Synthesis of Galactoclusters by Metal-Free Thiol 'Click Chemistry' and Their Binding Affinities forPseudomonas aeruginosaLectin LecA

Mannose-centered galactoclusters specific for lectin I of Pseudomonas aeruginosa (LecA) were synthesised by a combination of phosphoramidite chemistry and metal-free thiol click chemistry (i.e., thiol addition to acrylamide or nucleophilic displacement of bromine in a bromoacetamide group by a thiol function). These thiol click reactions were performed with microwave assistance in the presence of Et3N and with use of a reducing agent to avoid disulfide formation. Nine tetravalent galactoclusters containing different linkers (aliphatic, oligoethyleneglycol or aromatic) were synthesised with a DNA tag. Their binding to LecA was monitored in a DNA-based glycoarray and compared with that of a galactocluster synthesised by copper-catalyzed azide–alkyne cycloaddition. The results indicated stronger binding of all galactoclusters relative to the monovalent galactoside but slightly weaker binding than that shown by the galactocluster incorporating a triazole ring, due to a favourable interaction of the latter with proline 51 of LecA.