Exploration of the Amadori Rearrangement for Bioconjugation of Carbohydrates

For the investigation of the multi-facetted biological function of carbohydrates, which are ubiquitously found on cell surfaces as well as in soluble glycoconjugates, reliable methods to ligate saccharides to biomolecules and to specific labels are required. For many applications it is essential that such ligation chemistries are functional in an aqueous medium and without the requirement of protecting groups on the carbohydrate part - a considerable challenge in synthetic chemistry. It is the aim of this project to explore the Amadori rearrangement (AR) for the bioconjugation of saccharides to amines in order to add a new "natural" tool to the available bioconjugation methods. To date, a broad application of this reaction is not available, however, according to its mechanism, the scope of the Amadori rearrangement could well comprise a variety of useful conjugation reactions, including mono- as well as oligo- and polyvalent amines, and the carbohydrate decoration of functionalized surfaces. Here, the expertise of two groups from Germany and Austria will be combined to elaborate applications of the Amadori rearrangement in a selection of highly relevant examples. The tasks for the Austrian team of this application includes: 1. Development of a reliable protocol for employment of mono- as well as oligovalent amines to the Amadori rearrangement of suitable aldoheptoses. 2. Fabrication of complex glycoconjugates as well as glyco-decorated surfaces employing the AR. 3. Development of reaction protocol of the AR under physiological conditions in water. 4. Introduction of photosensitive substituents, i.a., diazirines and azobenzenes to an aldose starting material in order to obtain photoresponsive materials. By this strategy C-glycosidic linkages between the carbohydrate moiety and the aglycon part will be provided, which are known to be more stable against hydrolysis than the natural O- and N-linkages. Such structures can be embedded as monomeric units for the synthesis of glycopolymers, glycoclusters as well as -dendrimers. Furthermore decoration with a variety of reporter groups such as fluorescent dyes, affinity labels and other substituents as required for biological applications can be achieved easily by the developed method. Additionally, the concept will be investigated for the utility as site-selective peptide modification. The glycoconjugates synthesised in this project will be biologically evaluated as inhibitors of type 1-fimbriated E. coli bacteria by Prof. Lindhorst (Christiana Albertina University, Kiel, Germany). The method developed in this project will contribute a strategy to the repertoire of synthetic access of glycoconjugates mimetics and thereby extend the tool box of possible structures for promoting investigations in the field of Glycoscience in a variety of different research fields.

The ERA-CHEMISTRY project with the title Exploration of the Amadori Rearrangement for Bioconjugation of Carbohydrates evaluated the Amadori rearrangement as method for the chemical conjugation of saccharides to amines in order to add a tool to the available synthetic repertoire. The Austrian (sub) project in this collaborative research project has been majorly the development of a reliable synthetic protocol for the Amadori rearrangement as conjugation method for carbohydrate moieties to amino components. By this strategy a C-glycosyl type linkage between the carbohydrate moiety and the aglycon part is accessible, which is known to be much more stable against hydrolysis in cellular environment than the natural O- and N-linkages. In this respect, the scope and limitations of the Amadori Rearrangement were thoroughly investigated. A reliable protocol for the employment of mono-, di- and oligovalent amines as well as complex amino sugars as amino components in the Amadori rearrangement with suitable sugar substrates has been developed. Selected glycoconjugates synthesised in this project have been biologically evaluated as ligands for type 1-fimbriated E. coli bacteria in order to prove their suitability and potential application in the anti-adhesion therapy of bacterial infections. Additionally, selected compounds have been tested as inhibitors/ligands for a panel of different lectins, carbohydrate recognising proteins, from human, plant and bacterial sources. Furthermore, amino acids as well as short peptides have been successfully employed in the Amadori Rearrangement in order to probe the concept for its utility as site- selective peptide modification. Based on the results obtained from the biological evaluation of Amadori products as suitable inhibitors/ligands of lectines, such structures can find application in order to address medicinal and biochemical questions in the research field of Glycosciences.