Data mining and enantiophore studies on chiral stationary phases used in HPLC separation

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Resolution, determination of enantiomeric purity and chiral recognition mechanism of new xanthone derivatives on (S,S)-whelk-O1 stationary phase

The enantioresolution and determination of the enantiomeric purity of 32 new xanthone derivatives, synthesized in enantiomerically pure form, were investigated on (S,S)-Whelk-O1 chiral stationary phase (CSP). Enantioselectivity and resolution (α and R_S ) with values ranging from 1.41-6.25 and from 1.29-17.20, respectively, were achieved. The elution was in polar organic mode with acetonitrile/methanol (50:50 v/v) as mobile phase and, generally, the (R)-enantiomer was the first to elute. The enantiomeric excess (ee) for all synthesized xanthone derivatives was higher than 99%. All the enantiomeric pairs were enantioseparated, even those without an aromatic moiety linked to the stereogenic center. Computational studies for molecular docking were carried out to perform a qualitative analysis of the enantioresolution and to explore the chiral recognition mechanisms. The in silico results were consistent with the chromatographic parameters and elution orders. The interactions between the CSP and the xanthone derivatives involved in the chromatographic enantioseparation were elucidated.

Enantioresolution of chiral derivatives of xanthones on (S,S)-Whelk-O1 and L-phenylglycine stationary phases and chiral recognition mechanism by docking approach for (S,S)-Whelk-O1

The resolution of seven enantiomeric pairs of chiral derivatives of xanthones (CDXs) on (S,S)-Whelk-O1 and L-phenylglycine chiral stationary phases (CSPs) was systematically investigated using multimodal elution conditions (normal-phase, polar-organic, and reversed-phase). The (S,S)-Whelk-O1 CSP, under polar-organic conditions, demonstrated a very good power of resolution for the CDXs possessing an aromatic moiety linked to the stereogenic center with separation factor and resolution factor ranging from 1.91 to 7.55 and from 6.71 to 24.16, respectively. The chiral recognition mechanisms were also investigated for (S,S)-Whelk-O1 CSP by molecular docking technique. Data regarding the CSP-CDX molecular conformations and interactions were retrieved. These results were in accordance with the experimental chromatographic parameters regarding enantioselectivity and enantiomer elution order. The results of the present study fulfilled the initial objectives of enantioselective studies of CDXs and elucidation of intermolecular CSP-CDX interactions.

Enantioseparation and chiral recognition mechanism of new chiral derivatives of xanthones on macrocyclic antibiotic stationary phases

A chiral HPLC method using four macrocyclic antibiotic chiral stationary phases (CSPs) has been investigated for determination of the enantiomeric purity of fourteen new chiral derivatives of xanthones (CDXs). The separations were performed with the CSPs Chirobiotic T, Chirobiotic TAG, Chirobiotic V and Chirobiotic R under multimodal elution conditions (normal-phase, reversed-phase and polar ionic mode). The analyses were performed at room temperature in isocratic mode and UV and CD detection at a wavelength of 254 nm. The best enantioselectivity and resolution were achieved on Chirobiotic R and Chirobiotic T CSPs, under normal elution conditions, with R(S) ranging from 1.25 to 2.50 and from 0.78 to 2.06, respectively. The optimized chromatographic conditions allowed the determination of the enantiomeric ratio of eight CDXs, always higher than 99%. In order to better understand the chromatographic behavior at a molecular level, and the structural features associated with the chiral recognition mechanism, computational studies by molecular docking were carried out using VDock. These studies shed light on the mechanisms involved in the enantioseparation for this important class of chiral compounds.

Chiral recognition by enantioselective liquid chromatography: mechanisms and modern chiral stationary phases

An overview of the state-of-the-art in LC enantiomer separation is presented. This tutorial review is mainly focused on mechanisms of chiral recognition and enantiomer distinction of popular chiral selectors and corresponding chiral stationary phases including discussions of thermodynamics, additivity principle of binding increments, site-selective thermodynamics, extrathermodynamic approaches, methods employed for the investigation of dominating intermolecular interactions and complex structures such as spectroscopic methods (IR, NMR), X-ray diffraction and computational methods. Modern chiral stationary phases are discussed with particular focus on those that are commercially available and broadly used. It is attempted to provide the reader with vivid images of molecular recognition mechanisms of selected chiral selector-selectand pairs on basis of solid-state X-ray crystal structures and simulated computer models, respectively. Such snapshot images illustrated in this communication unfortunately cannot account for the molecular dynamics of the real world, but are supposed to be helpful for the understanding. The exploding number of papers about applications of various chiral stationary phases in numerous fields of enantiomer separations is not covered systematically.

Simple method for the prediction of the separation of racemates with high-performance liquid chromatography on Whelk-O1 chiral stationary phase

A simple method for the prediction of whether or not a racemate can be separated on a Whelk-O1 chiral stationary phase has been developed. In this approach, molecules are represented by counting the number of atom types of the neighbors spheres of the chiral center. A decision tree is then used to decide based on a few of these atom count descriptors whether a given racemate can be separated. High values of correct prediction were obtained, namely with more than 94% for training sets and of about 90% for cross-validation results. The same rate of correct prediction was also obtained on an external data set. The descriptors can be rapidly and easily retrieved by just counting the atom types around the chiral center by inspecting the chemical diagram of the molecule. Furthermore, the decision tree model can be applied through the use of a small set of rules that eventually predicts whether or not a racemate is separated. Due to its computational simplicity, the procedure is of interest for experimentalists that need to make rapid assessment of the separation without having to program or input complex formulas.

Enantiomeric separation of 2-aryloxyalkyl- and 2-arylalkyl-2-aryloxyacetic acids on a Penicillin G Acylase-based chiral stationary phase: Influence of the chemical structure on retention and enantioselectivity

The chiral recognition mechanism of Penicillin G Acylase (PGA) was investigated with a set of 18 new chiral acidic compounds. A series of 2-aryloxyalkyl- and 2-arylalkyl-2-aryloxyacetic acids in which the absolute configuration has been reported to exert a strong influence on pharmacological activity, were synthesized and analysed on PGA-based chiral stationary phase (CSP) and 11 racemates were completely resolved with a mobile phase composed of 50 mM phosphate buffer (pH 7.0). The influence of structural variations of analytes on retention and enantioselectivity was investigated by application of molecular modelling studies. Docking experiments were also carried out to rationalize the observed enantioselective behaviour. The computation approach revealed to be helpful in elucidating the molecular basis of the enantioselectivity observed on PGA-CSP.

Enantiomer separation of amino acids in immunoaffinity micro LC-MS

Chiral immunoaffinity microbore columns were directly interfaced with MS detection, and the effect of column length and temperature on the enantiomer separation of a number of underivatized aromatic and aliphatic amino acids was investigated utilizing an antibody chiral stationary phase that had been prepared by immobilizing a monoclonal anti-D-amino acid antibody onto silica. The stronger affinity of the antibody towards aromatic and bulky amino acids allowed separation of such analytes in a 0.75 x 150 mm column, while an increase in column length enabled separation of more weakly bound compounds. The strength of interaction between chiral selector and analytes could be modulated conveniently by lowering the temperature. For the first time, simultaneous enantiomer separation of mixtures of amino acids was achieved on antibody-based chiral stationary phases using extracted ion chromatograms.

The docking of chiral epoxides on the Whelk-O1 stationary phase: A molecular dynamics study

The docking of analytes on the Whelk-O1 chiral stationary phase is explored for two chiral epoxides in a hexane solvent. Density functional theory calculations are employed to develop flexible models for R/S-styrene oxide (phenyl oxirane) and (R,R/S,S)-stilbene oxide (2,3-diphenyl oxirane). Molecular dynamics simulations of the racemates in the presence of the Whelk-O1 chiral stationary phase reveal the distribution of the enantiomers at the interface. The importance of hydrogen bonding and ring-ring interactions is explored along with an examination of the major docking arrangements. The interactions between the Whelk-O1 molecules and the chiral epoxide enantiomers are quite distinct and consistent with the experimental elution orders [S.E. Schaus, B.D. Brandes, J.F. Larrow, M. Tokunage, K.B. Hansen, A.E. Gould, M.E. Furrow, E.N. Jacobsen, J. Am. Chem. Soc. 124 (2001) 1307] and separation factors [W.H. Pirkle, C.J. Welch, Tetrahedron: Asymm. 5 (1994) 777]. The impact of a polar solvent modifier is examined for R/S-styrene oxide where selectivity in 80:20 n-hexane:2-propanol is assessed.

Solvation of the Whelk-O1 chiral stationary phase: A molecular dynamics study

Density functional theory calculations and molecular dynamics simulations are employed to explore the solvation of the Whelk-O1 chiral stationary phase. First, a semi-flexible representation of the Whelk-O1 selective molecule is extracted from an extensive series of B3LYP/6-311+ G(2d,p) calculations. The resulting model is used to build a chiral surface, including end-caps, for molecular dynamics study of the interface between solvent and Whelk-O1. Three solvent environments in common use for Whelk-O1 HPLC have been examined: a normal-phase solvent of n-hexane/2-propanol; a reversed-phase solvent of water/methanol; and a supercritical solvent of CO(2) and methanol. In each case, we analyze the interface with an emphasis on solvent composition and solvent hydrogen bonding to the Whelk-O1 selector.

The role of molecular interaction fields on enantioselective and nonselective separation of chiral sulfoxides

The separation of a series of 23 asymmetric sulfoxides, including the three proton pump inhibitors (PPI) omeprazole, lansoprazole and pantoprazole was investigated by HPLC, under reversed-phase elution with amylose tris(3,5-dimethylphenylcarbamate), amylose tris[(S)-1-phenylethylcarbamate] and amylose tris(3,5-dimethoxyphenylcarbamate) chiral stationary phases, CSP1-3, respectively. The whole set of sulfoxides showed better enantioselectivity and enantioresolution on CSP1. However, the three PPI were enantioseparated only when using CSP1 and CSP3. It was observed an improved enantioselectivity and enantioresolution on CSP3. The mechanisms of retention were evaluated by molecular interaction fields (MIF) generated via GRID force field, which yielded the geometric reasons leading to the scenario outlined. The enantioselective and nonselective interactions are discussed in terms of the reported selectivity. The steric structural outline of the CSP nonselective interaction sites is of major importance to deliver the sulfoxides to the chiral selective sites where the enantioselective interactions take place.

Enantiophore modeling in 3D-QSAR. A data mining application on Whelk-O1 chiral stationary phase

A combination of the enantiophore concept described in a previous study and a quantitative structure enantioselective relationship (QSER) based on partial least squares (PLS) analysis is presented. In the present study, a comprehensive approach for describing the enantioselective binding properties of the Whelk-O1 chiral HPLC receptor is achieved using molecular descriptors calculated by the GRID program. The GRID descriptors allow us to describe the molecules in terms of their ability to form favorable interactions with independent chemical groups (probes) that can be related to receptor sites. For each molecule, we compute 120 enantiophore descriptors representing the energy contributions from all possible pairwise combinations of probes. The overall procedure was simplified by considering only the most energetically favorable locations and converting selected grid-point energies into alignment-independent descriptors. By using a training set of 143 diverse chiral compounds, an optimal PLS model requiring seven components was chosen by using the cross-validation method resulting in a correlation coefficient R2= 0.88 and a cross validated correlation coefficient Q2= 0.85. An interpretation of the model is proposed based upon a visual inspection of the regression coefficient plots. From these plots, the influence of particular molecular features for selective binding of solutes was estimated and used to outline the chiral recognition sites in the Whelk-O1 receptor. The predictive power of our model has been estimated by means of an external data set emphasizing the suitability of the procedure also for predictive aims.