How a Hacked Virus Is Bringing Us Closer to Artificial Photosynthesis

In new experiments, virus particles are tasked with transporting energy in organic light antennae.

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Oct 17 2015, 1:45pm

M19 visualization. Image: YouTube still

The question persists: Is a virus alive? Or is it simply a clever organic machine? It lacks most all of things we associate with "life": the ability to reproduce (outside of a host cell), biological componentry, a metabolism. All it is is DNA RNA, a single strand wrapped in a protein membrane. Through most of its existence, a virus is inert: nothing more than a "chemistry set."

But then, something happens. A virus meets a host cell. It comes to life, or so it seems. It sheds its membrane, revealing bare genetic code. The host cell is induced to replicate that code rather than its own. It's an invasion, a smooth synthesis with what is most certainly life.

Researchers at MIT have offered an answer, however indirectly. They have hijacked the hijacker, reprogramming virus particles to function as a structural scaffolding in a light-harvesting antenna system. The result, which is described in this week's Nature Materials, is a significant boost in the energy transport efficiency of an organic photovoltaic cell.

To be fair, the M13 virus used in the research has been put to use in all sorts of technologies, including medical imaging, batteries, and carbon nanotube-based photovoltaics. Left to its own device, M13 infects bacteria. The phage's utility owes to its convenient binding sites, which allow engineers to stick them together sort of like Legos. They're a building material, in other words.

Image: Belcher et al

The M13s are a bit more than Legos, however. The idea in organic photosynthesis is that photons smash into particles called chromophores, which pop out what are known as excitons. An exciton is a quasiparticle consisting of both an electron and an electron hole, which is the empty space in an atom or an atomic lattice where an electron might exist. Excitons are able to propagate through the genetically engineered virus particles with a relatively high efficiency.

"The use of programmable genes allows us to manipulate the positioning of the binding sites, thus multiplying the possibilities for creating intricate chromophore networks and for controlling the energy transfer," the researchers write.

The underlying problem is of keeping excitons from decaying too fast to be useful. Unstructured "free" chromophores can keep an exciton going for around 4 ns, but the best scientists have been able to do with engineered chromophors has been around 500 ps (picoseconds), which is around an eighth as long. The problem is that excitons in too close of proximity will "quench" each other, allowing their energy or energy potential to decay before it can be useful. Somehow, plants manage to transport excitons efficiently without running into this problem, or at least not to the same degree.

This last part is what researchers want to understand. Plants efficiently move around energy across networks of tightly packed chromophores without losing too much of it through quenching. The theory is that quantum effects allow for "non-local" jumps between relatively distant chromophores.

Physics World explains further:

The researchers created "nano-antennas" from the two types of viruses and immersed these in solutions containing two types of chromophores. One type acts as the "donor", which generates the exciton when hit by a photon, the other acts as an "acceptor", which collects excitons from surrounding donors and, when excited, emits fluorescent light. They illuminated the nano-antennas with light of a constant intensity and measured the intensity of the fluorescence, which revealed how many excitons had successfully reached an acceptor. The researchers used these results to calculate the distance the excitons had been able to diffuse in the two types of nanostructure.

With help from the viral substructure, the researchers were able to fine-tune their chromophore networks such that, while the excitons had relatively short lifespans, energy could be moved around fast enough to compensate. Nonetheless, plants still manage to keep excitons intact for much longer than what was seen in the experiment above. True artificial photosynthesis is still a ways off.