Influenza virus infection is initiated by the binding of hemagglutinin (HA), a viral carbohydrate-binding membrane protein, to sialic acid-containing oligosaccharides, such as GM3 trisaccharide [Neu5Aca(2,3)Galb(1,4)Glc], on the host cell surfaces.[1] Since this molecular-recognition process leads to the host cell-virus adhesion stage, molecules with a high affinity for the viral HA would be potent candidates for blockers of the influenza virus. Various types of HA blockers carrying sialooligosaccharides have been synthesized over the last decade.[2] Enhanced blocking effects by the multidentate (multivalent) glycoligands were often found by clustering [3] the sialooligosaccharides on macromolecular scaffolds, such as synthetic polymers, [4] cyclodextrins, [5] and dendrimers, [6] although the mechanism of this multiple interaction has not yet been fully investigated. In these macromolecular approaches, structural information on the spatial arrangement of the multiple binding sites of the protein is not usually accounted for in the design of ligands. Recently, it was found that ligands containing small oligodentate carbohydrate units, that are designed topologically to bind simultaneously to several binding sites of a single lectin, are very effective blockers. [7,8] The syntheses of the STARFISH dendrimer used to inhibit the adherence of verotoxins, is an excellent example of how detailed structure analysis has led to the creation of a strongly effective "multidentate" ligand. [7, 2e] Thus, on the basis of the HA structure, we created cyclic peptide scaffolds containing tridentate carbohydrate units as a potential HA blockers by using a convenient chemoenzymatic method. HA is a trimeric protein and each subunit contains one sialooligosaccharide binding site on the outer portion. From the three-dimensional structure of HA, as revealed by X-ray analysis, [9] these three binding sites are located on the apexes of an equilateral triangle, in which each binding pocket is separated by approximately 40-50 . To achieve a rational molecular design for an HA blocker, we focused on the use of cyclic peptides [10] as a scaffold because of the flexibility of the synthetic strategy and their biological compatibility in the human body.Herein, we report a chemoenzymatic synthesis of a series of cyclic peptides presenting three sialotrisaccharide units [Neu5Aca(2,3)Galb(1,4)Glc] by using transglutaminase and glycosyltransferase (Scheme 1). We also discuss the effect of peptide sequence on their biological activities.Scheme 1 shows the synthetic route to cyclic glycopeptides by means of a combined chemical and enzymatic strategy. In this case, a cyclic peptide 1 prepared by employing the Fmoc-Asp-ODmab method [11] on a chlorotrityl resin [12] was enzymatically coupled with 6-aminohexyl lactoside (4) using transglutaminase from guinea pig liver.[13] The transamination reaction between three Gln residues of cyclic peptide 1 and 4 (large excess) gave a mixture of mono-(1-m), bi-(1-b), and tri-substituted (1-t) derivatives. After...
