On-line enrichment of N-glycans by immobilized metal-affinity monolith for capillary electrophoresis analysis

A novel N-glycan enrichment strategy is presented using unexpected but strong interactions between the sulfonate groups brought by the fluorescent dye of glycans and the Zr⁴⁺ modified poly(ethylene glycol methacrylate phosphate (EGMP)-co-acrylamide (AM)-co-bis-acrylamide (BAA)) monolith. The poly (EGMP-co-AM-co-BAA) monolith was synthesized via ultraviolet (UV) irradiation and then functionalized with Zr⁴⁺. The obtained monolith was characterized with scanning electron microscopy and mercury intrusion porosimetry. Large through-pores and a continuous skeleton with high permeability were observed. The N-glycans were labeled with the 1-aminopyrene-3, 6, 8-trisulfonic acid (APTS) and enriched by the Zr⁴⁺ modified monolith through IMAC interaction. This enrichment step was then coupled off-line to capillary electrophoresis (CE) separation with laser induced fluorescence (LIF) detection. Successful preconcentration of the APTS labeled maltooligosaccharide ladder was achieved under optimized conditions. Enrichment factors obtained for the maltooligosaccharides ranged from 9 to 24 with RSDs from 2.0% to 9.2% (n = 3). Moreover, very good repeatabilities (<6.7%) were obtained for glucose oligomers (4-15 glucose units) corresponding to sizes expected for N-glycans, demonstrating the great potential of this Zr⁴⁺ modified monolith to enrich APTS labeled glycans from N-glycoproteins. The proposed method was then successfully applied for the enrichment of N-glycans released from Ribonuclease B, in which case all five expected oligomannose glycans (Man 5 to Man 9) were successfully enriched. Thanks to the advantage of the method to enrich selectively APTS-glycans compared to the commercial SPE columns composed of HILIC or PGC materials, the first proof of concept of on-line enrichment coupled to CE-LIF separation was demonstrated for maltooligosaccharides as well.