Differences in the complexation of sodium with methyl esterified carboxymethyl/methoxyacetyl-O-glucans in electrospray ionization-mass spectrometry

Methoxyacetates and (methoxycarbonyl)methyl ethers of alcohols are isomers where only the positions of C=0 and CH2 are interchanged. Therefore, methoxyacetates of methyl esterified carboxymethyl derivatives of glucans were expected to give equal response in electrospray ionization mass spectrometry which is a prerequisite for the quantitative analysis of substitution patterns in carboxymethyl celluloses (CMC). In order to fad out why this is not the case, corresponding derivatives of 1,2-ethanediol and trans-1,2-cyclohexanediol, resembling the 2,3-diol feature of glucose, where studied in ESI-MS. At high total concentration (10(-3) M) in pure acetonitrile, the methoxyacetates of the model compounds were strongly discriminated. With dilution, the relative intensities (RI) of the isomeric analytes progressively dropped to achieve similar to 2 at a total concentration of 10(-10) M. Similar retention in chromatography and surface activity and polarity parameters do not indicate to different surface chemistry of the constitutional isomers as predominant reason for this strong suppression. Quantum-chemical calculations (DFT) showed a difference of binding energies for Na+ between the two diol derivatives of about 50 kJ/mol. While the CM-ethers can involve the more basic carbonyls into the tetra-coordinated sodium complex, this is sterically not possible for the methoxyacetates. At addition of 10% water RI drastically decreased' probably due to change of conditional stability constants of the competing sodium complexes. Adjusting sodium molarities to 10(-4) M up to 10(-2) M caused typical salt effects, but will also influence the formation of sodium adducts. In case of mono- and substantially in di- and oligosaccharides, the number of conformations that can coordinate Na+ are more complex, and the discrepancy between the various substituent patterns decreases, but does not fully disappear.