Conformational Preferences of 3-(Dimethylazinoyl)propanoic Acid as a Function of pH and Solvent; Intermolecular versus Intramolecular Hydrogen Bonding

Abstract

The conformational equilibrium of 3-(dimethylazinoyl)propanoic acid (DMAPA, azinoyl ) N + (O -) has a weak pH-dependence in D 2 O, with a slight preference for trans in alkaline solutions. The acid ionization constants of the protonated amine oxide and carboxylic functional groups as determined by NMR spectroscopy were 7.9 × 10 -4 and 6.3 × 10 -6 , respectively. The corresponding value of K 1 /K 2 of 1.3 × 10 2 is not deemed large enough to provide experimental NMR evidence for a significant degree of intramolecular hydrogen bonding in D 2 O. Conformational preferences of DMAPA are mostly close to statistical (gauche/trans ) 2/1) in other protic solvents, e.g., alcohols. However, the un-ionized form of DMAPA appears to be strongly intramolecularly hydrogen-bonded and gauche in aprotic solvents.