Doctor of Philosophy (PhD)
The pollen of mugwort is a major contributor to hay fever in Europe and North America. The chemical structure of Art v 1, the major allergen of mugwort, has been elucidated. A notable motif in Art v 1 – characterized by clusters of contiguous â-arabinosides of hydroxyproline – was found to be a key recognition element for antibodies generated in response to the natural protein. This dissertation details the chemical synthesis of oligomers of â-arabinosides of hydroxyproline and the search to establish the minimal carbohydrate epitope of Art v 1. The â-arabinoside motif, being a 1,2-cis glycoside, presents a considerable challenge to organic synthesis. Methods for the stereoselective introduction of furanosides are not as well developed as for their pyranoside counterparts. The key issue pertaining to the formation of glycosidic bonds is the á/â selectivity at the anomeric carbon. To this end, we initially prepared N-tert-butoxycarbonyl-trans-4-hydroxy-L-proline allyl ester to be glycosylated by p-cresyl 2-O-benzyl-3,5-O-(di-tert-butylsilylene)-1-thio-á-L-arabinofuranoside, a conformationaly restricted donor developed for achieving high â-selectivity. Unfortunately, glycosylation with this donor led to complex mixtures and poor yields. Switching to the more robust 2,3,5-O-benzyl-1-thio-á-L-arabinofuranoside donor, we were able to obtain the Ara-Hyp monomer in 60% yield with 4:1 â:á selectivity using silver triflate and N-iodosuccinamide as activators. The â-Ara-Hyp monomer could be isolated by flash chromatography. A dimer of â-Ara-Hyp was prepared by deprotection of the N- and C- termini of the â-Ara-Hyp monomer respectively, after which peptide coupling of the two compounds was performed using HATU as coupling reagent to give the product, Boc-([â-L-Araf]Hyp)2-OAll, in 60% yield. Similar approaches were employed using a [2+1] or [1+2] fragment condensation strategy to produce the trimer, Boc-([â-L-Araf]Hyp)3-OAll, in 35% yield. The tetramer, Boc-([â-L-Araf]Hyp)4-OAll was produced using a [2+2] strategy in 49% yield. We’ve installed terminal amides on the oligomers to best mimic the extended peptide found in the natural allergen. Production of oligomer-specific building blocks (Ac-([â-L-Araf]Hyp)-OMe, Boc-([â-L-Araf]Hyp)-NHMe) allowed a more convergent synthesis towards the end-capped oligomers. With this strategy, end-capped dimer, trimer, and tetramer were synthesized by fragment condensation in 48%, 35%, and 15% respectively. The end-capped glycopeptides could then be fully deprotected by global debenzylation to give the final products in quantative yield. Circular dichroism spectra were obtained for the synthetic glycopeptides. Analysis of the CD spectra showed that the glycosylated proline oligomers exhibit a polyproline type II helical conformation. While CD spectrum of the monomer showed that it was unordered, the elliptical curve of dimer (ëmax = 220 nm, ëmin = 199 nm), trimer (ëmax = 222 nm, ëmin = 203 nm), and tetramer (ëmax = 220 nm, ëmin = 200 nm) all exhibited significant PPII characteristics. Nuclear magnetic resonance spectroscopy was employed to fully characterize all synthetic glycopeptides. Spectra obtained from 1H, 13C, and various 2D NMR was used for comparison with NMR data taken from the natural allergen.
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Xie, Ning, "Oligomers of beta-L-arabinosides of hydroxyproline : synthesis of the carbohydrate epitope of the Art v 1 allergen" (2013). LSU Doctoral Dissertations. 249.