The compactness of ribonuclease A and reduced ribonuclease A

The compactness of ribonuclease A with intact disulfide bonds and reduced ribonuclease A was investigated by synchrotron small-angle X-ray scattering. The Rg values and the Kratky plots showed that non-reduced ribonuclease A maintain a compact shape with a Rg value of about 17.3 A in 8 M urea. The reduced ribonuclease A is more expanded, its Rg value is about 20 A in 50 mM Tris-HCl buffer at pH 8.1 containing 20 mM DTT. Further expansions of reduced ribonuclease A were observed in the presence of high concentrations of denaturants, indicating that reduced ribonuclease A is more expanded and is in neither a random coil [A. Noppert et al., FEBS Lett. 380 (1996) 179-182] nor a compact denatured state [T.R. Sosnick and J. Trewhella, Biochemistry 31 (1992) 8329-8335]. The four disulfide bonds keep ribonuclease A in a compact state in the presence of high concentrations of urea.