Collagens are the most abundant family of extra cellular matrix (ECM) proteins, accounting for two-thirds of the dry mass of adult articular cartilage. Several subtypes have been identified including type II, IX, X, XI, VI, XII, and XIV collagen, however 80 – 90% of the collagen in the body consists of types I, II, and III. Type I collagen is a long thin protein which consists of three coiled subunits: two alpha 1 (I) (α1(I)) chains and one α2(I). Type I collagen forms part of the major structure of the bone, and mutations in the α1(I) or α2(I) genes lead to osteogenesis imperfecta, or brittle-bone disease.
The proline residue at position 5 of this peptide is isotopically labelled with carbon-13 (5) and nitrogen-15 (1), giving this peptide a mass increase of 6 compared to the unlabelled peptide