Carbohydrate--protein interactions by 'clicked' carbohydrate self-assembled monolayers

A Huisgen 1,3-dipolar cycloaddition 'click chemistry' was employed to immobilize azido sugars (mannose, lactose, [alpha]-Gal) to fabricate carbohydrate self-assembled monolayers (SAMs) on gold. This fabrication was based on preformed SAM templates incorporated with alkyne terminal groups, which could further anchor the azido sugars to form well-packed, stable, and rigid sugar SAMs. The clicked mannose, lactose, and [alpha]-Gal trisaccharide SAMs were used in the analysis of specific carbohydrate-protein interactions (i.e., mannose--Con A; ECL--lactose, ([alpha]-Gal--anti-Gal). The apparent affinity constant of Con A binding to mannose was (8.7 [+ or -] 2.8) x [10.sup.5] and (3.9 [+ or -] 0.2) x [10.sup.6] [M.sup.-1] measured by QCM and SPR, respectively. The apparent affinity constants of lactose binding with ECL and (x-Gal binding with polyclonal anti-Gal antibody were determined to be (4.6 [+ or -] 2.4) x [10.sup.6] and (6.7 [+ or -] 3.3) x [10.sup.6] [M.sup.-1], respectively by QCM. SPR, QCM, AFM, and electrochemistry studies confirmed that the carbohydrate SAM sensors maintained the specificity to their corresponding lectins and nonspecific adsorption on the clicked carbohydrate surface was negligible. This study showed that the clicked carbohydrate SAMs in concert with nonlabel QCM or SPR offered a potent platform for high-throughput characterization of carbohydrate-protein interactions. Such a combination should complement other methods such as ITC and ELISA in a favorable manner and provide insightful knowledge for the corresponding complex glycobiological processes.