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Stopped-flow spectroscopy, including fluorescence, absorption, and circular dichroism (CD), is a powerful technique widely used in biochemistry and biophysics to study rapid molecular interactions and reaction kinetics. By rapidly mixing reactants and measuring spectroscopic changes in real-time, it enables researchers to capture fast biochemical events on the millisecond timescale. Stopped-flow spectroscopy is often employed to monitor conformational changes, binding events, or complex formation, offering detailed insights into the mechanisms of molecular interactions. This real-time monitoring capability makes stopped-flow fluorescence ideal for investigating processes such as enzyme kinetics, protein folding, ligand binding, and molecular association or dissociation. The versatility of stopped-flow spectroscopy allows it to be applied to various biological systems, including protein-protein, protein-DNA/RNA, and protein-ligand interactions. It can be performed under controlled conditions, such as specific temperatures, pH levels, and ionic strengths, facilitating a thorough characterisation of the factors that influence molecular processes.

Stopped-flow spectroscopy is particularly well-suited for studying single turnover reactions, where a single catalytic cycle of an enzyme or molecular process is observed. In these experiments, reactants are mixed, and changes in absorbance or fluorescence are tracked to capture the kinetics of individual turnover events. This is especially useful for studying enzymes with discrete catalytic steps, enabling researchers to follow the formation and decay of reaction intermediates. As enzyme concentrations are often limiting in single turnover reactions, stopped-flow spectroscopy provides precise kinetic data for each step of the reaction pathway, offering valuable insights into enzyme mechanisms and the timing of individual steps.

Complementing this technique, chemical quench flow allows for the study of rapid reactions by freezing them at specific time points. In quench flow experiments, reactants are mixed, and the reaction is halted at precise intervals by adding a chemical reagent, allowing for the analysis of intermediates or products through methods like chromatography or mass spectrometry. This is particularly valuable for studying reactions that can not be monitored spectroscopically or those requiring detailed stepwise analysis of multiple species. Together, stopped-flow spectroscopy and chemical quench flow provide powerful tools for dissecting complex, fast biochemical reactions.