Thermally Denatured Ribonuclease A Retains Secondary Structure As Shown by FTIR

Fourier transform-infrared (FTIR) spectroscopy has been used to test for the presence of nonrandom structure in thermally denatured ribonuclease A (RNase A) at pH* 2.0 (uncorrected pH measured in D2O). The amide I spectral region of the native and thermally denatured protein was compared. A substantial decrease in the amount of beta-sheet and alpha-helix and a corresponding increase in the amount of turn and unordered structure was observed on thermal denaturation. The results indicate that thermally denatured RNase A contains significant amounts of secondary structure (11% helix and 17% beta-sheet), consistent with previous results reported for circular dichroism, and with a relatively compact structure, as revealed by dynamic light scattering. These results are in contrast to those of amide protection experiments reported recently [Robertson, A.D., & Baldwin, R.L. (1991) Biochemistry 30, 9907-9914] which indicated no stable hydrogen-bonded structure under these experimental conditions. Possible explanations for this apparent discrepancy are given.