Collagen-Based Structures Containing the Peptoid Residue N-Isobutylglycine (Nleu). 6. Conformational Analysis of Gly-Pro-Nleu Sequences by 1H NMR, CD, and Molecular Modeling

Molecular modeling, 1H NMR, and CD were employed to study the structure and stability of collagen-like triple helices composed of Gly-Pro-Nleu repeats. The compounds studied include the acetyl analogs Ac-(Gly-Pro-Nleu) n -NH2 (where n = 1, 6, 9) and the KTA conjugates KTA-[Gly-(Gly-Pro-Nleu) n -NH2]3 (where n = 1, 3, 6, 9 and KTA denotes the Kemp triacid). The presence of collagen-like assembled structures was supported by a consistent set of experimental observations, including the appearance of a distinct set of resonances, low hydrogen exchange rates for Gly NH, KTA signal splitting, cooperative melting transition, and analysis of NOESY cross peaks. In this regard, the concept of ensemble interchain NOEs was introduced and used to establish the close packing of Gly, Pro, and Nleu residues in triple helices composed of Gly-Pro-Nleu repeats. In addition, the ensemble interchain NOEs gave insight into the puckering of the Pro ring and the conformations accessible to the Nleu side chain. The effect of the KTA template on triple helicity was studied and shown to consist in a net gain in the free energy of triple-helix formation, as also seen for Gly-Pro-Hyp sequences. This free energy gain led to the induction of an assembled collagen-like structure in the KTA conjugate containing six Gly-Pro-Nleu repeats per chain and to an increase in thermal stability of the compound containing nine Gly-Pro-Nleu repeats per chain.