Stopped-flow spectroscopy enables the study of fast reactions in solution over timescales in the range of 1 millisecond to hundreds of seconds. A wide range of reactions can be investigated involving, for example, protein-protein interactions, ligand binding, electron transfer, fluorescence resonance energy transfer (FRET), protein folding, as well as enzyme, chemical or coordination reactions.

In most experimental set-ups, two reagents are rapidly mixed together and then ‘stopped’ in an observation cell. The sample cell is irradiated with monochromatic light and as the reaction proceeds the change in the signal (fluorescence, absorbance or CD) at a specific wavelength is recorded as a function of time.

Analysis of the resulting kinetic transient can determine reaction rates, complexity of the reaction mechanism, information on short-lived reaction intermediates etc. A series of stopped-flow experiments can be used to show the effect of parameters such as temperature, pH and reagent concentration on the kinetics of a reaction.