There is a growing need for biocompatible click reactions in order to prepare multifunctional conjugates, which are valuable molecules for innovative biomedical applications. In this context, we review the recent advances in the implementation of oxime ligation for the synthesis of multivalent or multicomponent systems. The value of these products is emphasized by their use in cell targeting, imaging, synthetic vaccines, and surface modifications.

There has been over the past decades a resurgence of the free-radical thiol-ene coupling (TEC) as a method for assembling crosslinked networks and polymer functionalization. On the other hand the use of TEC in carbohydrate chemistry, a field of special importance due to the key role of carbohydrates in living organisms, is represented only by a handful of papers. Nevertheless it appears that TEC possesses many if not all the attributes of a click process proceeding with the assistance of the greenest catalyst such as visible light. This tutorial review focuses on the application of TEC on different topics, all related to glycochemistry, including: (a) carbohydrate modification, (b) oligosaccharide and glycosyl amino acid synthesis, (c) assembly of glycoclusters on rigid molecular platforms (calixarene, cyclodextrin, silsesquioxane, dendrimer), (d) peptide and protein glycosylation. Also the very recent development in peptide glycosylation by the closely related thiol-yne chemistry is described.

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The detailed structure of the symbiotically important exopolysaccharide succinoglycan from Rhizobium meliloti Rm1021 was determined by mass spectrometry with electrospray ionization and collision-inauced dissociation of the octameric oligosaccharide repeating unit. Previously undetermined locations of the succinyl and acetyl modifications were determined, in respect to both the residue locations within the octamer and the carbon positions within the pyranose ring. Glycosidic linkages determined previously by methylation analysis were also verified.Nitrogen-fixing soil bacterial species of the genus Rhizobium colonize specific host plants through the induction and invasion of root hair nodules. Successful colonization involves a number of chemical messages that control this complex, multistep process. In many Rhizobium-legume symbioses, extracellular polysaccharides are required for nodule invasion (15). Mutants of Rhizobium meliloti RmlO21 that fail to produce the major extracellular polysaccharide of this strain, known as succinoglycan, induce the formation of nodules on alfalfa but are unable to invade these nodules (17). Subsequently, it was demonstrated that specific structural features of this succinoglycan are necessary for nodule invasion. The functional significance of the extracellular polysaccharide structures in nodule invasion motivated an investigation of the details of succinyl and acetyl substitution in R. meliloti succinoglycan (9,10,16,20).The earliest structural studies on R meliloti reported that the succinoglycan contained glucose, galactose, pyruvic acid, and O-acetyl groups in the proportion of 7:1:1:1 (3). The glycosyl linkage composition was determined by methylation analysis, and the O-acetyl group was reported to be located at carbon 6 (referred to herein as C6) of 3-and 4-0-linked glucose residues (3). At this time the succinate modification was not recognized, and the assignment of the O-acetyl group to a 3-0-linked glucose was done in error. Evidence for an octamer repeating unit of the succinoglycan was reported, and the linkage sequence in the main chain was determined by a modified Smith degradation method (13). The hexosyl residues were determined to be f-pyranosidic from the low optical rotation and the 'H nuclear magnetic resonance spectrum of the polysaccharide (13). Most of these structural details for R meliloti Rm1021 were subsequently confirmed as glycosylsequencing methodology developed (1). This report describes the structural characterization of the R. meliloti succinoglycan by electrospray ionization (ES) (7) and collision-induced dissociation and tandem mass spectrometry (CID). The specific residue locations in the octamer repeating unit of the succinyl and acetyl modifications are identified, interior glycosyl linkages are confirmed, and the ring positions for the acetyl and succinyl modifications are determined. Approximately 10 ,ug of starting material was used for methylation and to obtain both an ES mass spectrum and a collision-induced fragmentation spectrum. MA...