L-hydroxylysine dihydrochloride ((2S,5R)-5-Hydroxylysine dihydrochloride), an amino acid, is exclusive to collagen protein, which is formed by posttranslational hydroxylation of some lysine residues^[1]. In Vitro: L-hydroxylysine dihydrochloride ((2S,5R)-5-Hydroxylysine dihydrochloride), an amino acid found in collagen, is first identified by Van Slyke and Hiller in protein hydrosylates^[2].