Post-glycosylation

In an effort to address the medicinal chemist s need for new synthetic methods for the preparation of unnatural carbohydrates, new de novo methods for carbohydrate synthesis have been developed. These routes use asymmetric catalysis to set the sugar absolute stereochemistry, a palladium-catalyzed glycosylation reaction to stereoselectively control the anomeric center, and subsequent diastereoselective post-glycosylation to install the remaining sugar stereocenters. The utility of this method has been demonstrated by the syntheses of several classes of mono-, di- and tri-saccharides. 

This de novo approach to carbohydrates has also been applied to oligosaccharides (Schemes 9 to 14). Key to the success of this approach is the development of a mild palladium-catalyzed glycosylation (Scheme 6) in combination with the use of the previously developed highly stereoselective enone reduction and dihydroxylation reaction (Scheme 3) as post-glycosylation transformation for the installation of manno stereochemistry 19). 

An example of the use of a post-glycosylation oxidation strategy is presented by the recent synthesis of monomeric and dimeric repeats of the zwitterionic Type 1 capsular polysaccharide from Streptococcus pneumonia (Spl) depicted in... 

Novel Strategies in the Post-glycosylation of Pre-formed Polymers. 46... 

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) has been widely used in the post-glycosylation of pre-formed polymers, for which the protected aUcyne monomers can be first polymerized by various LRP strategies followed by removal of trimethylsilyl (TMS) protection groups using tetrabutylammonium fluoride (TBAF)/ acetic acid for click reaction with azido functional sugars (Fig. 3) [59, 60]. This approach avoids the use of hazardous azide-functionalized monomers and utilizes the diversity of well-documented azido functional sugars [59]. 

While electrospray is used for molecules of all molecular masses, it has had an especially marked impact on the measurement of accurate molecular mass for proteins. Traditionally, direct measurement of molecular mass on proteins has been difficult, with the obtained values accurate to only tens or even hundreds of Daltons. The advent of electrospray means that molecular masses of 20,000 Da and more can be measured with unprecedented accuracy (Figure 40.6). This level of accuracy means that it is also possible to identify post-translational modifications of proteins (e.g., glycosylation, acetylation, methylation, hydroxylation, etc.) and to detect mass changes associated with substitution or deletion of a single amino acid. 

Both ChEs undergo several post-translational modifications, including glycosylation and glycosylphosphatidy-linositolation (GPI), phosphorylation and carbamylation. 

Experimentation showed that the protein was not glycosylated and that the sequence at the iV-amino acid terminus corresponded to that expected. The C-terminus sequence, however, did not correspond to that predicted and these data were interpreted in terms of the presence of a heterogeneous, truncated, protein. A study of the tryptic digest fragments from this protein with matrix-assisted laser desorption ionization (MALDI) with post-source decay enabled the authors to suggest the positions at which the parent protein had been truncated. 

Post-translation modification Changes that occur to proteins after peptide-bond formation has occurred, e.g. glycosylation and acylation. 

Post-translational modification of proteins plays a critical role in cellular function. For, example protein phosphorylation events control the majority of the signal transduction pathways in eukaryotic cells. Therefore, an important goal of proteomics is the identification of post-translational modifications. Proteins can undergo a wide range of post-translational modifications such as phosphorylation, glycosylation, sulphonation, palmitoylation and ADP-ribosylation. These modifications can play an essential role in the function of the protein and mass spectrometry has been used to characterize such modifications. 

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