Another role of proline: stabilization interactions in proteins and protein complexes concerning proline and tryptophane

Proline-tryptophan complexes derived from experimental structures are investigated by quantum chemical procedures known to properly describe the London dispersion energy. We study two geometrical arrangements: the ''L-shaped'', stabilized by an H-bond, and the ''stacked-like'', where the two residues are in parallel orientation without any H-bond. Interestingly, the interaction energies in both cases are comparable and very large (similar to 7 kcal mol(-1)). The strength of stabilization in the stacked arrangement is rather surprising considering the fact that only one partner has an aromatic character. The interaction energy decomposition using the SAPT method further demonstrates the very important role of dispersion energy in such arrangement. To elucidate the structural features responsible for this unexpectedly large stabilization we examined the role of the nitrogen heteroatom and the importance of the cyclicity of the proline residue. We show that the electrostatic interaction due to the presence of the dipole, caused by the nitrogen heteroatom, contributes largely to the strength of the interaction. Nevertheless, the cyclic arrangement of proline, which allows for the largest amount of dispersive contact with the aromatic partner, also has a notable-effect. Geometry optimizations carried out for the ''stacked-like'' complexes show that the arrangements derived from protein structure are close to their gas phase optimum geometry, suggesting that the environment has only a minor effect on the geometry of the interaction. We conclude that the strength of proline non-covalent interactions, combined with this residue's rigidity, might be the explanation for its prominent role in protein stabilization and recognition processes.