Quantum theory of the structure and bonding in proteins : Part 15. The threonine dipeptide

The energies of the conformations of the threonine dipeptide are determined by adding the energetic effects of the methyl group to the results for the serine dipeptide. The results are then checked by explicit ab initio molecular orbital computations on the important low-energy conformations. Analysis of these results yields the non-bonded repulsions, the torsion barriers and the hydrogen bonds.

There are many similarities between the serine results and those for threonine. In particular, the computations show that the +60° value of χ ₁ is again important in the helix bridge region of the Ramachandran map. On the other hand, there is a pronounced difference between serine and threonine in that the methyl group of the latter causes the form with χ₁ = 180° to be a high-energy conformation over all areas of the Ramachandran map except the left sheet region. These and other points are in accord with the experimental data for five proteins.