Hydrogen-tunneling in biologically relevant small molecules: the rotamerizations of α-ketocarboxylic acids

Quantum mechanical tunneling governs the C-O bond rotamerization of simple alkyl and aryl carboxylic acid conformers at cryogenic temperatures. In this study, we report tunneling investigations on a series of electronically different α-ketocarboxylic acids including glyoxylic, pyruvic, cyclopropylglyoxylic, and phenylglyoxylic acid in solid Ar and Ne as host materials at temperatures ranging from 3 to 20 K. The higher-lying rotamers generated through photoirradiation with wavelengths of λ = 313 nm or λ > 850 nm convert to their low-energy conformers through hydrogen-tunneling, as evident from the time evolution of their infrared spectra, and the complete suppression of this process by deuteration. The conversion rates sensitively depend on the choice of matrix material and the tunneling half-lives range from a few hours to several days and are higher in Ne than in Ar for glyoxylic, pyruvic, and cyclopropylglyoxylic acid. The advent of tunneling in α-ketocarboxylic acids dominates their conformational preferences and conceivably also the reactivity of biologically and pharmacologically relevant acid congeners.