= Type IV collagen
<Type IV collagen> is a network-forming <collagen> of the <basement membrane>, the specialized <extracellular matrix> underlying an <epithelium> and supporting other tissue interfaces. Triple-helical molecular units associate into a sheet-like scaffold and are reinforced by intermolecular <covalent bonds>.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[2009, abstract and Fig.3, basement-membrane network architecture]{ref}
The chemistry of that reinforcement was identified using <mass spectrometry> and <nuclear magnetic resonance>. Researchers isolated linked <peptides> from adjoining units and identified a <sulfilimine bond> between a <methionine> residue and a <hydroxylysine> residue. Chemical reduction broke the linkage and recovered the constituent <peptides>, supplying a further test of the proposed bond.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[Table 1 and Figs.1–3: exact mass, NMR, reduction and crosslink location]{ref}
The example connects three scales: the identity of an <amino acid> side chain, a chemical link between <proteins>, and stabilization of a supporting <tissue> network. Knowing only that a specimen contains <collagen> leaves these structural and chemical distinctions unresolved.
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