NANOSECOND TIME-RESOLVED ABSORPTION AND POLARIZATION DICHROISM SPECTROSCOPIES

Time-resolved measurements of the light absorption and polarization dichroism of protein residue and prosthetic group chromophores provide a real-time probe for transient structural states of enzymes. Most transient structures represent thermal excursions from equilibrium and are too ephemeral for study. In many enzyme systems, however, a rapid perturbation, such as a laser pulse, can drive a perceptible fraction of the sample into metastable states corresponding to kinetic intermediates. The spectral detection and characterization of such metastable structures and their decay paths are a major component in understanding the mechanism of enzyme function. The polarization dichroism spectroscopies used in time-resolved spectral studies of enzyme kinetics include linear dichroism (LD), natural circular dichroism (CD), and magnetic circular dichroism (MCD) spectro- scopies. With time-resolved absorption and polarization dichroism spectroscopies, nonequilibrium kinetic intermediates of enzymes can be studied under conditions of temperature and solvation that approach those of biological function. © 1993, Academic Press, Inc.