The characterization of the glucono-δ-lactone-carboxylic acid equilibrium in the products of chitin-active lytic polysaccharide monooxygenases

Modern biorefining of cellulose and chitin requires the use of glycoside hydrolases (GHs) and lytic polysaccharide monooxygenases (LPMOs). LPMOs use molecular oxygen and two electrons from an external electron donor to cleave glycosidic bonds, by a mechanism that entails oxidization of the C1-carbon of glucose and N-acetylglucosamine, respectively, to produce δ-lactones. The equilibrium between these δ-lactones and their aldonic acids, which dominate at neutral pH, is of importance, since the former are potential inhibitors of GHs. We have used 13C NMR to obtain more insight into the properties of the oxidized compounds and have studied the properties of N,N′,N″-triacetylchitotrionic acid, using 13C NMR at various pHs and temperatures. Thus, we have determined the pKa of the aldonic acid to be 2.88 ± 0.05. The equilibrium constant for lactone at pH 1.41 was 0.137 ± 0.008 corresponding to a ΔG° of 4.9 ± 0.2 kJ/mol. Using Van't Hoff analysis ΔH° and ΔS° were determined to be 19.5 ± 1.6 kJ/mol and 49.0 ± 5.4 J/K mol, respectively.