Antenna-Reaction Center Interface: Organization and Delivery
PARC will draw upon the fundamental knowledge of native, biohybrid and synthetic antenna complexes, both architectures and processes, to design and assemble LH arrays to integrate into target solar-conversion systems. The ultimate aim is fabrication of LH assemblies that absorb 90% of the incident photons over any specified subset or full span of the 350 to 900 nm region of the solar spectrum, delivering the energy to a target site or sites where it is trapped with a quantum yield of 80% or greater. The assemblies will be stable over a timescale of months, and will expand the physical size of purely biological LH arrays ten-fold from 100-500 nm to 5000 nm. The specific goals are as follows:
- Self-assembling macromolecular arrays will be based on tunable pigments incorporated into designer maquette polypeptides or engineered native antenna proteins.
- The functionality of the arrays will be extended to include sites of energy trapping and photochemistry at defined positions.
- The mesoscale architecture of artificial assemblies will be controlled using lithographic approaches to interface the synthetic and biohybrid components.
The overall aim is the fabrication of micron-scale arrays for efficient solar light harvesting, energy transfer and trapping that can be used for chemical processes or generation of photocurrent.
Plants and photosynthetic bacteria have macromolecular antenna systems that use protein scaffolding to exquisitely control the spatial relationships of the tetrapyrrole (chlorophyll, bacteriochlorophyll) cofactors. The protein matrix also helps tune the properties of the cofactors to aid, or control, the funneling of excitation energy toward specific sites, such as to adjacent light-harvesting complexes and ultimately to the photochemical reaction center. An alternative strategy has been employed in chlorosomes found in certain bacteria. This antenna system is in essence a cofactor aggregate formed through specific pigment-pigment contacts without protein-cofactor interactions. A feature common to all the natural light-harvesting systems is that the structural and energetic characteristics have derived from evolutionary pressures to perform requisite antenna functions under diverse environmental conditions. Although these systems generally function in a robust manner, questions remain as to whether antenna size, organization, and efficiency are optimal, or even close to optimal.
Furthermore, such systems can be compromised by mutations that alter the structural or energetic characteristics that have evolved. In designing synthetic light-harvesting complexes for solar-energy conversion strategies, it is desirable to explore systems that maximize the overall functional characteristics of the natural photosynthetic antennas while minimizing structural complexity.
Theme 3 proposes a new and versatile strategy for preparing novel mesoscale antenna systems. The strategy merges designer peptide maquettes that will provide the scaffoldings to house tetrapyrrole (porphyrin, chlorin, bacteriochlorin) chromophores whose electronic properties (photophysical, redox) and chemical attributes (solubility, attachment motifs, etc.) are under synthetic design control. Additional tuning can be provided by the peptide environment itself, thereby exploiting the approach utilized by the native photosynthetic systems wherein the protein serves as a smart matrix to provide electronic as well as structural control of functional characteristics. The diversity of architectures possible will provide unique opportunities to explore the fundamental size, architectural, and chromophore requirements for the efficient harvesting and funneling of solar energy.
The overall goal is to address a range of key issues that ultimately underlie the ability of an antenna system to efficiently capture solar light and deliver the energy to the photochemical site. Such issues relate to optimum size (including number of chromophores), architecture (linear, cyclic, dendrimeric, lateral versus layered), chromophore type and combination (porphyrin, chlorin, bacterichlorin), interchromophore electronic coupling (weak or strong), and the means to promote photochemical robustness and incorporation of a photoprotective function. It is difficult to address such fundamental questions at the required microscopic level solely from studies of the natural photosynthetic systems given the complexity of their design and function. Similarly, it is extremely difficult to see how these issues can be addressed adequately using systems prepared by covalent synthesis or self-assembly techniques given the current state-of-the-art and any optimistic projection of where such methods will be five years from now. The overall goal of the proposed project is to provide the required platform for addressing key unresolved questions such as those outlined above that underlie antenna function.
Chromophores. The pigments will include porphyrins, chlorins and bacteriochlorins, which contain zero, one and two reduced pyrrole rings, respectively. Porphyrins absorb most strongly in the near-UV region and weakly in the visible region. Chlorins and bacteriochlorins have somewhat weaker violet-blue absorbance and comparable green-orange absorbance as porphyrins, but additionally have strong absorption in the red and near-infrared regions, respectively. Methodology now exists for placing virtually any pattern of peripheral groups on these macrocycles to tune electronic properties such as absorption wavelengths, as illustrated at the right. These chromophores will form the active sites for energy capture and transfer to be incorporated into the maquette scaffolds described below.
Maquette Protein Scaffolds. Sculptors traditionally test novel designs in simpler, smaller-scale forms they call maquettes. Similarly, maquette proteins can be designed to offer a vehicle to explore structural and engineering requirements and tolerances of a given function that have been observed in vastly complex natural proteins.
The proposed work on antenna construction will involve the progressive study of (1) single four-helix-bundle maquetttes (A below) to optimize the chemistry for binding synthetic tetrapyrroles, (2) small maquette assemblies (B) having multicofactor binding to allow energy-transfer studies, and (3) large multi-maquette architectures derived from the incorporation of multiplecharge patches (C) or covalent-linkage units (D) on two four-helix maquette modules to encourage multimerization into larger scale assemblies.
Characterization of Synthetic Protein Scaffolds, Tetrapyrroles, and Maquettes. The synthetic peptides, isolated tetrapyrroles, maquettes, and maquette-maquette assemblies prepared will be subjected to detailed physicochemical studies. These include static and time resolved (femtosecond and slower) photophysical measurements, electrochemical studies, resonance Raman spectroscopy, surface-science techniques, and density functional theory calculations.
Theoretical Studies of Light-Harvesting Efficiency and Energy Funneling. The spectroscopic and photophysical studies of the maquettes will be accompanied by theoretical investigations. These studies will examine questions related to optimization of energy flow, determination of optimal sizes, and the role of structural stability or disorder.
Collectively, the studies aggressively explore the idea that the antenna functions performed by sophisticated natural photosynthetic systems may be achieved in protein-tetrapyrrole architectures that are much simpler and more readily assembled. The project provides a unique approach to elucidate fundamental principles that underpin solar light harvesting and energy funneling. The overriding goal is to open the path to simple, robust light-harvesting systems with efficiencies equal to or better than the native photosynthetic antenna and that will contribute to revolutionary advances in artificial systems for solar-energy conversion.