J Phys Chem B. 2022 Nov 10. doi: 10.1021/acs.jpcb.2c04986. Online ahead of print.
ABSTRACT
MA’AT analysis uses ensembles of redundant experimental NMR spin-coupling constants, parametrized J-coupling equations obtained from density functional theory (DFT) calculations, and circular statistics to produce probability distributions of molecular torsion angles in solution and information on librational motions about these angles (Meredith et al., J. Chem. Info. Model. 2022, 62, 3135-3141). Current DFT methods give nearly quantitative two- and three-bond JHH, JCH, and 1JCC values for use in MA’AT analysis of saccharides. In contrast, the accuracy of DFT-calculated one-bond 1JCH and 1JCC values is more difficult to determine, preventing their use in MA’AT modeling. This report describes experimental and computational studies that address this problem using two approaches (Strategies 1 and 2). Differences [1JCHcalc – 1JCHexp] (Strategy 1) ranged from -1.2 to 2.5 Hz, giving an average difference of 0.8 ± 1.7 Hz. Percent differences ranged from -0.8% to 1.6%, giving an average % difference of 0.5 ± 1.1%. In comparison, [1JCHMA’AT – 1JCHexp] (Strategy 2) ranged from -1.8 to 0.2 Hz, giving an average difference of -1.2 ± 0.7 Hz. Percent differences ranged from -1.2% to 0.1%, giving an average % difference of -0.8 ± 0.5%. Strategy 1 gave an average difference of 2.1 Hz between calculated and experimental 1JCC values, with an average % difference of 5.1 ± 0.2%. Calculated 1JCC values were consistently larger than experimental values. Strategy 2 also gave calculated 1JCC values that were larger than the experimental values, with an average difference of 2.3 ± 0.6 Hz, and an average % difference of 5.6 ± 1.6%. The findings of both strategies are similar and indicate that 1JCH values in saccharides can be calculated nearly quantitatively, but 1JCC values appear to be consistently overestimated by ∼5% using current DFT methods.
PMID:36356177 | DOI:10.1021/acs.jpcb.2c04986
