Cooperative Hydration of Pyruvic Acid in Ice

Abstract

About 3.5 ± 0.3 water molecules are still involved in the exothermic hydration of 2-oxopropanoic acid (PA) into its monohydrate (2,2-dihydroxypropanoic acid, PAH) in ice at 230 K. This is borne out by thermodynamic analysis of the fact that Q_H(T) = [PAH]/[PA] becomes temperature independent below ∼250 K (in chemically and thermally equilibrated frozen 0.1 ≤ [PA]/M ≤ 4.6 solutions in D₂O), which requires that the enthalpy of PA hydration (ΔH_H ∼ −22 kJ mol^-1) be balanced by a multiple of the enthalpy of ice melting (ΔH_M = 6.3 kJ mol^-1). Considering that: (1) thermograms of frozen PA solutions display a single endotherm, at the onset of ice melting, (2) the sum of the integral intensities of the ¹δ_PAH and ¹δ_PA methyl proton NMR resonances is nearly constant while, (3) line widths increase exponentially with decreasing temperature before diverging below ∼230 K, we infer that PA in ice remains cooperatively hydrated within interstitial microfluids until they vitrify.