Amide Cis-Trans Isomerization in Aqueous Solutions of Methyl N-Formyl-D-glucosaminides and Methyl N-Acetyl-D-glucosaminides: Chemical Equilibria and Exchange Kinetics

摘要

Amide cis-trans isomerization (CTI) in methyl 2-deoxy-2-acylamido-D-glucopyranosides was investigated by ~1H and ~(13)C NMR spectroscopy. Singly ~(13)C-labeled methyl 2-deoxy-2-formamido-D-glucopyranoside (MeGlcNFm) anomers provided standard ~1H and ~(13)C chemical shifts and ~1H-~1H and ~(13)C-~(13)C spin-coupling constants for cis and trans amides that are detected readily in aqueous solution. Equipped with this information, doubly ~(13)C-labeled methyl 2-deoxy-2-acetamido-D-glucopyranoside (MeGlcNAc) anomers were investigated, leading to the detection and quantification of cis and trans amides in this biologically important aminosugar. In comparison to MeGlcNFm anomers, the percentage of cis amide in aqueous solutions of MeGlcNAc anomers is small (～23% for MeGlcNFm versus ～1.8% for MeGlcNAc at 42 ℃) but nevertheless observable with assistance from ~(13)C-labeling. Temperature studies gave thermodynamic parameters ΔG°, ΔH°, and ΔS° for cis-trans interconversion in MeGlcNFm and MeGlcNAc anomers. Cisltrans equilibria depended on anomeric configuration, with solutions of a-anomers containing less cis amide than those of β-anomers. Confirmation of the presence of cis amide in MeGlcNAc solutions derived from quantitative ~(13)C saturation transfer measurements of CTI rate constants as a function of solution temperature, yielding activation parameters E_(act), ΔG°~≠, ΔH°~≠, and ΔS°~≠ for saccharide CTI. Rate constants for the conversion of trans to cis amide in MeGlcNFm and MeGlcNAc anomers ranged from 0.02 to 3.59 s~(-1) over 31-85 ℃, compared to 0.24-80 s~(-1) for the conversion of cis to trans amide over the same temperature range. Energies of activation ranged from 16-19 and 19-20 kcal/mol for the cis→ trans and trans→ cis processes, respectively. Complementary DFT calculations on MeGlcNFm and MeGlcNAc model structures were conducted to evaluate the effects of an acyl side chain and anomeric structure, as well as C2-N2 bond rotation, on CTI energetics. These studies show that aqueous solutions of GlcNAc-containing structures contain measurable amounts of both cis and trans amides, which may influence their biological properties.