When MIT phenoms Seth Lloyd and Angela Belcher put their heads together to create the perfect peanut butter cup, you know we are going to be there to take a bite. Lloyd, of quantum computer fame, realized that certain features of the kinds of viruses which Belcher builds are ideally dimensioned for trying increase the efficiency of photosynthetic energy transport via quantum effects. When he mentioned that to her, she said her lab was already making them. A short time later, the team had their prize: quantum viruses genetically engineered for optimal exciton transport.
What are excitons you might ask? Technically speaking, they are neutral quasiparticles consisting of an electron and an electron hole bound by an electrostatic Coulomb force. They are formed when a photon is absorbed by insulators or semiconductors, and can transport energy on the smallest of scales without transporting net charge.
There is now considerable evidence that proteins, including those which harness various chromophore molecules, act as semiconductors — in many cases even so-called quantum critical semiconductors. When a photon hits a photosynthetic chromophore, an exciton is generated just like it might in more familiar semiconductor materials. It then hops along additional chromophores until it bumps into a reaction center where the energy is used to string together molecules from freely diffusible CO2 plucked straight from the air.
The magic comes into play during this hopping stage. The wavelike nature of the particle provides a mechanism for it to simultaneously explore multiple pathways and ultimately resolve the optimal route. If the spacing of the chromophores, and the lifetimes of their excitons, are not “just so,” then the particle takes much longer to arrive at the reaction center. Much the same situation applies to electron tunneling through proteins in the mitochondrial respiratory chain. Lloyd whimsically describes these general phenomena as examples of the Quantum Goldilocks Effect: “Natural selection tends to drive quantum systems to the degree of quantum coherence that is ‘just right’ for attaining maximum efficiency.”
Lloyd notes that the total excitonic lifetime in photosynthesis, which is on the order of nanoseconds, spans six orders of magnitude in going down to the fastest measurable femtosecond events. The overall transfer time from absorption in the photosynthetic antenna harvesting system to capture in the reaction center is a few tens of picoseconds. In extending the classical Goldilocks principle of biology into this quantum system, Lloyd would have it that natural selection has brought about a convergence of the relevant timescales by adding the necessary quantum processes in between photosynthetic events.
What may make all that a tough pill to swallow whole is that overall the full photosynthetic ecosystem of the larger chloroplastic endosymbiont of the cell is far from perfect. While Lloyd maintains that the level of quantum coherence and complexity must be exactly right to efficiently transport 100% of the excitons generated at the antenna to the reaction center (something that he says can occur), that doesn’t imply there is ever 100% efficiency of anything. For example, we previously noted that while the so-called theoretical photocurrent efficiency for photosystem II often gets quoted at 95%, in the real world it is more like 5% — and when you try to take a closer look at where that number comes from you inevitably ask yourself, 5% of what?
At this point you might be wondering what all this has to do with viruses. Belcher’s group had previously been able to bind chromophores known as zinc porphyrins to the M13 virus, and also use them to explore various solar, electrolysis, and battery applications. Zinc porphyrins can naturally form in our blood cells when there isn’t enough iron around to get incorporated in the porphyrin core. In chlorophyll, incidentally, a magnesium atom is used at the heart of much the same basic porphyrin cofactor.
To make the new quantum M13 virus, Belcher instead used a few of the more exotic new chromophores — Alexa Fluors 488 as the acceptor and 594 as the donor. These synthetic molecules can be made with narrow and well-separated absorption bands, and the proper spectral overlaps for efficient energy transfer. They were bound to the virus via the primary amino groups of the viral pVIII coat protein.
The net result was that the genetically-enhanced viral antenna system achieved a 68% longer diffusion length and a fourfold increase in the number of donors transferring energy to acceptors. Traveling at effectively double the speed, the excitons migrated significantly further before dissipating. To observe the light-harvesting events and verify that quantum coherence was enhancing transport the group used laser spectroscopy and theoretical modeling of exciton dynamics.
- By John Hewitt on October 16, 2015 at 3:52 pm