Dielectric relaxation of aqueous solutions of ribonuclease A in the absence and presence of urea

The dielectric spectra of aqueous solutions of ribonuclease A (RNase) have been measured in the frequency domain over the whole absorption region between 2 MHz and 72 GHz at 20 degrees C in two solvents, pure water and 8 mol/l urea solution. These environmental conditions promote the native and unfolded state of the protein, respectively. The results are discussed with regard to (i) the low and (ii) the high frequency part of the spectra. The contribution to (i) which is attributable to the tumbling motion of RNase is changed by the presence of urea, indicating that the RNase moieties are increased in polarity, but only insignificantly in size. From (ii) it is concluded that addition of urea leads to a change in the overall water structure and dynamics which is accompanied by an increased fraction of non-associated solvent molecules. These are likely to be instrumental in the denaturation process.