A high‐resolution 13C‐nmr study of collagenlike polypeptides and collagen fibrils in solid state studied by the cross‐polarization–magic angle‐spinning method. Manifestation of conformation‐dependent 13C chemical shifts and application to conformational characterization

Abstract We have recorded high‐resolution 13 C‐nmr spectra of collagen fibrils in the solid state by the cross‐polarization–magic‐angle‐spinning(CP–MAS)method and analyzed the spectra with reference to those of collagenlike polypeptides. We used two kinds of model polypeptides to obtain reference 13 C chemical shifts of major amino acid residues of collagen (Gly, Pro, Ala, and Hyp): the 3 1 ‐helical polypeptides (Gly) n II, (Pro) n II, (Hyp) n , and (AlaGlyGly) n II, and the triple‐helical polypeptides (ProGlyPro) n and (ProAlaGly) n . Examination of the 13 C chemical shifts of these polypeptides, together with our previous data, showed that the 13 C chemical shifts of individual amino acid residues are the same, within experimental error (±0.5 ppm), among different polypeptides with different primary sequences, if the conformations are the same. We found that the 13 C chemical shifts of Ala residues of the 3 1 ‐helical (AlaGlyGly) n and triple‐helical (ProAlaGly) n are significantly displaced, compared with those of the α‐helix, β‐sheet, and silk I form, and can be utilized as excellent probes to examine conformational features of collagen‐like polypeptides. Further, the 13 C chemical shifts of Gly and Pro residues in the triple‐helical polypeptides are substantially displaced from those found in (Gly) n II and (Pro) n II of the 3 1 ‐helix, reflecting further conformational change from the 3 1 ‐helix to the supercoiled triple helix. In particular, the 13 C chemical shifts of Gly C  O carbons of the triple‐helical polypeptides are substantially displaced upfield (4.1–5.1 ppm), with respect to those of the 3 1 ‐helical polypeptides. These displacements are interpreted by that Gly C  O of the former is not involved in NH …︁ O  C hydrogen bonds, while this carbon of the latter is linked by these kinds of hydrogen bonds. On the basis of these 13 C chemical shifts, as reference data for the collagenlike structure, we were able to assign the 13 C‐nmr peaks of Gly, Ala, Pro, and Hyp residues of collagen fibrils, which are in good agreement with the values expected from the model polypeptides mentioned above. We also discuss a plausible conformational change of collagen fibrils during denaturation.