Exploration of Mononucleotides as Fixed Ligands towards Chiral Discrimination of Hexose Monosaccharides by the Kinetic Method

Abstract

The most recent version of the kinetic method, i.e. fixed ligand method, is applied towards chiral discrimination of three pairs of enantiomeric hexose monosaccharides under mass spectral conditions. Naturally occurring mononucleotides are used as fixed ligands (FL) and the amino acids are selected as the chiral references (ref) to discriminate the analyte (A), the enantiomers of glucose, mannose and galactose. Chiral discrimination is achieved by investigating the collision-induced dissociation spectra of trimeric complex ion, [Ni(II)(FL)(ref)(A)-H](+) generated by electrospraying the solution mixture of D- or L- analyte (A), FL, amino acid (ref) and NiCl(2). The relative abundance of product ions resulting from the competitive loss of reference amino acid and analyte are considered for measuring the degree of chiral discrimination by applying the kinetic method. L-Asp, L-Thr, L-Glu, L-Trp and L-Ser are found as suitable reference compounds. Among the tested mononucleotides (5'AMP, 5'GMP, 5'CMP, 5'UMP and 5'TMP), 5'GMP is found to be the best for the studied analytes. Chiral discrimination is found to depend on the nature of the monosaccharide, the functional groups present in the side chain of reference amino acids and the configuration of reference amino acids.

Complete Hexose Isomer Identification with Mass Spectrometry

“…Chiral separation in the fixed ligand kinetic method comes from Equation 7, where R fixed is the fixed ligand branching ratio, R is the gas constant, Δ(ΔG) is the change in Gibbs' free energy, and T eff is the average temperature of the activated complexes [17,18,41,[46][47][48]. Small differences in free energy between the diastereomeric fragment ion complexes will result in larger differences in their branching ratio, R fixed , values.…”

“…Previous monosaccharide work with the Cooks' kinetic method has focused on the R chiral-fixed term for enantiomeric pairs of the Bcommon^sugars (galactose, glucose, mannose, and fructose) [20,41,46,49]. However, none of these studies have incorporated any of the Brare^sugars (allose, altrose, gulose, idose, talose, psicose, sorbose, and tagatose) into their results.…”

“…It is important to note that traces of formic acid were added to the 5′GMPNa 2 stock solution and allowed to sit at room temperature for 2 wk prior to the experiment [46].…”

“…Chiral references and divalent metal cations were initially chosen based on their known ability from previous studies to form the trimeric ion complexes in high abundances, as well as undergo competitive dissociation to form both diastereomeric fragment ions [20,40,41,46,47,49]. Furthermore, transition metals contain d-electronic orbitals that increase chiral recognition as well as being oxophilic, which is preferred for monosaccharide analytes.…”

“…Furthermore, transition metals contain d-electronic orbitals that increase chiral recognition as well as being oxophilic, which is preferred for monosaccharide analytes. Chiral references should have similar size and functional groups (OH) as the analyte of interest [20,40,41,46,47,49]. Although boron readily forms cis-diols with the hydroxyl groups of monosaccharides [38,50,51], B III proved useless as the central metal cation to form the desired trimeric ion complex.…”