  Proteins are minimally frustrated polymers. However, for realistic protein
models non-native interactions must be taken into account. In this paper we
analyze the effect of non-native interactions on the folding rate and on the
folding free energy barrier. We present an analytic theory to account for the
modification on the free energy landscape upon introduction of non-native
contacts, added as a perturbation to the strong native interactions driving
folding. Our theory predicts a rate-enhancement regime at fixed temperature,
under the introduction of weak, non-native interactions. We have thoroughly
tested this theoretical prediction with simulations of a coarse-grained protein
model, by employing an off-lattice $C_\alpha$ model of the src-SH3 domain. The
strong agreement between results from simulations and theory confirm the non
trivial result that a relatively small amount of non-native interaction energy
can actually assist the folding to the native structure.
