Conformational Analysis of n→π* Interactions in Collagen Triple Helix Models

Ab initio calculations of three models of collagen at positions Pro-Pro-Gly (1), Pro-Gly-Pro (2), and Gly-Pro-Pro (3) were performed to assess the conformational variation of n ->pi* contributions to the stability of the collagen triple helix. Full conformational analyses by relaxed potential-energy scans of the Psi dihedral angle of the central residue in models 1, 2, and 3 revealed the presence of several n ->pi* interactions. In model 2, with Gly as the central residue, both the Phi and Psi dihedral angles of Gly were scanned. Most minima of each model contained one or two n ->pi* interactions, with pyramidalization at the pi* carbon. We also observed pyramidalization at the n ->pi* donor amide nitrogens. Minima with hydrogen-bond or non-native n ->pi* interactions compete with the collagen stabilizing n ->pi* interactions. The collagen-like n -> re-pi* conformation was found as the global minimum only in model 3. The global minimum of 1 had a 5-membered ring hydrogen bond with an additional weak n -> si-pi* interaction. The global minimum of 2 was in the extended conformation. We predict that the n ->pi* interactions found in native collagen, while individually small, cumulatively contribute to the stability of the triple helix conformation of collagen.