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Quinine carbamate chiral stationary phases: Systematic optimization of steric selector‐selectand binding increments and enantioselectivity by quantitative structure‐enantioselectivity relationship studies
Author(s) -
Lämmerhofer Michael,
Franco Pilar,
Lindner Wolfgang
Publication year - 2006
Publication title -
journal of separation science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.72
H-Index - 102
eISSN - 1615-9314
pISSN - 1615-9306
DOI - 10.1002/jssc.200600111
Subject(s) - steric effects , chemistry , enantiomer , carbamate , elution , stereochemistry , quantitative structure–activity relationship , chromatography , organic chemistry
A series of quinine carbamate‐based chiral stationary phases (CSPs) differing solely in the carbamate residue have been devised and a congeneric set of N ‐3,5‐dinitrobenzoyl (DNB) amino acids (AAs) was separated into enantiomers on these CSPs by HPLC using a buffered hydro‐organic mobile phase. Some details on retention and chiral recognition mechanisms have been investigated by application of quantitative structure‐property relationship (QSPR) studies using the linear free energy relationship methodology, i. e. , the extrathermodynamic approach. Retention factors of the high affinity enantiomer (log k 2 ) and enantioselectivities (log α) were correlated with Taft's steric parameter as structural descriptor for the variability in the carbamate and AA residues, and statistically significant QSPR models could be obtained. They confirmed that the variance in the dependent variable (log k 2 , log α) is mainly associated with the steric bulkiness of the selectors' carbamate and of the AA residues. The retention factor of the second eluted enantiomers and the enantioselectivity first increased with steric bulkiness of the carbamate and AA residues, which may be explained by stronger dispersive interactions. After reaching an optimum, the dependent variable, however, declined with further increase of the steric bulkiness of the substituents, probably because of steric hindrance. The variability of the retention factors of the first eluted enantiomer could not be explained by steric descriptors. Instead, it has become obvious that the retention arises mainly from interactions of the DNB‐AA and the quinine carbamate backbone, as it turned out to be more or less constant and solely to a minute amount modulated by the carbamate residue and the AA side chain. The QSPR models were fully in agreement with an earlier postulated chromatographically and spectroscopically derived hypothetical selector‐selectand binding model.