科研之友
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摘要
Methyl alpha- and beta-D-glucopyranuronides singly labeled with C-13 at C1-C6 were prepared from the corresponding C-13-labeled methyl D-glucopyranosides, and multiple NMR J-couplings (J(HH), J(CH) and J(CC)) were measured in their protonated and ionized forms in aqueous ((H2O)-H-2) solution. Solvated density functional theory (DFT) calculations of J-couplings in structurally related model compounds were performed to determine how well the calculated J-couplings matched the experimental values in saccharides bearing an ionizable substituent. Intraring J(HH) values in both uronide anomers, including (3)J(H4,H5), are unaffected by solution pD, and COOH ionization exerts little effect on J(CH) and J(CC) except for (1)J(C1,H1), (1)J(C4,H4), (1)J(C5,H5), (1)J(C5,C6) and (2)J(C3,C5), where changes of up to 5 Hz were observed. Some of these changes are associated with changes in bond lengths upon ionization; in general, better agreement between theory and experiment was observed for couplings less sensitive to exocyclic C-O bond conformation. Titration of H-1 and C-13 chemical shifts, and some J-couplings, yielded a COOH pK(a) of 3.0 +/- 0.1 in both anomers. DFT calculations suggest that substituents proximal to the exocyclic COOH group (i.e., the C-4-O-4 bond) influence the activation barrier to C5-C6 bond rotation due to transient intramolecular H-bonding. A comparison of J-couplings in the glucopyranuronides to corresponding J-couplings in the glucopyranosides showed that more pervasive changes occur upon conversion from a COOH to a CH2OH substituent at C6 than from COOH ionization within the uronides. Twelve J-couplings are affected, with the largest being (1)J(C5,C6) (similar to 18 Hz larger in the uronides), followed by (2)J(C6,H6) (similar to 2.5 Hz more negative in the uronides).
- 出版日期2012-11-2
