Institute for Analytical Chemistry, University of Vienna, Vienna, Austria.
J Sep Sci. 2012 Jul;35(13):1560-72. doi: 10.1002/jssc.201200103.
The intramolecular distances of anion and cation exchanger sites of zwitterionic chiral stationary phases represent potential tuning sites for enantiomer selectivity. In this contribution, we investigate the influence of alkanesulfonic acid chain length and flexibility on enantiomer separations of chiral acids, bases, and amphoteric molecules for six Cinchona alkaloid-based chiral stationary phases in comparison with structurally related anion and cation exchangers. Employing polar-organic elution conditions, we observed an intramolecular counterion effect for acidic analytes which led to reduced retention times but did not impair enantiomer selectivities. Retention of amphoteric analytes is based on simultaneous double ion pairing of their charged functional groups with the acidic and basic sites of the zwitterionic selectors. A chiral center in the vicinity of the strong cation exchanger site is vital for chiral separations of bases. Sterically demanding side chains are beneficial for separations of free amino acids. Enantioseparations of free (un-derivatized) peptides were particularly successful in stationary phases with straight-chain alkanesulfonic acid sites, pointing to a beneficial influence of more flexible moieties. In addition, we observed pseudo-enantiomeric behavior of quinine and quinidine-derived chiral stationary phases facilitating reversal of elution orders for all analytes.
手性固定相中两性离子的阴离子和阳离子交换位点的分子内距离代表了对映体选择性的潜在调谐位点。在本研究中，我们研究了烷磺酸链长和柔性对六种基于金鸡纳生物碱的手性固定相对手性酸、碱和两性分子的对映体分离的影响，并与结构相关的阴离子和阳离子交换器进行了比较。在采用极性有机洗脱条件下，我们观察到酸性分析物的分子内抗衡离子效应，这导致保留时间缩短，但不影响对映体选择性。两性分析物的保留基于其带电官能团与两性选择器的酸性和碱性位点的同时双重离子配对。在强阳离子交换位点附近的手性中心对于碱基的手性分离至关重要。空间位阻较大的侧链有利于游离氨基酸的分离。在具有直链烷磺酸位点的固定相中，游离（未衍生化）肽的对映体分离特别成功，这表明更灵活的部分具有有益的影响。此外，我们观察到奎宁和辛可宁衍生的手性固定相对映体行为的假对映体，有利于所有分析物的洗脱顺序反转。
