Glycopeptides as oligosaccharide mimics: high affinity sialopeptide ligands for sialoadhesin from combinatorial libraries

Two different sialic acid containing glycopeptide (sialopeptide) libraries were synthesized using the portion mixing method and ladder synthesis. The libraries were attached via an IMP spacer and a photolabile linker to PEGA(1900) resin in order to facilitate rapid and unambiguous structural analysis of hits by MALDI-TOFMS. One library contained a lactamized sialic acid moiety at the N terminus of a pentapeptide, while a second library displayed a sialic acid residue at the center of a heptapeptide. The sialopeptide libraries were screened against the recombinant binding domain (SnD1) of a sialic acid binding Ig-like protein, sialoadhesin (Siglec-1). No ligands were identified from the lactamized sialic acid library, underscoring the importance of the carboxylic acid moiety for binding. Screening of the second gave few distinct hits (approximately 0.03% of library) with a high consensus. The high-affinity ligands contained, in most cases, a WG motif following the sialylated Thr. The strength of binding of selected ligands was determined by surface plasmon resonance. The best sialopeptide ligand, WLLT(Sa)WGT, exhibited micromolar affinity of SnD1; >10 times the affinity of SnD1 to 3'-sialyl lactose.