University of St Andrews, UK
An optical physicist sees beyond fluorescent labels.
Many a molecular biologist likes to watch molecules move around inside living cells, particularly in real time. The job is usually done by tethering a fluorescent tag to interesting biological molecules and following their movements by means of the tag’s glow. But fluorescent tags are often bigger than the molecules they label, so frequently perturb their movements. Better to watch intracellular dramas without millstones around the actors’ necks. But how?
A twist on ‘Raman scattering’ may hold the answer. Normally, when a laser is shone at a molecule, the molecule scatters most of the light at the same frequency at which it was emitted by the laser. A tiny amount — Raman scattered light — is scattered at different frequencies. These frequencies indicate the chemical bonds in the molecule, and can thus identify it as a fingerprint identifies a person. If only Raman signals were stronger, they would be suitable for real-time microscopy on a molecular scale.
A second laser provides the twist — and the necessary amplification. Sunney Xie of Harvard University and his colleagues have found that another laser can enhance the contrast of an image, improving the sensitivity over previous studies by four orders of magnitude (C. W. Freudiger et al. Science 322, 1857–1861; 2008). For this to work, the two lasers must coincide on the sample, and the difference in their frequencies must exactly match that of a specific molecular vibration of a certain chemical bond in the sample. The background noise is eliminated and the signal is amplified.
This method is both versatile and powerful; the authors used it to observe the uptake of omega-3 fatty acids by human lung-cancer cells and the changing distribution of two drugs as they were absorbed by mouse skin. I think this could spur the development of tag-free molecular movie machines for all.