= Hydroxylysine
<Hydroxylysine> is a hydroxylated derivative of the <amino acid> <lysine>. In the important collagen-associated form, a <hydroxyl group> is attached at carbon 5 of the side chain. Free 5-hydroxylysine has formula $\mathrm{C_6H_{14}N_2O_3}$, and its connectivity can be written
$$
\mathrm{HOOC{-}CH(NH_2){-}CH_2{-}CH_2{-}CH(OH){-}CH_2{-}NH_2}.
$$
This formula specifies the arrangement of the groups but does not by itself distinguish the possible <stereoisomers>. https://www.ebi.ac.uk/chebi/CHEBI:18040[ChEBI entry for erythro-5-hydroxy-L-lysine: definition, formula, connectivity and stereochemistry]{ref}
In <collagen>, <hydroxylysine> occurs as a modified <amino acid> residue within a <protein>. A concrete example is the <sulfilimine bond> linking hydroxylysine-211 of one <Collagen IV> unit to methionine-93 of another. <Mass spectrometry> and <nuclear magnetic resonance> identified that <sulfur>–<nitrogen> connection.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[abstract and Figures 1–3]{ref} The example shows why a modified side chain matters: changing a particular residue changes the chemistry available for joining <protein> units. Free <hydroxylysine> and a residue incorporated into <collagen> must nevertheless be distinguished, because incorporation places its backbone in <peptide bonds>.
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