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Researchers Explore New Methods to Capitalize on Nature’s Light-Harvesting Secrets

The green sulfur bacterium lives in the chilly waters of the Black Sea. In order to eek out its solitary existence, this life form scavenges energy from the weak sunlight available to it at a depth of more than 250 feet.

Seen in grey, the DNA DX-tile forms a scaffolding allowing for the precise placement of dye molecule chromophores, which self-assemble on the scaffold in characteristic J configurations, seen in green. Blue and red chromophores represent donor and acceptor molecules, respectively. Credit: The Biodesign Institute at Arizona State University

Plants carry out the same remarkable trick, collecting radiant energy from the sun and converting it to biological energy vital for growth.  This process — perfected over billions of years — is called photosynthesis.

Hao Yan and Neal Woodbury, from ASU’s Biodesign Institute and colleagues from Harvard and MIT, are currently exploring new methods in order to capitalize on Nature’s light-harvesting secrets. Their new study summaries the design of a synthetic system for energy gathering, conversion and transport that could point the way to innovations in materials science, nanotechnology, solar energy and photonics.

This multi-institute collaborative effort demonstrates a nice use of DNA nanotechnology to spatially control and organize chromophores for future excitonic networks.

Hao Yan, ASU’s Biodesign Institute

Light Moves

A system for the programmed assembly of light-gathering elements or chromophores has been described in research published in the advanced online issue of the journal Nature Materials. In natural systems like photosynthetic bacteria and plants, the spatial organization of densely packed chromophores is important for efficient, directed energy transfer. Such biological systems assemble chromophores in an accurate manner on rigid scaffolds made up of protein.

Practically all life on earth depends indirectly or directly on photosynthesis. The organisms using it competently transport the energy of sunlight from receptors, which collect photons from sunlight, to reaction centers where the energy could be harnessed — a performance effortlessly rivaling the most effective manmade solar cells.

Efforts to understand natural light harvesting systems in photosynthetic microbes and plants date back at least a century. Even though the phenomena have been understood in a wider outline, the details turn out to be complicated and the challenges in producing synthetic analogues have been significant.

Plants execute photosynthesis by transforming photons of light striking their chromophores into another form of energy called an exciton. An exciton refers to an energetic state of a molecule, or closely coupled group of molecules after they are excited by light absorption. Excitons are valuable in both research efforts to duplicate the process and in natural photosynthesis, since they can carry energy from one molecule to another, energy that can eventually be used to power the movement of electrons.

Solar energy is expected to majorly contribute to the global energy supply over the next century, as society moves away from the use of fossil fuels. Researchers can achieve this by learning how to capture, transfer and then store solar energy with maximum efficiency at reasonable cost.

Designing from Nature

In the present study, dye molecules responsive to specific ranges of light energy are employed as synthetic chromophores. By employing DNA as a scaffold, it is possible for the relative positions of the dye molecules to be accurately controlled, better mimicking natural systems.

This DNA scaffolding is capable of being self-assembled from six strips of single-stranded DNA whose base pairing properties make it to produce the desired structure. The form, which has become a backbone in the field of DNA nanotechnology, is called a double crossover- or DX-tile. It is frequently used as a simple building block for programmed synthetic DNA assemblies.

The outlined method permits the optimum arrangement of chromophores to be modeled, developing a light-harvesting circuit capable of efficiently carrying the energy of an absorbed photon over distance along the DNA architecture with minimal energy loss all through the way.

The ability to model and build molecular circuits for gathering light energy and moving it around in a controlled fashion, opens the door for the design and development of a variety of nano-scale devices that are powered and controlled by light.

Neal Woodbury, from ASU’s Biodesign Institute

The resulting synthetic circuit permits the absorption spectra of the chromophores to be finely tuned in a manner that is similar to natural light-harvesting systems.  This can be attained in part by accurately controlling the orientation of dye molecules and their distance from each other.

Quantum Leap

Researchers have recently determined that part of the success of natural photosynthetic systems is because of strange physical effects belonging to the quantum world. It turns out that in photosynthetic organisms comprising of multiple chromophores packed firmly together, it is possible to share light excitation between molecules. This feature, called quantum coherence, can majorly improve the efficiency of energy transfer. This is one reason why plants and photosynthetic bacteria are so good at it.

The efficiency of biological systems and nanomachines in capturing light and then transporting energy is due to the highly ordered nanoscale architecture of photoactive molecules. In the last few decades, the use of DNA as a template for the arrangement of functional elements such as organic dyes into precise arrays has experienced speedy improvement.

In the present study, the self-assembling properties of DNA and chromophores were exploited in order to accurately determine the locations for the J-aggregate chromophore assemblies on the DX-tile. These J-aggregate chromophore assemblies comprise of light-gathering characteristics similar to the natural light-harvesting antennas employed by photosynthetic purple bacteria.

The initial step was to recognize the size range of chromophore dye aggregates that could effectively self-assemble on a length of double-stranded DNA, while continuing to retain efficient energy transfer properties. Modeling established the fact that the minimal DNA length essential for accommodating a stable J-aggregate of chromophores was eight base pairs.

Next, a circuit made up of four chromophore aggregates placed on the DX-based tile was designed, modeled, and then optimized, employing principles of quantum dynamics in order to guide the rational assembly of multiple discreet dye aggregates within a DNA DX-tile. The chromophore aggregates were computationally explored in order to identify sequence designs displaying fast exciton transport properties.

This was followed by synthesizing the optimal circuit design and using sophisticated methods of florescence spectroscopy to precisely characterize the results. Additional investigations attempted to accurately characterize the molecular organization of chromophores within a single J-aggregate.

The researchers projected that an aggregate of six dye molecules would assemble per eight base pair segment of DNA, a result, which associated well with earlier estimates of 8-12 dye molecules for every single turn of DNA’s double-helical ladder. A separation distance of two base pairs was determined in order to provide the finest excitonic coupling between adjacent chromophore aggregates. The resulting circuit presented properties of energy transport reliable with modeling predictions.

Future Light

The success is considered to be another demonstration of the versatility and power of a bottom-up approach to the assembly of nano-scale architectures. Specifically, the design of excitonic circuits such as the one described could result in new applications going beyond light-harvesting technology, including advances in information and communications technology, and advances in fields ranging from the transportation, environment, manufacturing, energy and healthcare.

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