  Chain-like macromolecules in solution, whether biological or synthetic,
transform from a spatially extended conformation to a compact one upon change
of temperature or solvent qualities. This sharp transition plays a key role in
various phenomena, including DNA condensation, protein folding, and the
behaviour of polymer solutions. In biological processes such as DNA
condensation the collapse is sensitively induced by a small amount of added
molecules. Here we derive a general criterion for the effect of such agents on
conformational transitions. We find two different scenarios depending on chain
stiffness. If the persistence length --the characteristic distance along which
the chain retains its direction-- is smaller than the range of attractive
correlations induced by the agent (typically up to several nanometres), the
chain contracts gradually. Stiffer chains undergo sharp collapse. We thereby
suggest that the enhanced rigidity of double-stranded DNA as compared to the
single strand is a prerequisite for sharp, controllable conformational
transitions.
