  We consider equilibrium folding transitions in lattice protein models with
and without side chains. A dimensionless measure, $Omega_{c}$, is introduced to
quantitatively assess the degree of cooperativity in lattice models and in real
proteins. We show that larger values of $\Omega_{c}$ resembling those seen in
proteins are obtained in lattice models with side chains (LMSC). The enhanced
cooperativity in LMSC is due to the possibility of denser packing of side
chains in the interior of the model protein. We also establish that
$\Omega_{c}$ correlates extremely well with (\sigma = (T_{\theta} -T_{f}
)/T_{\theta}), where (T_{\theta}) and (T_{f}) are collapse and folding
transition temperatures, respectively. These theoretical ideas are used to
analyze folding transitions in various real proteins. The values of $\Omega
_{c}$ extracted from experiments show a correlation with $\sigma $. We conclude
that the degree of cooperativity can be expressed in terms of the single
parameter $\sigma $, which can be estimated from experimental data.
