Circular Dichroism Evidence for the Presence of Burst-Phase Intermediates on the Conformational Folding Pathway of Ribonuclease A

Refolding of the very-fast-folding unfolded species (Uvf) of disulfide-intact bovine pancreatic ribonuclease A has been monitored by circular dichroism (CD) at 222 and 275 nm at 0.9 or 2.6 M guanidine hydrochloride, pH 7.0, and 5 degrees C. The refolding of Uvf represents a purely conformational folding process which is not complicated by cis-trans proline isomerization. The data indicate that there are at least two intermediates on the refolding pathway of Uvf and that both intermediates form in the burst phase when the refolding is monitored by CD. At the initiation of folding, Uvf is converted to a largely unfolded intermediate, termed Iu, which then undergoes a hydrophobic collapse to form the molten-globule-like intermediate I phi. The CD values obtained for Iu and I phi indicate that IU has no significant secondary structure and presumably differs from Uvf by a local structural rearrangement, while I phi has a substantial population of secondary and tertiary structures, about 40%-50% of that of native.