Heterogeneity of depolymerized heparin SEC fractions: to pool or not to pool?

Understanding the effect of the counterion on the reverse-phase ion-pair high-performance liquid chromatography (RPIP-HPLC) resolution of heparin-related saccharide anomers

Reverse-phase ion-pair high-performance liquid chromatography (RPIP-HPLC) is an increasingly popular chromatographic technique for the separation of charged compounds, including oligosaccharides derived from the glycosaminoglycans (GAGs) heparin and heparan sulfate (HS). This family of heparin disaccharides has been shown to be useful compounds to probe the details of the RPIP-HPLC separation mechanism, the aspects of which are still being debated. In this manuscript, the effects of ion-pairing reagent (IPR) concentration, counterion, and mobile phase pH on the quality of the RPIP-UPLC separation were examined with particular emphasis on how these factors impact the separation of the disaccharide anomers. These results highlight the role of the IPR counterion and demonstrate that the resolution of the disaccharide anomers can be minimized by conducting the separation at low pH, simplifying chromatographic analysis and improving resolution. The results presented herein can also provide insights into strategies for developing more sensitive and efficient reverse-phase separations for other charged analytes including larger GAG oligosaccharides.

Heparin characterization: challenges and solutions

Although heparin is an important and widely prescribed pharmaceutical anticoagulant, its high degree of sequence microheterogeneity and size polydispersity make molecular-level characterization challenging. Unlike nucleic acids and proteins that are biosynthesized through template-driven assembly processes, heparin and the related glycosaminoglycan heparan sulfate are actively remodeled during biosynthesis through a series of enzymatic reactions that lead to variable levels of O- and N-sulfonation and uronic acid epimers. As summarized in this review, heparin sequence information is determined through a bottom-up approach that relies on depolymerization reactions, size- and charge-based separations, and sensitive mass spectrometric and nuclear magnetic resonance experiments to determine the structural identity of component oligosaccharides. The structure-elucidation process, along with its challenges and opportunities for future analytical improvements, is reviewed and illustrated for a heparin-derived hexasaccharide.

Hyphenated techniques for the analysis of heparin and heparan sulfate

The elucidation of the structure of glycosaminoglycan has proven to be challenging for analytical chemists. Molecules of glycosaminoglycan have a high negative charge and are polydisperse and microheterogeneous, thus requiring the application of multiple analytical techniques and methods. Heparin and heparan sulfate are the most structurally complex of the glycosaminoglycans and are widely distributed in nature. They play critical roles in physiological and pathophysiological processes through their interaction with heparin-binding proteins. Moreover, heparin and low-molecular weight heparin are currently used as pharmaceutical drugs to control blood coagulation. In 2008, the health crisis resulting from the contamination of pharmaceutical heparin led to considerable attention regarding their analysis and structural characterization. Modern analytical techniques, including high-performance liquid chromatography, capillary electrophoresis, mass spectrometry, and nuclear magnetic resonance spectroscopy, played critical roles in this effort. A successful combination of separation and spectral techniques will clearly provide a critical advantage in the future analysis of heparin and heparan sulfate. This review focuses on recent efforts to develop hyphenated techniques for the analysis of heparin and heparan sulfate.

Insights into the capillary electrophoresis separation of heparin disaccharides from nuclear magnetic resonance, pKa, and electrophoretic mobility measurements

Determination of the pK(a) values of heparin disaccharide functional groups can provide insights into the nature of glycosaminoglycan (GAG)-protein interactions and prove useful for optimization of the charged-based separations typically used in GAG analysis. In order to gain a better understanding into the capillary electrophoresis (CE) separation process, the pK(a) values of the carboxylate and primary amine moieties of 11 heparin disaccharide standards were determined through (1)H NMR detected pH titrations. These pK(a) values were used to calculate the effective net charge of each disaccharide and compared to the electrophoretic mobilities measured by CE. Although a different migration order had been reported by other researchers, our results indicate a strong positive correlation between the two measurements, consistent with the migration order observed in our CE separations. The effect of mutarotation was also examined by (1)H NMR. Mutarotation equilibrium constants favored the alpha anomer over the beta conformation. pK(a) values determined for both anomers of the four disaccharide standards containing a GlcN primary amine indicated that the beta anomer of the GlcN residue was more acidic. Partial separation of these anomers was achieved in CE separations using either formic acid or phosphate buffer.