Aggregation of capped hexaglycine strands into hydrogen-bonding motifs representative of pleated and rippled β-sheets, collagen, and polyglycine I and II crystal structures. A density functional theory study.

We compare the energies and enthalpies of inter-action of three- and seven-stranded capped polyglycine aggregates in both the pleated and rippled antiparallel and parallel β-sheet structures as well as the collagenic (three-strand) or polyglycine II-like (seven-strand) forms using density functional theory at the B3LYP/D95(d,p) level. We present the overall interaction energies as broken down into pure H-bonding between the strands at the geometries they assume in the aggregates and the distortion energies required to achieve those geometries starting from the fully relaxed single strands. While the antiparallel sheets represent the most stable structures for both the three- and seven-strand structures, the pure H-bonding interactions are the smallest for these structures. The overall interaction energies are dominated by the energy required to distort the relaxed polyglycine strands rather than the H-bonding energies. The antiparallel β-sheet constrained to C(s) symmetry has a lower enthalpy, but higher energy, of interaction than the fully optimized structure.