Polymorphism of Collagen Triple Helix Revealed by 19F NMR of Model Peptide [Pro-4(R)-Hydroxyprolyl-Gly]3-[Pro-4(R)-Fluoroprolyl-Gly]-[Pro-4(R)-Hydroxyprolyl-Gly]3

We have characterized various structures of (Pro-Hyp R -Gly)3-Pro-fPro R -Gly-(Pro-Hyp R -Gly)3 in the process of cis–trans isomerization and helix–coil transition by exploiting the sole 19F NMR probe in 4(R)-fluoroproline (fPro R ). Around the transition temperature (T m), we detected a species with a triple helical structure distinct from the ordinary one concerning the alignment of three strands. The 19F–19F exchange spectroscopy showed that this misaligned and that the ordinary triple helices were interchangeable only indirectly via an extended monomer strand with all-trans peptide bonds at Pro–fPro R , Pro–Hyp R , and Gly–Pro in the central segment. This finding demonstrates that the helix–coil transition of collagen peptides is not described with a simple two-state model. We thus elaborated a scheme for the transition mechanism of (Pro-Hyp R -Gly) n that the most extended monomer strand can be the sole source both to the misaligned and correctly folded triple-helices. The staggered ends could help misaligned triple helices to self-assemble to higher-order structures. We have also discussed the possible relationship between the misaligned triple helix accumulating maximally at T m and the kinetic hysteresis associated with the helix–coil transition of collagen.