Slow Motion Picture of Protein Inactivation During Single-Droplet Drying: A Study of Inactivation Kinetics of l-Glutamate Dehydrogenase Dried in an Acoustic Levitator

A novel technique is presented to allow measurement of the kinetics of protein inactivation during drying of an acoustically levitated single droplet. Droplets/particles are removed from the acoustic field after various times during drying, and the state of the protein within them is analyzed. The influence of drying air temperature, relative humidity, buffer concentration, and the presence of a substrate on the inactivation of glutamate dehydrogenase is described. The kinetics of inactivation showed three distinct phases. The first phase of constant drying rate demonstrated little protein inactivation in the solution droplet. After the critical point of drying, a second phase was distinguishable when the surface temperature has risen sharply, but there is still only little inactivation of the protein in the solid particle. An onset point of rapid inactivation of the protein marked the start of the third phase that proceeded with approximately first-order rate kinetics. In the case of l-glutamate dehydrogenase, the evidence suggests that the residual moisture content of the solid and not the temperature alone determines the point of onset of protein inactivation. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:2239-2249, 2012