THERMALLY DENATURED RIBONUCLEASE-A RETAINS SECONDARY STRUCTURE AS SHOWN BY FTIR

Fourier transform-infrared (FTIR) spectroscopy has been used to test f or the presence of nonrandom structure in thermally denatured ribonucl ease A (RNase A) at pH 2.0 (uncorrected pH measured in D2O). The amid e 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 unordere d structure was observed on thermal denaturation. The results indicate that thermally denatured RNase A contains significant amounts of seco ndary structure (11% helix and 17% beta-sheet), consistent with previo us results reported for circular dichroism, and with a relatively comp act structure, as revealed by dynamic light scattering. These results are in contrast to those of amide protection experiments reported rece ntly [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 discr epancy are given.