Computational Study of Packing a Collagen-Like

The lateral packing of a collagen-like molecule, CHJO(Gly-L-Pro-L-Pro),NHCH,, has been examined by energy minimization with the ECEPPN force jield. Two current packing models, the Smith collagen microjibril twisted equilateral pentagonal model and the quasi-hexagonal packing model, have been extensively investigated. In treating the Smith microjibril model, energy minimization was carried out on various conformations including those with the symmetry of equivalent packing, i.e., in which the triple helices were arranged equivalently with respect to each other. Both models are based on the experimental observation of the characteristic axial periodicity, D = 67 nm, of light and dark bands, indicating that, if any superstructure exists, it should consist of jive triple helices. The quasi-hexagonal packing structure is found to be energetically more favorable than the Smith microjibril model by as much as 31.2 kcal/mol of five triple helices. This is because the quasi-hexagonal packing geometry provides more nonbonded interaction possibilities between triple helices than does the Smith microjibril geometry. Our results are consistent with recent x-ray studies with synthetic collagen-like molecules and rat tail tendon, in which the data were interpreted as being consistent with either a quasi-hexagonal or a square-triangular structure.