Quasi-elastic light scattering by diffusional fluctuations in RNase solutions

Using measurements of quasi-elastic light scattering spectra, we have investigated diffusional fluctuations of RNase. The diffusion coefficient for individual protein molecules, together with the corresponding calculated effective molecular radius R(eff), were determined. Between room temperature and the point of irreversible denaturation at 63.5 degrees C, R(eff) increased from 20-250 A. This is comparable to the plateau in R(eff) of 300 A reached after about 200 min following chemical denaturation in 10 M urea. The measurements indicated the presence of a large size component even in the freshly prepared and chromatographically purified solutions. From the diffusion constants deduced for this large component we obtained effective sizes from 1000-5000 A. Concentration and temperature dependent measurements exclude the possibility that these large particles are impurities and indicate that they are the result of aggregations of RNase molecules.