A chromatographic and computational study on the driving force operating in the exceptionally large enantioseparation of N-thiocarbamoyl-3-(4′-biphenyl)-5-phenyl-4,5-dihydro-(1H) pyrazole on a 4-methylbenzoate cellulose-based chiral stationary phase

Molecular Dynamics Simulations of Amylose- and Cellulose-Based Selectors and Related Enantioseparations in Liquid Phase Chromatography

In the last few decades, theoretical and technical advancements in computer facilities and computational techniques have made molecular modeling a useful tool in liquid-phase enantioseparation science for exploring enantioselective recognition mechanisms underlying enantioseparations and for identifying selector-analyte noncovalent interactions that contribute to binding and recognition. Because of the dynamic nature of the chromatographic process, molecular dynamics (MD) simulations are particularly versatile in the visualization of the three-dimensional structure of analytes and selectors and in the unravelling of mechanisms at molecular levels. In this context, MD was also used to explore enantioseparation processes promoted by amylose and cellulose-based selectors, the most popular chiral selectors for liquid-phase enantioselective chromatography. This review presents a systematic analysis of the literature published in this field, with the aim of providing the reader with a comprehensive picture about the state of the art and what is still missing for modeling cellulose benzoates and the phenylcarbamates of amylose and cellulose and related enantioseparations with MD. Furthermore, advancements and outlooks, as well as drawbacks and pitfalls still affecting the applicability of MD in this field, are also discussed. The importance of integrating theoretical and experimental approaches is highlighted as an essential strategy for profiling mechanisms and noncovalent interaction patterns.

HPLC Enantioseparation of Rigid Chiral Probes with Central, Axial, Helical, and Planar Stereogenicity on an Amylose (3,5-Dimethylphenylcarbamate) Chiral Stationary Phase

The chiral resolving ability of the commercially available amylose (3,5-dimethylphenylcarbamate)-based chiral stationary phase (CSP) toward four chiral probes representative of four kinds of stereogenicity (central, axial, helical, and planar) was investigated. Besides chirality, the evident structural feature of selectands is an extremely limited conformational freedom. The chiral rigid analytes were analyzed by using pure short alcohols as mobile phases at different column temperatures. The enantioselectivity was found to be suitable for all compounds investigated. This evidence confirms that the use of the amylose-based CSP in HPLC is an effective strategy for obtaining the resolution of chiral compounds containing any kind of stereogenic element. In addition, the experimental retention and enantioselectivity behavior, as well as the established enantiomer elution order of the investigated chiral analytes, may be used as key information to track essential details on the enantiorecognition mechanism of the amylose-based chiral stationary phase.

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.

Triptycene derivatives as chiral probes for studying the molecular enantiorecognition on sub-2-μm particle cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase

Two chiral triptycene derivatives were analyzed on the Chiralpak IB-U column packed with cellulose tris(3,5-dimethylphenylcarbamate)-based sub-2-μm diameter particles. Under normal-phase conditions, sub-minute baseline enantioseparations were obtained. Differences in structural elements and chromatographic behavior of the investigated compounds were evaluated to identify the interactions that drive the chiral discrimination process. From the evaluation of the experimental chromatographic data, it was found that hydrogen bond formation is essential for the separation of enantiomers.

High-performance liquid chromatography enantioseparation of chiral 2-(benzylsulfinyl)benzamide derivatives on cellulose tris(3,5-dichlorophenylcarbamate) chiral stationary phase

Recently it has been reported that immobilized chlorinated-type chiral stationary phases based on cellulose tris(3,5-dichlorophenylcarbamate) are able to express an outstanding enantioselectivity towards the structure of 2-(benzylsulfinyl)benzamide. We now introduce two homologue series of chiral sulfoxides based on the same 2-(sulfinyl)benzoyl core as the prototype of new selectands for HPLC, whose enantioselectivity could be modulable through the replacement of the benzyl group with an unbranched alkyl chain varying in length from 1 to 5 carbon atoms. HPLC parameters such as mobile phase composition and column temperature have been carefully evaluated in order to get pertinent structure-enantioselectivity relationships. The enantiomer elution order was unambiguously determined by a combined strategy involving theoretical and experimental procedures. Two cases of temperature-dependent inversion of the elution order of enantiomers in the operative temperature range of chiral chromatographic support were observed.

Recent studies of docking and molecular dynamics simulation for liquid-phase enantioseparations

Liquid-phase enantioseparations have been fruitfully applied in several fields of science. Various applications along with technical and theoretical advancements contributed to increase significantly the knowledge in this area. Nowadays, chromatographic techniques, in particular HPLC on chiral stationary phase, are considered as mature technologies. In the last thirty years, CE has been also recognized as one of the most versatile technique for analytical scale separation of enantiomers. Despite the huge number of papers published in these fields, understanding mechanistic details of the stereoselective interaction between selector and selectand is still an open issue, in particular for high-molecular weight chiral selectors like polysaccharide derivatives. With the ever growing improvement of computer facilities, hardware and software, computational techniques have become a basic tool in enantioseparation science. In this field, molecular docking and dynamics simulations proved to be extremely adaptable to model and visualize at molecular level the spatial proximity of interacting molecules in order to predict retention, selectivity, enantiomer elution order, and profile noncovalent interaction patterns underlying the recognition process. On this basis, topics and trends in using docking and molecular dynamics as theoretical complement of experimental LC and CE chiral separations are described herein. The basic concepts of these computational strategies and seminal studies performed over time are presented, with a specific focus on literature published between 2015 and November 2018. A systematic compilation of all published literature has not been attempted.

Recognition Mechanisms of Chiral Selectors: An Overview

Stereospecific recognition of chiral molecules plays an important role in nature as the basis of the interaction of chiral bioactive compounds with the chiral target structures. In separation sciences such as chromatographic and capillary electromigration techniques, interactions between chiral analytes and chiral selectors, i.e., the formation of transient diastereomeric complexes in thermodynamic equilibria, are the basis for chiral separations. Due to the large structural variety of chiral selectors, different structural features contribute to the overall chiral recognition process. This introductory chapter briefly summarizes the present understanding of the structural enantioselective recognition processes for various types of chiral selectors.

HPLC Enantioseparations with Polysaccharide-Based Chiral Stationary Phases in HILIC Conditions

In contrast to achiral hydrophilic interaction liquid chromatography (HILIC), which is a popular and largely applied technique to analyze polar compounds such as pharmaceuticals, metabolites, proteins, peptides, amino acids, oligonucleotides, and carbohydrates, the introduction of the HILIC concept in enantioselective chromatography has been relatively recent and scarcely debated. In this chapter, the HILIC enantioseparations carried out on polysaccharide-based chiral stationary phases are grouped and discussed. Another objective of this chapter is to provide a comprehensive overview and insight into the experimental conditions needed to operate under HILIC mode. Finally, to stimulate and facilitate the application of this chromatographic technique, a detailed experimental protocol of a chiral resolution on a chlorinated cellulose-based chiral stationary phase under HILIC conditions is described.