Solid-state NMR study reveals Collagen I structural modifications of amino-acid side chains upon fibrillogenesis.

In vivo, collagen I, the major structural protein in human body, is found assembled into fibrils. In the present work, we study a high concentrated collagen sample in its soluble, fibrillar and denatured states using one and two dimensional 1H-13C solid-state NMR spectroscopy. We interpret 13C chemical shifts variations in terms of dihedral angle conformation changes. Our data show that fibrillogenesis increases the side chain and backbone structural complexity. Nevertheless, only three to five rotameric equilibria are found for each amino acid residue indicating a relatively low structural heterogeneity of collagen upon fibrillogenesis. Using side chain statistical data we calculate equilibrium constants for a great number of amino acids residues. Moreover, based on a 13C quantitative spectrum we estimate the percentage of residues implicated in each equilibrium. Our data indicate that fibril formation greatly affects hydroxyproline and proline prolyl pucker ring conformation. Finally, we discuss the implication of these structural data and propose a model in which the attractive force of fibrillogenesis comes from a structural reorganization of 10% to 15% of the amino acids. These results allow us to further understand collagen's self-assembling process and fibrillar structure.