Thermal and Chemical Denaturation of Staphyloccocal Nuclease: A Dynamic Light Scattering Study

Thermal and chemical denaturation data are presented from dynamic light scattering (DLS) experiments in which both wild-type and a quadruple mutant form of the small globular protein staphylococcal nuclease (SN) were studied. Previous studies on this particular mutant, known as mutant 62, indicate it has a smaller solvent- accessible surface area, and hence, more compact denatured state than wild-type SN. We performed DLS experiments at temperatures from $23^{\circ}$C--$55^{\circ}$C for dilute solutions of SN in buffer for GuHCl concentrations from 0--2 M. Diffusivities were measured and protein sizes were calculated. A dramatic increase in protein size occurs at temperatures ranging from $44.4^{\circ}$C -- $53.0^{\circ}$C as the GuHCl concentration was increased. Protein size is also dependent on GuHCl concentration at temperatures below this dramatic denaturation event. These and other results will be presented as well as their implications for the existence of intermediate states and models for denaturation in this system.