The internal rotational barriers about NCα and CαC backbone bonds of polypeptides

In many studies on the protein folding problem it is assumed that the internal rotational barriers about NC alpha and (CC)-C-alpha backbone bonds in unfolded polypeptides are quite small, around 0.7 kcal/mol, of an order comparable to the energy of kT at normal temperature (where k is Boltzmann's constant and T is the temperature in K) and hence that rotations about these bonds occur almost freely. Here it is highlighted that such consideration is an unfortunate mistake. Approximate values for the rotational barriers of NC alpha and (CC)-C-alpha bonds are suggested from computations of U(phi, psi) potential energy surface (PES) maps of a number of oligopeptides by a semiempirical method for conformational analysis. The proposed values are about 16 kcal/mol for NC alpha bonds and 6 kcal/mol for (CC)-C-alpha bonds. The values of the same barriers estimated from some ab initio quantum-mechanical PES maps for several dipeptides available in literature are also highlighted.