  Most helix-coil transition theories can be characterized by a set of three
parameters: energetic, describing the (free) energy cost of forming a helical
state in one repeating unit; entropic, accounting for the decrease of entropy
due to the helical state formation; and geometric, indicating how many
repeating units are affected by the formation of one helical state. Depending
on their effect on the helix-coil transition, solvents or co-solutes can be
classified with respect to their action on these parameters. Solvent
interactions that alter the entropic cost of helix formation by their osmotic
action can affect both the stability (transition temperature) and the
cooperativity (transition interval) of the helix-coil transition. A consistent
inclusion of osmotic pressure effects in a description of helix-coil transition
for poly(L-glutamic acid) in solution with polyethylene glycol can offer an
explanation of the experimentally observed linear dependence of transition
temperature on osmotic pressure as well as the concurrent changes in the
cooperativity of the transition.
