Secondary structure formation precedes tertiary structure in the refolding of ribonuclease A

The kinetics of refolding of ribonuclease A were monitored by circular dichroism (CD), tyrosine fluorescence and absorbance in the -40 to -10 degrees C range using a methanol cryosolvent. The native-like far-ultraviolet CD signal returned in the dead-time of the mixing, whereas the native absorbance and fluorescence signals returned in a multiphasic process at rates several orders of magnitude more slowly. Thus the secondary structure was formed much more rapidly than the tertiary structure. In addition, the absorbance signal showed evidence of an early intermediate in which one, or more, tyrosine residues was in a transiently more polar environment. A total of four kinetic phases were observed by absorbance in refolding, the slowest two of which had energies of activation consistent with proline isomerization. A refolding scheme involving initial hydrophobic collapse, concurrent with secondary structure formation, followed by much slower rearrangement to the native tertiary structure is proposed.