Denatured states of ribonuclease A have compact dimensions and residual secondary structure.

Using small-angle X-ray scattering and Fourier transform infrared spectroscopy, we have determined that the thermally denatured state of native ribonuclease A is on average a compact structure having residual secondary structure. Under strongly reducing conditions, the protein further unfolds into a looser structure with larger dimensions but still retains a comparable amount of secondary structure. The dimensions of the thermally and chemically denatured states of the reduced protein are different but both are more compact than is predicted for a random coil of the same length. These results demonstrate that thermal denaturation in ribonuclease A is not a simple two-state transition from a native to a completely disordered random coil state.