Design and Synthesis of a Macrocyclic Phospholipid




Mitchell, Gavin Maxwell

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The membrane-spanning phospholipids of the domain Archaea are postulated to provide membrane stability; this thesis reports the design and synthesis of a synthetic membrane-spanning macrocyclic lipid to test this hypothesis. Protected glutaric anhydride reacted with 10-undecyn-1-ol to produce a glutarate monoester. Copper-mediated azide-alkyne coupling (CuAAC) using 1,5-diazido-3-oxapentane(bis-azide) afforded a dicarboxylicacid with a hydrophobic chain of sufficient length to span a 35 Å bilayer membrane. The carboxylic acids were each esterified with an equivalent of 10-undecyn-1-ol. After optimization an 87% yield was obtained in the closure of the 72-membered macrocycle with the bis-azide using CuAAC. Deprotection and coupling with p-nitrophenyl phosphorodichloridate completed the synthesis. Two other phospholipids, a linear bolaamphiphile derived from the precursor to the second click reaction, and a linear amphiphile created from a glutarate bis-dodecyl ester, were also synthesized to provide controls for probing the orientation of the macrocyclic phospholipid in the bilayer membrane of vesicles. The amphiphile, linear bolaamphiphile, and macrocyclic bolaamphiphile were synthesized in 4, 5, and 6 steps with yields of 19, 4, and 5% respectively. The hydrophilic head group of the macrocyclic phospholipid was designed to release p-nitrophenolate in basic conditions from the p-nitrophenylphosphate head group to produce an absorbance at 400 nm. This assay was expected to elucidate the membrane-spanning orientation of the phospholipid in the bilayer membrane of vesicles. The final target compound failed to release p-nitrophenolate under basic conditions and underwent phosphate elimination to produce an α, β-unsaturated ester instead. Although the macrocyclic lipid produced associates with membranes and may be membrane-spanning, this lipid design was unable to reveal its membrane orientation.



macrocyclic, phospholipid, membrane-spanning, membrane-stabilizing, synthetic archaea lipid, bolaamphiphile