Acetyl-Terminated and Template-Assembled Collagen-Based Polypeptides Composed of Gly-Pro-Hyp Sequences. 3. Conformational Analysis by 1H-NMR and Molecular Modeling Studies

Using 1D and 2D 1H-NMR and molecular modeling, we characterized the conformational features of template-assembled collagen-like polypeptides of the type KTA-[Gly-(Gly-Pro-Hyp) n -NH2]3 (n = 1, 3, 5, 6; KTA denotes the Kemp triacid) and of the corresponding acetylated single-chain polypeptides Ac-(Gly-Pro-Hyp) n -NH2 (n = 1, 3, 5, 6, 9) in water. We established the presence of triple-helical conformations on the basis of consistent experimental observations including the appearance of a set of distinct assembled resonances and the measurement of low hydrogen-exchange rates for the assembled Gly NH of the longer chain analogs. In addition, following the pioneering work of Brodsky et al. we proved the consistency of the NOESY spectra with the interchain NOEs anticipated by the X-ray model for triple-helical (Gly-Pro-Hyp) sequences. For the KTA-terminated structures the triple helicity is further supported by the KTA signal splitting detected for KTA-[Gly-(Gly-Pro-Hyp) n -NH2]3 (n = 3, 5, 6) and caused by the triple-helical screw symmetry which breaks the rotational symmetry of KTA. Thermal melting studies indicate that the KTA template leads to a significant gain in the free energy of triple-helix formation. This free energy gain results in a remarkable increase of the thermal stabilities of the KTA terminated compounds as compared to the acetyl analogs. Our NMR results are fully consistent with our previous investigations based on CD, UV, and optical rotation spectroscopic methods.