Physics scholar talks on energy transfer studies
Physicist Phil Nelson gave a lecture about the history and promise of Fluorescent Resonant Energy Transfer on Tuesday evening in the Abelson-Bass-Yalem physics building. The talk, titled, “The Physics, Biology, and Technology of Resonant Energy Transfer,” explored the discovery of FRET, in the early 20th century and examined how it is being used to improve the technology of tomorrow.
Nelson, a professor at the University of Pennsylvania, began his lecture by describing the history of FRET. The phenomenon was first encountered by Günther Cario and James Franck in 1922. The scientists conducted an experiment where they used a laser to excite a mixture of two gases: thallium vapor and mercury vapor. When they modified the wavelength of the laser to excite only the mercury vapor, the mixture paradoxically emitted energy that could only have come, instead, from the thallium vapor. Upon further study it was discovered that the energy was being transferred from one compound to the other by what would come to be known as FRET.
Over the next 10 years, scientists conducted more experiments, greatly enhancing their understanding of the phenomenon. They found that FRET would only occur if the energy levels of the “donor” atoms (the particles that can absorb the energy being produced) are similar to the energy levels of the “acceptor” atoms (the particles that end up emitting energy).
As time continued, researchers uncovered more examples of interactions that take advantage of FRET. DNA is able to coil into tightly-packed loops because of FRET; it is also responsible for the function of a family of genetically encoded chemical indicators used extensively in biology which change color in the presence of calcium.
Nelson explained that for many years after its discovery, physicists did not fully understand how FRET worked, even as researchers continued to use it in their experiments. This problem arose from the then-emerging field of quantum physics, which looked like the perfect way to describe the behavior.
The field of quantum physics had been built on observations that the electrons in atoms could only occupy discrete, quantized (hence quantum) energy levels. When hit by the right wavelength of light, an electron would be excited into a higher energy state, then eventually drop back down to its lowest energy state (the “ground state”), emitting its own light from the energy loss. Quantum physics seemed like the perfect tool to describe FRET: discrete energy transfer on a small scale with only two particles to consider.
When physicists tried to do just that, they found that their models didn’t predict the donor-to-acceptor behavior of FRET. Instead, they predicted that the energy would be transferred back and forth between donor and acceptor again and again until being randomly emitted. This “slosh[ing] back and forth” of energy was a poor approximation of the observed “one-way” energy transfer of FRET.
Nelson then showed how theorists were able to modify the original approximation to accurately describe FRET behavior. The original approximation assumed that the acceptor and the donor atoms were completely isolated from other particles, and assumed that the donor would always transfer its energy to the acceptor. To account for cases where the donor simply returns to its ground state without exciting the acceptor, another term was added to the equation describing the donor particle. They also added a term to account for the influence of other particles on the exchange.
Finally, Nelson explained how paradoxes in plant biology were solved by recognizing that plant cells take advantage of FRET to operate efficiently. One of the first people to make this connection was W.A. Arnold, a physicist at the University of California, Berkeley. In 1931 Arnold became a research assistant for Robert Emerson, a prominent biologist who had extensively studied photosynthesis. Emerson suspected that one of the pigments in an alga he was working with, phycocyanin, was somehow involved in the alga’s photosynthesis. The alga was exposed to a wavelength of light that the phycocyanin was able to absorb, but which the chlorophyll was not.
Despite this, the alga was still able to photosynthesize energy. Emerson asked Arnold to see whether the phycocyanin was responsible for the photosynthesis, or if the energy from the phycocyanin was somehow being transferred to the chlorophyll. Arnold agreed, and after a few experiments he concluded that the phycocyanin was transferring the energy to the chlorophyll.
Arnold brought the problem to one of his professors, renowned physicist J. Robert Oppenheimer. Oppenheimer had a hunch that the behavior was a result of FRET, and the two agreed to write a paper about it. About two years later, Oppenheimer published the paper himself without consulting Arnold. He showed that the alga were able to absorb more frequencies of light by taking advantage of FRET, one of the first documented examples of the phenomenon in nature.
Nelson concluded the lecture with a slide showing a 2010 paper which proposed to make more efficient solar panels by mimicking the structure of plants. The paper outlined how FRET could be used to design solar panels which would collect energy from more wavelengths of light, greatly increasing their capacity to generate electricity.