Far-ultraviolet circular dichroism and uronic acid components of anticoagulant deca-, dodeca-, tetradeca-, and octadecasaccharide heparin fractions.

The therapeutic anticoagulant action of heparin is mediated by the ability of a multifunctional octadecasaccharide region of the molecule to bind to and differentially alter the conformational integrity of antithrombin, and the sugar sequence of the primary binding domain is known. Low ultraviolet circular dichroism spectroscopy of heparin-derived anticoagulant octa-, deca-, dodeca-, tetradeca-, and octadecasaccharides has been useful in elucidating the nature of the sugars that are contained within the second functional domain of the octadecasaccharide. The difference between the spectra of the molar ellipticity of the above sequential oligosaccharides was taken to be the CD spectrum of the corresponding additional disaccharide unit(s). Optical models of the component disaccharides of heparin were derived from CD spectra of heparins having a high degree of sulfation, synthetic glycosides of N-acetylglucosamine, and glycosides of component uronic acids. These were sufficiently distinct in magnitude and spectral position to warrant interpretation of the experimental difference CD spectra. The uronic acids of the disaccharides between deca- and octamer, dodeca- and decamer, and tetradeca- and dodecamer were thereby ascribed to sulfated iduronate, unsulfated iduronate, and glucuronate residues, respectively, while those of the tetrasaccharide between the octadeca- and tetradecasaccharide were tentatively assigned to sulfated iduronate moieties. Interpretation of the difference CD spectra on the basis of the optical models was less certain in regard to the amino sugar components. It appears that the amino sugar derivative between the dodeca- and decamer was N-acetylglucosamine, while the other disaccharides of the octa- to octadecasaccharide probably contained the N-sulfated derivative. A speculative disaccharide sequence drawn from these data indicates that relatively less strongly anionic disaccharides, having nonsulfated uronic acid moieties and N-acetylglucosamine, were flanked by trisulfated disaccharide units, constituting a structural element similar to that which contains the primary binding domain of the anticoagulant.