  Using three-dimensional Go lattice models with side chains for proteins, we
investigate the dependence of folding times on protein length. In agreement
with previous theoretical predictions, we find that the folding time grows as a
power law with the chain length N with exponent $\lambda \approx 3.6$ for the
Go model, in which all native interactions (i.e., between all side chains and
backbone atoms) are uniform. If the interactions between side chains are given
by pairwise statistical potentials, which introduce heterogeneity in the
contact energies, then the power law fits yield large $\lambda$ values that
typically signifies a crossover to an underlying activated process.
Accordingly, the dependence of folding time is best described by the stretched
exponential \exp(\sqrt{N}). The study also shows that the incorporation of side
chains considerably slows down folding by introducing energetic and topological
frustration.
