NMR study of 13C-kinetic isotope effects at 13C natural abundance to characterize oxidations and an enzyme-catalyzed reduction

13C-kinetic isotope effects (KIEs) of four cinnamyl alcohol oxidations and a xylose reductase-catalyzed cinnamyl aldehyde reduction have been determined by 13C NMR using competition reactions with reactants at natural 13C-abundance. Differences in KIEs among oxidations indicate dissimilarities between the respective hydrogen transfers. Their mechanistic implications are discussed. A low primary KIE of the enzymatic reduction is consistent with a kinetically complex mechanism in which steps other than the chemical step of hydride transfer from NADH are slow.