Chiral differentiation of some cyclic β‐amino acids by kinetic and fixed ligand methods

Abstract

Differentiation of beta-amino acid enantiomers with two chiral centres was investigated by kinetic method with trimeric metal-bound complexes. Four enantiomeric pairs of beta-amino acids were studied: cis-(1R,2S)-, cis-(1S,2R)-, trans-(1R,2R)- and trans-(1S,2S)-2-aminocyclopentanecarboxylic acids (cyclopentane beta-amino acids), and cis-(1R,2S)-, cis-(1S,2R)-, trans-(1R,2R)-, and trans-(1S,2S)-2-aminocyclohexanecarboxylic acids (cyclohexane beta-amino acids). The results showed that the choice of metal ion (Cu(2+), Ni(2+)) and chiral reference compound (alpha- and beta-amino acids) had an effect on the enantioselectivity. Especially, aromaticity of the reference compound was noted to enhance the enantioselectivity. The fixed-ligand kinetic method, a modification of the kinetic method, was then applied to the same beta-amino acids, with dipeptides used as fixed ligands. With this method, dipeptide containing an aromatic side chain enhanced the enantioselectivity.

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.…”

“…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.…”

“…The fixed ligands chosen were L-Phe -Gly, 5′CMP, and 5′GMP, based on previous literature [41,[46][47][48]. Fixed ligands must contain some inherent chirality and, preferably, an aromatic group that could increase chiral discrimination from cation-π interactions with the central, divalent metal cation [32,46,47].…”

Enantiomerek királis elválasztása és megkülönböztetése

“…Többféle királis tömegspektrometriás módszer ismert, úgymint kinetikus módszer [92][93][94][95][96][97][98][99][100][101][102][103][104][105], rögzített-ligandum kinetikus módszer [102,[105][106][107][108], ion/molekula reakciók (gazda/vendég komplexek) [109][110][111], valamint egyéb módszerek [112,113]. Ezek közül, a trimer fémkomplexeken alapuló kinetikus módszer [92][93][94][95] …”