Reversible thermal denaturation of staphylococcal nuclease: a Fourier transformed infrared spectrum study

The secondary structures of staphylococcal nuclease (SNase) have been assigned and semiquantitatively estimated from the deconvoluted Fourier transformed infrared (FTIR) spectrum. The changes in the secondary structures accompanying unfolding and refolding of SNase during reversible thermal denaturation up to 70 degrees C are followed by FTIR measurements. Only slight perturbation was observed up to 35 degrees C. The unfolding transition temperatures of beta-structure and alpha-helix are almost the same at 48.0-48.5 degrees C. During refolding the formation of the beta-structure follows the same pathway but that of the alpha-helix does not, although it recovers its original content almost completely. The final thermally denatured state at high temperature (60-70 degrees C) contains nonrandom structures in the complicated interaction, energetically resembling many kinds of structures. The occurrence of local conformational change before the final cooperative transition to the unfolded state during thermal denaturation as judged by FTIR spectra indicates that the unfolding and refolding of SNase may not follow the typical two-state model.