DESIGN, SYNTHESIS AND CHARACTERIZATION OF HELICAL OPIOID GLYCOPEPTIDES AND FLUORESCENT DERIVATIVES INCLUDING OPTIMIZATION OF SERINE GLYCOSYLATION UTILIZING SUGAR ACETATES
Committee ChairPolt, Robin
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PublisherThe University of Arizona.
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AbstractOur effort to provide an efficient route to serine glycosides with utility in glycopeptide synthesis has led to the identification of two particularly effective promoters of O-glycosylation. Indium(III) bromide and scandium(III) triflate were shown to be superior promoters of microwave accelerated O-glycosylation utilizing peracetyl carbohydrate donors. 247, 249 These Lewis acids afforded several advantages over previously described promoters including, increased yields, tolerance to moisture, decreased environmental toxicity, ease of work up, and increased reproducibility. Both affected the microwave accelerated glycosylation of Fmoc-ser-OH with sugar peracetates providing superior yields to previously reported methods. For larger scale work the two step route involving the glycosylation of Fmoc-Ser-OBn followed by removal of the benzyl protecting group via hydrogenolysis was preferred. Of the two Lewis acids, the minimally active indium (III) bromide was preferred, as it afforded slightly higher yields and was effective in catalytic quantities. Three groups of helical DAMGO glycopeptide analogs were synthesized in order to provide a better understanding of the structure activity relationships of these opioid peptides. Although the introduction of the amphipathic helix significantly affected binding of the DAMGO message, there was no correlation between binding affinity at the individual opioid receptors and the degree of helicity. In general, addition of the helical address imparted increased affinity for the kappa receptor. The nature of the linker connecting the N-terminal DAMGO sequence and the C-terminal helical address effected binding affinity only slightly. Successive addition of positive charges to the address increased binding at all three opioid receptors until a maximum was reached at a positive two address charge. Although, the amphipathic helix was shown to moderate receptor selectivity, the native mu preference of the DAMGO message was retained Two groups of fluorescent analogs of the mixed δ / μ opioid agonist MD100 were prepared. Within the first series, the fluorescent label was attached to the interior of the address sequence employing the pNZ moiety as a secondary protecting group. The second series of analogs was based on NovaTag™ resin, and allowed for attachment of the fluorophore at the carboxy terminus. The influence on helicity imparted by fluorophore conjugation depended on the nature and point of attachment of the label. The disruption of secondary structure associated with attachment of the fluorescent correlated with decreased binding affinity at the individual opioid receptors. Preliminary in vivo results were encouraging. The least parent like of the MD100 fluorescent analogs was shown to be taken up into endothelial cells. This suggests that the labeled glycopeptides are likely to cross the blood-brain barrier.