Residual structure in a staphylococcal nuclease fragment. Is it a molten globule and is its unfolding a first-order phase transition?

Temperature-induced unfolding of staphylococcal nuclease and its large fragment, which lacks 13 C-terminal amino acid residues, was studied calorimetrically, and by CD and fluorescence spectroscopy. It was shown that, in contrast to the full length protein which includes two domains and unfolds in two distinct stages under some conditions, the fragment unfolds in one stage. Unfolding of the fragment proceeds in the same temperature range in which the N-terminal beta-barrel domain unfolds in the full length staphylococcal nuclease. Therefore, the fragment is initially partly unfolded. It retains a stable N-terminal domain which unfolds co-operatively with significant heat absorption. Unfolding of the fragment can be regarded as a first-order phase transition, but its initial state certainly does not represent a molten globule, as it was believed.