Collagen is one of most abundant protein of the extracellular matrix in mammals [1]. Its primary chemical structure is composed of the Xaa-Yaa-Gly amino acid triad, where Xaa and Yaa are mostly occupied by L-proline (Pro or P) and 4(R)-hydroxy-L-proline (Hyp or O) amino acids, respectively [2]. The unique characteristic of collagen is the ability of single peptide strands to acquire a polyproline type II (PPll) helical conformation, in which three of those strands can come together and form a stable triple helix [3]. The triple helical structural motif is critical for the subsequent self-assembly into high order fibers and fibrous networks in nature.
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