Chain collapse can occur concomitantly with the rate-limiting step in protein folding.

We have directly characterized the extent of chain collapse early in the folding of protein L using time-resolved small angle X-ray scattering. We find that, immediately after the initiation of refolding, the protein exhibits dimensions indistinguishable from those observed under highly denaturing, equilibrium conditions and that this expanded initial state collapses with the same rate as that of the overall folding reaction. The observation that chain compaction need not significantly precede the rate-limiting step of folding demonstrates that rapid chain collapse is not an obligatory feature of protein folding reactions.