we developed a general synthetic framework to selectively attach spatially addressable QDs on 3D wireframe DNA origami scaffolds using interfacial control of the QD surface.

Observation of conformational dynamics in single light-harvesting proteins from cryptophyte algae.

Photosynthetic organisms use pigment-protein complexes to capture the sunlight that powers most life on earth. Within these complexes, the position of the embedded pigments is all optimized for light harvesting. At the same time, the protein scaffold undergoes thermal fluctuations that vary the structure, and, thus, photophysics, of the complexes.

Ligand-induced transmembrane conformational coupling in monomeric EGFR.

Single pass cell surface receptors regulate cellular processes by transmitting ligand-encoded signals across the plasma membrane via changes to their extracellular and intracellular conformations. This transmembrane signaling is generally initiated by ligand binding to the receptors in their monomeric form. While subsequent receptor-receptor interactions are established as key aspects of transmembrane signaling, the contribution of monomeric receptors has been challenging to isolate due to the complexity and ligand-dependence of these interactions.

Bioinspired Supercharging of Photoredox Catalysis for Applications in Energy and Chemical Manufacturing.

For more than a decade, photoredox catalysis has been demonstrating that when photoactive catalysts are irradiated with visible light, reactions occur under milder, cheaper, and environmentally friendlier conditions. Furthermore, this methodology allows for the activation of abundant chemicals into valuable products through novel mechanisms that are otherwise inaccessible. The photoredox approach, however, has been primarily used for pharmaceutical applications, where its implementation has been highly effective, but typically with a more rudimentary understanding of the mechanisms involved in these transformations.

Photoprotective conformational dynamics of photosynthetic light-harvesting proteins.

Under high light conditions, excess energy can damage the machinery of oxygenic photosynthesis. Plants have evolved a series of photoprotective processes, including conformational changes of the light-harvesting complexes that activate dissipation of energy as heat. In this mini-review, we will summarize our recent work developing and applying single-molecule methods to investigate the conformational states of the light-harvesting complexes.

Tuning Optical Absorption and Emission Using Strongly Coupled Dimers in Programmable DNA Scaffolds.

Molecular materials for light harvesting, computing, and fluorescence imaging require nanoscale integration of electronically active subunits. Variation in the optical absorption and emission properties of the subunits has primarily been achieved through modifications to the chemical structure, which is often synthetically challenging. Here, we introduce a facile method for varying optical absorption and emission properties by changing the geometry of a strongly coupled Cy3 dimer on a double-crossover (DX) DNA tile.

Observation of robust energy transfer in the photosynthetic protein allophycocyanin using single-molecule pump-probe spectroscopy.

Photosynthetic organisms convert sunlight to electricity with near unity quantum efficiency. Absorbed photoenergy transfers through a network of chromophores positioned within protein scaffolds, which fluctuate due to thermal motion. The resultant variation in the individual energy transfer steps has not yet been measured, and so how the efficiency is robust to this variation has not been determined.

Protein-Protein Interactions Induce pH-Dependent and Zeaxanthin-Independent Photoprotection in the Plant Light-Harvesting Complex, LHCII.

Plants use energy from the sun yet also require protection against the generation of deleterious photoproducts from excess energy. Photoprotection in green plants, known as nonphotochemical quenching (NPQ), involves thermal dissipation of energy and is activated by a series of interrelated factors: a pH drop in the lumen, accumulation of the carotenoid zeaxanthin (Zea), and formation of arrays of pigment-containing antenna complexes. However, understanding their individual contributions and their interactions has been challenging, particularly for the antenna arrays, which are difficult to manipulate in vitro.

Concerted Differential Changes of Helical Dynamics and Packing upon Ligand Occupancy in a Bacterial Chemoreceptor.

Transmembrane receptors are central components of the chemosensory systems by which motile bacteria detect and respond to chemical gradients. An attractant bound to the receptor periplasmic domain generates conformational signals that regulate a histidine kinase interacting with its cytoplasmic domain. Ligand-induced signaling through the periplasmic and transmembrane domains of the receptor involves a piston-like helical displacement, but the nature of this signaling through the >200 Å four-helix coiled coil of the cytoplasmic domain had not yet been identified.

Spectrally-tunable femtosecond single-molecule pump-probe spectroscopy.

Single-molecule spectroscopy has been extensively used to investigate heterogeneity in static and dynamic behaviors on millisecond and second timescales. More recently, single-molecule pump-probe spectroscopy emerged as a method to access heterogeneity on the femtosecond and picosecond timescales. Here, we develop a single-molecule pump-probe apparatus that is easily tunable across the visible region and demonstrate its utility on the widely-used fluorescent dye, Atto647N.

Ultrafast energy transfer between lipid-linked chromophores and plant light-harvesting complex II.

Light-Harvesting Complex II (LHCII) is a membrane protein found in plant chloroplasts that has the crucial role of absorbing solar energy and subsequently performing excitation energy transfer to the reaction centre subunits of Photosystem II. LHCII provides strong absorption of blue and red light, however, it has minimal absorption in the green spectral region where solar irradiance is maximal. In a recent proof-of-principle study, we enhanced the absorption in this spectral range by developing a biohybrid system where LHCII proteins together with lipid-linked Texas Red (TR) chromophores were assembled into lipid membrane vesicles.