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

Abstract

Methoxyacetates and (methoxycarbonyl)methyl ethers of alcohols are isomers where only the positions of CO 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 find 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 (10−3M) 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 ∼2 at a total concentration of 10−10M. 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 50kJ/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 10−4M up to 10−2M 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.