Kinetic crystallography by Raman microscopy.

Raman spectra, obtained using a Raman microscope, offer a unique and incisive approach to follow interactions and reactions inside a single crystal under soak-in or soak-out conditions. The utility of this approach derives from the finding that the Raman spectra from single macromolecular crystals, under normal (non-resonance) conditions, are extremely stable, with a low "light background," and provide ideal platforms for Raman difference spectroscopy. In turn, this allows the interrogation of sub-molecular changes in very large and complex macromolecular environments. There is often great synergy with X-ray crystallography, with the Raman spectroscopist providing crystallography colleagues with the best soak-in conditions to generate a targeted intermediate for flash freezing and X-ray analysis. On the other hand, X-ray structures at points along a reaction pathway provide invaluable benchmarks for interpreting the Raman data from populations seen by Raman to be changing in real-time. These principles will be illustrated by two reactions: the first involves a complex, branching reaction pathway underlying the inhibition of β-lactamases by clinically important pharmaceutical compounds, where different combinations of drug and enzyme function in different regions of the pathway. The second shows how temporal data can be derived for several events in the initiation step of RNA synthesis-more specifically, when one GTP molecule is joined to one ATP molecule to form a G∙A dimer in the active site of a 115,000 Dalton crystalline RNA polymerase. Finally, we will summarize the extension of Raman microscopy to nucleic acid crystals and the information that has been obtained for RNA-based enzymes. This article is part of a Special Issue entitled: Protein Structure and Function in the Crystalline State.