Office of Postgraduate Studies and Research
presents
Investigations into the Novel Behaviour and Chemical Characteristics of 2,6-Pyridine Diacetic Acid an Oral Defence by Ms. Shivani Boodram

2,6-Pyridine Diacetic Acid (PDA) is an underexploited member of the pyridine dicarboxylic family of ligands. Thus, it may have similar benefits to its analogue 2,6 pyridine dicarboxylic acid (dipic). Dipic is a favoured ligand because of its low toxicity, amphiphilicity and chelation to biologically relevant metals. Prior to this study, the use of PDA for macrocycle formation was not yet reported. A novel macrocyclic compound (4,10-Dioxa-7,17-diaza-bicyclo[11.3.1] heptadeca-1(16),13(17),14-triene-3,11-dione) was synthesised using PDA as a component. However, the macrocyclic product yield remained low despite several attempts at optimisation. Consequently, further studies on PDA were done which lead to the first report of a solvent-dependent intramolecular proton transfer (IPT) and acid-base equilibria in PDA. These phenomenon were found to be contributing factors to a solid state reversible, structural transformation and a spontaneous double decarboxylation. Differential scanning calorimetry (DSC) on PDA, exhibited several reversible phase transitions, with the most pronounced change occurring at 168 K. A reversible structural transformation, from the monoclinic system to the triclinic one, was confirmed via temperature dependent X-ray diffraction studies. In water, pH dependant absorbance shifts were observed. NMR spectroscopy provided evidence of the solvent dependent, irreversible conversion of PDA to the to 2,6-dimethylpyridine via sequential, facile decarboxylations. The ΔS‡ for each decarboxylation were 39.53 ± 3.87 and 23.56 ± 2.84 JK-1mol-1 respectively. Finally, the rate constants for observed D₂O exchange with the α-carbon protons were determined as 1.02 ± 0.12 x 10-3 s-1 and 6.13 ± 0.12 x 10^-4 s^-1 with a double exponential fit.