Commercial gelatins are often produced from bovine and porcine bone and skin, and consist mainly of partially hydrolyzed collagen type 1α1, 1α2 and, to a lesser extent 3α1 (only for skin).
Collagen is the most abundant protein in mammals and as the main component of connective tissues, collagen is an important structural protein. The fibrillar collagens 1 and 3 are composed of hetero- or homotrimer helices with an approximate length of 300 nm. Almost their entire primary structures consist of continuous Gxy amino acid triplet repeats, where G represents glycine and x and y represent any amino acid (often proline, alanine and hydroxyproline, which is a post-translationally modified amino acid). The high hydroxyproline content (approximately 10%), mostly at the 3rd position in the Gxy repeats, is a discriminating feature of collagens in comparison to other proteins. The amino acid sequence of the Gxy-domain is highly conserved, also within the clade which includes collagen 1 and 3. However, evolutionary divergence has offered the opportunity to select unique targets, which can be analyzed using LC-MS, to distinguish between collagens from different animal species and between different related collagen types and to investigate the authenticity of collagen-containing products such as gelatin.