Peptides intermolecular linking

Kuboki Y, Tsuzaki M, Sasaki S, Liu CF and Mechanic GL (1981) Location of the intermolecular cross-links in bovine dentine collagen, solubilization with trypsin and isolation of cross-link peptides containing dihydroxylysinonorleucine and pyridinoline. Biochem Biophys Res Comm 102, 119-126. 

Simple proteins are composed only of amino-acids linked together via a peptide bond as illustrated above. Note that the molecule as such now has an N-terminal end (free amino group) and a C-terminal end (free carboxylic acid group). Peptides and proteins are differentiated by size and interaction of the molecule—proteins have a peptide backbone which is sufficient for long distance intermolecular interactions to take place"11. This phenomenon occurs at about 2000-3000 molecular weight and above. 

Involvement of esters in the intermolecular cross-links accounts for the dissolution of mature collagen by hydroxylamine, hydrazine or alkali in the presence of hydrogen-bond breakers. Similarly, the participation of hexoses accounts for the dissolution of collagen by periodic acid and a hydrogen-bond breaker. Recently, it has been shown that collagen can be completely dissolved by some proteolytic enzymes (see Section II). These enzymes also cleave the intramolecular cross-links (Kiihn et al, 1963a Rubin et al., 1963). It appears therefore that either the crosslinks include a peptide chain or the chain close to the cross-links contains pepsin- and trypsin-sensitive bonds. 

Proteins consist of 20, in some bacteria 22, different amino acids covalently linked with each other by peptide bonds. The amino acid side chains form many hydrophobic bonds, intermolecular hydrogen bridge bonds, and salt bridges within the protein chains or with another partner molecule. 

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