  Collapse of the polypeptide backbone is an integral part of protein folding.
Using polyglycine as a probe, we explore the nonequilibrium pathways of protein
collapse in water. We find that the collapse depends on the competition between
hydration effects and intra-peptide interactions. Once intra-peptide van der
Waal interactions dominate, the chain collapses along a nonequilibrium pathway
characterized by formation of pearl-necklace-like local clusters as
intermediates that eventually coagulate into a single globule. By describing
this coarsening through the contact probability as a function of distance along
the chain, we extract a time-dependent length scale that grows in linear
fashion. The collapse dynamics is characterized by a dynamical critical
exponent $z=0.5$ that is much smaller than the values of $z=1-2$ reported for
non-biological polymers. This difference in the exponents is explained by the
instantaneous formation of intra-chain hydrogen bonds and local ordering that
may be correlated with the observed fast folding times in proteins.
