  We report circular dichroism measurements on the helix-coil transition of
poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding
agent. Using small angle neutron scattering, PEG solutions have been
characterized and found to be well described by the picture of a transient
network of mesh size $\xi$, usual for semi-diluted chains in good solvent. We
show that the increase of PEG concentration stabilizes the helices and
increases the transition temperature. But more unexpectedly we also notice that
the increase of crowding agent concentration reduces the mean helix extent at
the transition, or in other words reduces its cooperative feature. This result
cannot be accounted for by an entropic stabilization mechanism. Comparing the
mean length of helices at the transition and the mesh size of the PEG network,
our results strongly suggest two regimes: helices shorter or longer than the
mesh size.
