Interpretation of the low-angle meridional neutron diffraction patterns from collagen fibres in terms of the amino acid sequence

Measurement of fully corrected, low angle meridional neutron diffraction intensities from native collagen fibres, in a full range of H₂O/D₂O contrasts, is described. The observed contrast dependence of the intensities of the first 12 orders of 670 A (D) axial periodicity is fitted to a general quadratic theory of contrast variation. The observed first order contrast dependence is compared to predictions based on the amino acid sequence, assuming different extents of chemical H/D exchange, and found to be consistent with complete non-carbon linked H/D exchange except for 1 to 1.6 hydrogen atoms per gly-X-Ytriplet involved in H-bonding. Both X-ray and neutron diffraction data in a variety of contrasts are consistent with a unique model for the axially projected structure of native collagen fibrils based on the amino acid sequence. This model if characterized by average axial residua translations in the NH₂ and COOH terminal, non-triple helical telopeptides, expressed as multiples of the triple helical residue translation, of 0.85 ± 0.05 and 0.7 ± 01 respectively, with D = 235 ± 1 residues.