Stopped‐flow circular dichroism: A new fast‐kinetic system

Circular dichroism, units AA, where A € = (BL -CR) = A A / ( c M .~~~) CD SFCD stopped-flow circular dichroism AAT aspartate amino-transferase (pig-heart, a-isomer) CV crystal violetA combined stopped-flow circular dichroism system has been developed with sensitivity 2 X Applications are briefly described for the study of rapid transient processes involved in interactions of macromolecules and ligands.The method allows direct observation of rapid conformational events and provides a new level of investigation into the mechanism of ligand-induced conformational changes.The use of absorption and fluorescence optics with stopped-flow systems is well established, l , * but the parameters are not intrinsically sensitive to conformation.The conformational basis of CD is well founded in theory3 and experiment4 and CD has been widely used to study the equilibrium properties of proteins5*6 and nucleic acids7J.Conventional CD instruments have time constants 1-10 sec, and cannot be used for rapid transient work.Because of the direct interrelationship between sensitivity and time constant, a thousand-fold increase in the former has been sought to permit measurements at millisecond resolution.The experimental technique requires a) high-frequency modulation, b) light levels lop W, c) rapid response time of the electronic system.These points will be considered systematically e1sewhere.QThe schematic diagram of the apparatus is shown in Figure