Factors that Affect the Folding Ability of Proteins

The folding ability of a heteropolymer model for proteins subject to Monte Carlo dynamics on a simple cubic lattice is shown to be strongly correlated with the energy gap between the native state and the structurally dissimilar part of the spectrum. We consider a number of estimates of the energy gap that can be determined without simulation, including the gap in energy between the native and first excited fully compact states for sequences with fully compact native states. These estimates are found to be more robust predictors of folding ability than a parameter $\sigma$ that requires simulation for its evaluation: $\sigma = 1 - T_f/T_\theta$, where $T_f$ is the temperature at which the fluctuation of the order parameter is at its maximum and $T_\theta$ is the temperature at which the specific heat is at its maximum. We show that the interpretation of $T_\theta$ as the collapse transition temperature is not correct in general and that the correlation between $\sigma$ and the folding ability arises from the fact that $\sigma$ is essentially a measure of the energy gap.