FT-IR STUDIES ON THERMAL-DENATURATION PROCESSES OF RIBONUCLEASES-A AND RIBONUCLEASES-S IN H2O AND D2O SOLUTIONS

Thermal denaturation processes of bovine pancreatic ribonucleases A and S in neutral aqueous solutions were studied by Fourier transform-infrared (FT-IR) difference spectroscopy and Fourier self-deconvolution. Although the infrared spectra of the ribonucleases are almost identical at room temperature, they show different temperature dependences. From the plots of absorbance changes of the amide I band against temperature, it is shown that (1) denaturation occurs abruptly at 60-degrees-C for ribonuclease A and at 52-degrees-C for ribonuclease S in H2O solution, although gradual absorbance changes also take place below (and probably also above) the denaturation temperatures, and (2) the denaturation of ribonuclease A is reversible, whereas that of ribonuclease S is not. All the deconvoluted bands, except for very broad bands arising from the denatured proteins, show abrupt and simultaneous losses of intensity at the denaturation temperatures. These results are basically in agreement with the two-state transition model.