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Heparin is a strongly sulfated glycosaminoglycan containing alternating glucosamine-derived and uronic acid residues, including iduronic acid and glucuronic acid. Chains vary in length and chemical substitution, so a preparation is not adequately described by one small-molecule molecular formula. Sulfation and carboxylate groups contribute substantial negative electric charge.^[ref]

A defined fragment can test which structural details matter. In 1983, J. Choay and collaborators synthesized related four- and five-sugar fragments and verified their structures with nuclear magnetic resonance. The five-sugar fragment bound antithrombin and enhanced its inhibition of factor Xa, while the tested four-sugar fragment did not.^[ref]

The experiment shows why counting sulfate groups or measuring total charge cannot by itself specify biological activity: a particular arrangement of residues also matters. This is a molecular structure–function example, distinct from selecting a clinical dose or predicting every property of an unfractionated preparation.

J. Choay is the first author of a 1983 study testing which defined fragments of heparin support binding to antithrombin. The team synthesized related four- and five-sugar structures, checked their chemical structures by nuclear magnetic resonance, and measured binding and enhancement of factor Xa inhibition.^[ref]

The tested five-sugar fragment formed an equimolar complex with antithrombin and enhanced its inhibitory activity; the four-sugar fragment did not show the same behavior. Comparing chemically defined fragments helped separate a particular residue arrangement from the heterogeneous mixture of chains in a heparin preparation. Structure confirmation and activity measurement answered different questions, and both were needed for the argument.^[ref]