COLLAGEN-BASED STRUCTURES CONTAINING THE PEPTOID RESIDUE N-ISOBUTYLGLYCINE (NLEU) .6. CONFORMATIONAL-ANALYSIS OF GLY-PRO-NLEU SEQUENCES BY H-1-NMR, CD, AND MOLECULAR MODELING

Molecular modeling, H-1 NMR, and CD were employed to study the structu re and stability of collagen-like triple helices composed of Gly-Pro-N leu 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 Kem p triacid). The presence of collagen-like assembled structures was sup ported by a consistent set of experimental observations, including the appearance of a distinct set of resonances, low hydrogen exchange rat es for Gly NH, KTA signal splitting, cooperative melting transition, a nd analysis of NOESY cross peaks. In this regard, the concept of ensem ble interchain NOEs was introduced and used to establish the close pac king of Gly, Pro, and Nleu residues in triple helices composed of Gly- Pro-Nleu repeats. In addition, the ensemble interchain NOEs gave insig ht into the puckering of the Pro ring and the conformations accessible to the Nleu side chain. The effect of the KTA template on triple heli city 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. Th is free energy gain led to the induction of an assembled collagen-like structure in the KTA conjugate containing six Gly-Pro-Nleu repeats pe r chain and to an increase in thermal stability of the compound contai ning nine Gly-Pro-Nleu repeats per chain.