New Raman Method Measures Extremely Low Frequency Acoustic Vibrations

A new acoustic method allows researchers to obtain Raman spectra of low-frequency vibrational modes that are not usually accessible to the analytical technique. Reuven Gordon, Skyler Wheaton, and Ryan M. Gelfand of the University of Victoria, in British Columbia, call the new method extraordinary acoustic Raman, or EAR (Nat. Photonics 2014, DOI: 10.1038/nphoton.2014.283). To obtain the spectra, they combine the beams from two lasers at slightly different wavelengths to generate a “beat frequency” that modulates the electric field in laser tweezers holding a nanoparticle or protein in place. The modulation excites Raman-active vibrations that cause mechanical stretching in the particle or protein. These vibrations provide information about the size, shape, and mechanical properties of the sample. In the case of proteins, the spectra could be used to study phenomena such as allostery, in which ligand binding at one site causes conformational changes that affect binding sites elsewhere.