By Patricia Daukantas
Most molecular biologists stare down at throngs of their tiny subjects the way an aerial photographer captures a large pack of runners at a marathon. Steven Block wants to focus on a single molecule, just like zooming in to study the guy who broke out of the pack to win the marathon four times.
That’s how Block, a professor of both biology and physics at Stanford University, set the stage for his Wednesday morning CLEO plenary talk on single-molecule biophysics with optical tweezers. I’m not an expert on biology by any stretch -- even my high-school biology class is sadly out of date now -- but I’ll try to convey what he said as best I can.
RNA polymerase, the enzyme that produces RNA, is a sophisticated nanomachine, and scientists would like to know how it works. Humans have three or four kinds of RNA polymerase; it’s the stuff that makes our cells differentiate themselves by function, even though each chromosome has the same DNA. On the scale of proteins, RNA polymerase is pretty big -- about 3,300 amino acids -- but on the scale of things in general, it’s pretty small -- roughly 10 nm big.
Optical tweezers are “the closest thing humans have made to a tractor beam,” Block said after showing his grad-school-days video of a single bacterium stuck in an optical trap. His experiments with then-grad-student Will Greenleaf and colleagues, as I understand them, involved setting up two tiny dielectric spheres in side-by-side traps and stretching a single DNA molecule back and forth between them. They did this in order to study riboswitches, which are non-coding messenger RNA (mRNA) strands that control gene expression by changing structure when they selectively bind to a molecule. More experiments are forthcoming, even though Greenleaf is now a postdoc at Harvard University.
David Awschalom, the QELS plenary speaker from the University of California at Santa Barbara, talked about something else that’s darned tiny: single electron spins in semiconductors.
Much like photons, electron spin ensembles exhibit coherence in doped semiconductors. Much research into semiconductor spins has been done with low-temperature ensembles, Awschalom said, but tremendous progress has been made over the last five years into the study of single spins in solid-state matter.
Diamond -- that glittering crystal of carbon -- is a CMOS-compatible (both p- and n-type) semiconductor with remarkable thermal properties. Awschalom and his colleagues study synthetic diamonds with certain impurities called nitrogen-vacancy centers, in which two neighboring points of the carbon crystal lattice are replaced by a nitrogen atom and a gap with no atom. (Diamond gemstones with many of these impurities look yellowish.)
Again, the way I understand these experiments, the team shone polarized light through a diamond at room temperature and used a confocal microscope to spatially map the photoluminescence pattern and thus measure the single spins. In a paper published last December in Science, the team described how these single spins can flip on the order of 1 ns, which is about five times faster than the RAM in a modern desktop computer operates. Paradoxically, performance improves with increasing temperature -- not the way conventional electronic devices work.
Arrays of these tiny spins within diamonds could have many uses in quantum computing and communications -- and many other kinds of defects in diamonds have yet to be explored. To keep up with Awschalom’s research group, check out http://www.physics.ucsb.edu/~awschalom.