Computational studies in enantioselective liquid chromatography: Forty years of evolution in docking- and molecular dynamics-based simulations

Molecular insights into chromatography: Automated workflows for the virtual design of methacrylate-based chromatography resins

Computational chemistry provides invaluable insights into the behaviors and properties of various materials at the molecular level. This capability is of particular interest in chromatography where adsorbents engage with target molecules through intricate interactions. However, the broad integration of molecular simulations into the field of chromatography has been notably limited, despite significant achievements in previous studies. One potential reason is the requirement for considerable expertise to effectively configure these simulations, presenting a significant barrier to entry. In this context, workflow management systems (WMSs) provide a viable solution by allowing experts to automate complex simulation tasks, making them accessible to the wider research community without necessitating in-depth knowledge of the simulation process. This manuscript outlines the creation and application of two automated workflows designed to generate comprehensive all-atom models of methacrylate-based chromatography resin surfaces and to rapidly calculate binding free energies with peptides as target molecules. These innovations represent a significant advancement in the field by streamlining the simulation process, enhancing predictive accuracy, and making complex molecular modeling more accessible to researchers across disciplines. By publishing these workflows, we aim to catalyze molecular modeling in the field of chromatography by encouraging scientists to utilize and build upon our work.

Conformational changes in polysaccharide-based chiral selectors induced by mobile phase composition: Effects on enantioselective retention and enantiomer elution order reversal

Despite having identical physicochemical properties, chiral molecules require effective separation techniques due to their distinct pharmacological effects. Polysaccharide-based chiral stationary phases (CSPs) are widely used for chiral separations in liquid chromatography; however, the mechanisms of chiral recognition are not well understood. This research explored the adsorption, retention, and chiral recognition mechanisms of three amylose-based CSPs: Chiralpak ID, IF, and IG. The effect of mobile phase composition on enantioselective retention was examined using four acyloin-type chiral solutes in normal-phase mode. For pantolactone (PL) and methyl mandelate (MM), reversals in enantiomer elution order were observed with ID and IG sorbents, respectively, at 2 vol.% isopropanol (iPrOH). As the iPrOH concentration increased, the adsorption of MM enantiomers reached an energetic barrier at this concentration, causing discontinuities in the enthalpy-entropy compensation. Conversely, while the reversal behavior of PL was also attributed to conformational changes in the ID polymer, it did not encounter an energetic barrier and thus remained in line with the enthalpy-entropy compensation. For the IF sorbent, no significant changes in enantioselective retention or enthalpic curves were noted. Nevertheless, a reversal was observed for benzoin (B) enantiomers on the IF sorbent at 10 vol.% iPrOH. It was postulated that the IF sorbent contains two chiral sites with opposing recognition abilities, and their relative contributions to the apparent enantioselectivity of B are influenced by the iPrOH concentration. These findings highlight the importance of conformational changes in chiral selectors, driven by mobile phase composition, in chiral recognition mechanisms. Understanding these effects is crucial for developing predictive models of chiral retention and enhancing optimization of chiral separation processes.

Molecular modeling study to unravel complexation of daclatasvir and its enantiomer by β-cyclodextrins. Computational analysis using quantum mechanics and molecular dynamics

A computational study was performed to unravel mechanisms underlying capillary electrophoresis enantioseparations of daclatasvir and its (R,R,R,R)-enantiomer with native and methylated β-cyclodextrins (β-CDs) as chiral selectors. Considering the enantioseparation results as benchmark, the structures of β-CD and seven methylated β-CDs were optimized by quantum mechanics, and their topography and computed molecular properties were compared. Furthermore, the electron charge density distribution of the macrocycles was also evaluated by calculating the molecular electrostatic potential of pivotal regions of native and methylated β-CDs. The function of hydrogen bonds in the complexation process of daclatasvir and the CDs was derived from quantum mechanics analysis and confirmed by molecular dynamics, as orthogonal computational techniques. The presence of a round-shaped cavity in the CDs used as chiral selector appeared as a necessary requirement for the enantioseparation of daclatasvir and its (R,R,R,R)-enantiomer. In this regard, it was confirmed that the round shape of the CDs is sustained by hydrogen bonds formed between adjacent glucopyranose units and blocking rotation of the linking glycosidic bonds. The presence of hydroxy groups at the 6-position of the glucopyranose units and the concurrent absence of hydroxy groups at the 2-position were evidenced as important factors for enantioseparation of daclatasvir and its enantiomer by methylated β-CDs.

Enantioselective Separation and Pharmacokinetics of a Chiral 1,4-Dihydropyrimidine Derivative in Rats: A Combined Chromatography and Docking Approach

Chirality in 1,4-Dihydropyrimidines influences their pharmacological properties and synthetic strategies. Enantiomers of chiral drugs often exhibit different pharmacokinetic profiles. Therefore, separating and studying individual enantiomers is crucial to optimize drug efficacy and safety. Enantiomeric separation of ±4-(4-chlorophenyl)-6-methyl-2-oxo-N-(O-toyl)-1,2,3,4-tetrahydropyrimidine-5-carboxamide (DP-1), which is a 1,4-Dihydropyrimidine derivative is achieved on CHIRALCEL® OD-H column (particle size: 5 μm, inner diameter: 4.6 mm, length:150 mm), following by investigating the kinetic properties of (R) and (S) enantiomers. The separation was achieved with a mobile phase composed of 70% (v/v) isopropyl alcohol and 30% (v/v) n-hexane. For the bioanalytical study, acetonitrile was used to precipitate the rat plasma samples and validated the method according to USFDA guidelines. The validated bioanalytical method was then successfully applied to determine the pharmacokinetic parameters of the drug in biological samples. Molecular modeling techniques, specifically docking simulations, were employed to predict the elution order of DP-1 enantiomers. The docking results revealed moderate binding interactions between the enantiomers and the chiral stationary phase (CSP), which aligns with the theoretical expectation that stronger interactions lead to longer retention times on the column.

All-atom modeling of methacrylate-based multi-modal chromatography resins for Langmuir constant prediction of peptides

In downstream processing, the intricate nature of the interactions between biomolecules and adsorbent materials presents a significant challenge in the prediction of their binding and elution behaviors. This complexity is further heightened in multi-modal chromatography (MMC), which employs two distinct binding mechanisms. To gain a deeper understanding of the involved interactions, simulating the adsorption of biomolecules on resin surfaces is a focal point of ongoing research. However, previous studies often simplified the adsorbent surface, modeling it as a flat or slightly curved plane without including a realistic backbone structure. Here, we introduce and validate two novel workflows aimed at predicting peptide binding behaviors in MMC, specifically targeting methacrylate-based resins. Our first achievement was the development of an all-atom model of a commercial MMC resin surface, incorporating its polymethacrylic backbone. Furthermore, we established and tested a workflow for rapid calculations of binding free energies (ΔG) with 10 linear peptides as target molecules. These ΔG calculations were effectively used to predict Langmuir constants, achieving a high coefficient of determination (R²) of 0.96. In subsequent benchmarking tests, our model outperformed established, simpler resin surface models in terms of predictive capabilities.

A journey in unraveling the enantiorecognition mechanism of 3,5-dinitrobenzoyl-amino acids with two Cinchona alkaloid-based chiral stationary phases: The power of molecular dynamic simulations

BACKGROUND

Innovations in computer hardware and software capabilities have paved the way for advances in molecular modelling techniques and methods, leading to an unprecedented expansion of their potential applications. In contrast to the docking technique, which usually identifies the most stable selector-selectand (SO-SA) complex for each enantiomer, the molecular dynamics (MD) technique enables the consideration of a distribution of the SO-SA complexes based on their energy profile. This approach provides a more truthful representation of the processes occurring within the column. However, benchmark procedures and focused guidelines for computational treatment of enantioselectivity at the molecular level are still missing.

RESULTS

Twenty-eight molecular dynamics simulations were performed to study the enantiorecognition mechanisms of seven N-3,5-dinitrobenzoylated α- and β-amino acids (DNB-AAs), occurring with the two quinine- and quinidine-based (QN-AX and QD-AX) chiral stationary phases (CSPs), under polar-ionic conditions. The MD protocol was optimized in terms of box size, simulation run time, and frame recording frequency. Subsequently, all the trajectories were analyzed by calculating both the type and amount of the interactions engaged by the selectands (SAs) with the two chiral selectors (SOs), as well as the conformational and interaction energy profiles of the formed SA-SO associates. All the MDs were in strict agreement with the experimental enantiomeric elution order and allowed to establish (i) that salt-bridge and H-bond interactions play a pivotal role in the enantiorecognition mechanisms, and (ii) that the π-cation and π-π interactions are the discriminant chemical features between the two SOs in ruling the chiral recognition mechanism.

SIGNIFICANCE

The results of this work clearly demonstrate the high contribution given by MD simulations in the comprehension of the enantiorecognition mechanism with Cinchona alkaloid-based CSPs. However, from this research endeavor it clearly emerged that the MD protocol optimization is crucial for the quality of the produced results.

Histidine-modified pillar[5]arene-functionalized mesoporous silica materials for highly selective enantioseparation

Chiral enantiomers, especially the enantiomers of chiral drugs often exhibit different pharmacological activity, metabolism and toxicity, thus it is of great research significance to scientifically and reasonably develop single chiral drugs with low toxicity and high efficiency. Among them, high performance liquid chromatographic techniques based on chiral stationary phases (CSPs) has become one of the most attractive methods used to evaluate the enantiomeric purity of single-enantiomers compound of pharmacological relevance. In this work, pillar[5]arene functionalized with L- and D-histidine, respectively, were modified on the surface of mesoporous silica as novel chiral stationary phases called L/DHis-BP5-Sil. Notably, L/D-histidine had the characteristics of low steric hindrance and easy derivatization. Although the π-π interaction of imidazole group was weaker than that of benzene ring, the benzene ring bonding imidazole-conjugated ring in the structure produced better enantioseparation effect. The results showed that L/DHis-BP5-Sil can separate a variety of complex structural enantiomers with excellent reproducibility, thermal stability and separation performance. Hence, the unique advantage of the highly selective separation of L/DHis-BP5-Sil provides new insights into the enantioseparation field.

Update on chiral recognition mechanisms in separation science

The stereospecific analysis of chiral molecules is an important issue in many scientific fields. In separation sciences, this is achieved via the formation of transient diastereomeric complexes between a chiral selector and the selectand enantiomers driven by molecular interactions including electrostatic, ion-dipole, dipole-dipole, van der Waals or π-π interactions as well as hydrogen or halogen bonds depending on the nature of selector and selectand. Nuclear magnetic resonance spectroscopy and molecular modeling methods are currently the most frequently applied techniques to understand the selector-selectand interactions at a molecular level and to draw conclusions on the chiral separation mechanism. The present short review summarizes some of the recent achievements for the understanding of the chiral recognition of the most important chiral selectors combining separation techniques with molecular modeling and/or spectroscopic techniques dating between 2020 and early 2024. The selectors include polysaccharide derivatives, cyclodextrins, macrocyclic glycopeptides, proteins, donor-acceptor type selectors, ion-exchangers, crown ethers, and molecular micelles. The application of chiral ionic liquids and chiral deep eutectic solvents, as well as further selectors, are also briefly addressed. A compilation of all published literature on chiral selectors has not been attempted.

Recent developments in molecular modeling tools and applications related to pharmaceutical and biomedical research

In modern pharmaceutical and biomedical research, molecular modeling represents a useful tool to explore processes and their mechanistic bases at the molecular level. Integrating experimental and virtual analysis is a fruitful approach to study ligand-receptor interaction in chemical, biochemical and biological environments. In these fields, molecular docking and molecular dynamics are considered privileged techniques for modeling (bio)macromolecules and related complexes. This review aims to present the current landscape of molecular modeling in pharmaceutical and biomedical research by examining selected representative applications published in the last years and highlighting current topics and trends of this field. Thus, a systematic compilation of all published literature has not been attempted herein. After a brief overview of the main theoretical and computational tools used to investigate mechanisms at molecular level, recent applications of molecular modeling in drug discovery, ligand binding and for studying protein conformation and function will be discussed. Furthermore, specific sections will be devoted to the application of molecular modeling for unravelling enantioselective mechanisms underlying the enantioseparation of chiral compounds of pharmaceutical and biomedical interest as well as for studying new forms of noncovalent interactivity identified in biochemical and biological environments. The general aim of this review is to provide the reader with a modern overview of the topic, highlighting advancements and outlooks as well as drawbacks and pitfalls still affecting the applicability of theoretical and computational methods in the field of pharmaceutical and biomedical research.

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.

Quantum Chemical Investigation of the Interaction of Thalidomide Monomeric, Dimeric, Trimeric, and Tetrameric Forms with Guanine DNA Nucleotide Basis in DMSO and Water Solution: A Thermodynamic and NMR Spectroscopy Analysis

Thalidomide (TLD) was used worldwide as a sedative, but it was revealed to cause teratogenicity when taken during early pregnancy. It has been stated that the (R) enantiomer of TLD has therapeutic effects, while the (S) form is teratogenic. Clinical studies, however, demonstrated the therapeutic efficacy of thalidomide in several intractable diseases, so TLD and its derivatives have played an important role in the development and therapy of anticancer drugs. Therefore, it is important to know the molecular mechanism of action of the TLD, although this is still not clear. In what molecular interactions are concerned, it is known that drug molecules can interact with DNA in different ways, for example, by intercalation between base pairs. Furthermore, the ability of the TLD to interact with DNA has been confirmed experimentally. In this work, we report a theoretical investigation of the interaction of the R and S enantiomers of TLD, in its monomeric, dimeric, trimeric, and tetrameric forms, with guanine (GUA) DNA nucleotide basis in solution using density functional theory (DFT). Our initial objective was to evaluate the interaction of TLD-R/S with GUA through thermodynamic and spectroscopic study in dimethyl sulfoxide (DMSO) solvent and an aqueous solution. Comparison of the experimental 1H nuclear magnetic resonance (NMR) spectrum in DMSO-d6 solution with calculated DFT-PCM-DMSO chemical shifts revealed that TLD can undergo molecular association in solution, and interaction of its dimeric form with a DNA base ((TLD)2-GUA and (TLD)2-2GUA, for example) through H-bond formation is likely to take place. Our results strongly indicated that we must consider the plausibility of the existence of TLD associations in solution when modeling the complexation of the TLD with biological targets. This is new information that may provide further insight into our understanding of drug binding to biological targets at the molecular level.

Microsampling and enantioselective liquid chromatography coupled to mass spectrometry for chiral bioanalysis of novel psychoactive substances

In this paper, the development of efficient enantioselective HPLC methods for the analysis of five benzofuran-substituted phenethylamines, two substituted tryptamines, and three substituted cathinones is described. For the first time, reversed-phase (eluents made up with acidic water-methanol solutions) and polar-ionic (eluent made up with an acetonitrile-methanol solution incorporating both an acidic and a basic additive) conditions fully compatible with mass spectrometry (MS) detectors were applied with a chiral stationary phase (CSP) incorporating the (+)-(18-crown-6)-tetracarboxylic acid chiral selector. Enantioresolution was achieved for nine compounds with α and R_S factors up to 1.32 and 5.12, respectively. Circular dichroism (CD) detection, CD spectroscopy in stopped-flow mode and quantum mechanical (QM) calculations were successfully employed to investigate the absolute stereochemistry of mephedrone, methylone and butylone and allowed to establish a (R)<(S) enantiomeric elution order for these compounds on the chosen CSP. Whole blood miniaturized samples collected by means of volumetric absorptive microsampling (VAMS) technology and fortified with the target analytes were extracted following an optimized protocol and effectively analysed by means of an ultra-high performance liquid chromatography-MS system. By this way a proof-of-concept procedure was applied, demonstrating the suitability of the method for quali-quantitative enantioselective assessment of the selected psychoactive substances in advanced biological microsamples. VAMS microsamplers including a polypropylene handle topped with a small tip of a polymeric porous material were used and allowed to volumetrically collect small aliquots of whole blood (10 μL) independently from its density. Highly appreciable volumetric accuracy (bias, in the -8.7-8.1% range) and precision (% CV, in the 2.8-5.9% range) turned out.

Ferrocene derivatives with planar chirality and their enantioseparation by liquid-phase techniques

In the last decade, planar chiral ferrocenes have attracted a growing interest in several fields, particularly in asymmetric catalysis, medicinal chemistry, chiroptical spectroscopy and electrochemistry. In this frame, the access to pure or enriched enantiomers of planar chiral ferrocenes has become essential, relying on the availability of efficient asymmetric synthesis procedures and enantioseparation methods. Despite this, in enantioseparation science, these metallocenes were not comprehensively explored, and very few systematic analytical studies were reported in this field so far. On the other hand, enantioselective high-performance liquid chromatography has been frequently used by organic and organometallic chemists in order to measure the enantiomeric purity of planar chiral ferrocenes prepared by asymmetric synthesis. On these bases, this review aims to provide the reader with a comprehensive overview on the enantioseparation of planar chiral ferrocenes by discussing liquid-phase enantioseparation methods developed over time, integrating this main topic with the most relevant aspects of ferrocene chemistry. Thus, the main structural features of ferrocenes and the methods to model this class of metallocenes will be briefly summarized. In addition, planar chiral ferrocenes of applicative interest as well as the limits of asymmetric synthesis for the preparation of some classes of planar chiral ferrocenes will also be discussed with the aim to orient analytical scientists towards 'hot topics' and issues which are still open for accessing enantiomers of ferrocenes featured by planar chirality.

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.

Investigation on improvement of enantioseparation in capillary electrophoresis based on maltodextrin by chiral ionic liquids

Taking advantage of chiral ionic liquids, this study deals with the improvement of the enantioseparation performance of a traditional chiral selector (maltodextrin) in capillary electrophoresis. Herein, two polyhydroxy compound-based chiral ionic liquids, namely tetramethylammonium-D-gluconic acid and tetramethylammonium-shikimic acid were designed and utilized as additives for chiral separation for the first time. The synergistic systems provided much better enantioseparations of twelve model drugs compared to the single maltodextrin system. These model analytes contained analgesics, antidepressants, antiallergic drugs, antifungal drugs, antihypertensive drugs, and antiparkinsonian drugs. After optimizing the separation conditions, the chiral recognition mechanism was probed by means of ultraviolet spectroscopy, nuclear magnetic resonance, and molecular modeling. The results of spectroscopic and computational analyses were in good consistency with enantioseparation outcomes. Finally, the proposed method was successfully used for the determination of the enantiomeric purity of duloxetine hydrochloride.

Simulation of gas chromatographic separation based on random diffusion

色谱分离过程中的粒子扩散问题是色谱动力学研究的基础,深入理解粒子的扩散行为对优化分离操作条件、提升色谱性能和开发新型色谱柱尤为关键。现有的模拟方法多集中于局部过程的热力学研究,而整体的扩散分离过程报道并不多见。为此,该文基于微尺度受限空间内随机扩散的方法,通过动态追踪粒子的运动轨迹,实现粒子在气相色谱开管柱内的扩散全过程模拟。基于前期烷烃同系物的分离模拟研究,结合Kovats保留指数,分别建立了吸附步数与温度、吸附步数与碳数的函数关系,由此获得不同类型的同系物在不同温度条件下的分离参数系统。以醇类同系物的分离验证模拟的可靠性,结果表明保留时间的相对误差基本控制在5%以内,而峰宽相对误差在0.75%~60%之间。峰宽误差较大的原因在于:(1)参数化计算过程中未能充分迭代以及使用外推法;(2)模型中忽略了醇分子之间的氢键作用。该文提出的模拟方法虽然可以准确地预测色谱保留时间以及合理描述色谱峰的基本形貌特征,但尚有进一步发展空间,特别是增加对分子间相互作用的细节处理,例如可参考分子力学的方法建立分子间势函数和吸附步数的关系,利用分子力学计算的能量来取代参数化的吸附步数,从而实现更为精确的分离过程模拟。总体而言,该文所提出的模拟方法为优化色谱分离操作条件和开发新型色谱分离技术提供了有价值的参考。

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.

Chiral separation and molecular simulation study of six antihistamine agents on a coated cellulose tri-(3,5-dimethylphenycarbamate) column (Chiralcel OD-RH) and its recognition mechanisms

Enantiomeric separation of six antihistamine agents was first systematically investigated on a cellulose-based chiral stationary phase (CSP), that is, cellulose tris-(3,5-dimethyl phenyl carbamate) (Chiralcel OD-RH), under the reversed-phase mode. Orphenadrine, meclizine, terfenadine, dioxopromethazine, and carbinoxamine enantiomers were completely separated under the optimized mobile phase conditions with resolutions of 5.02, 1.93, 1.68, 1.67, and 1.54, respectively. Mequitazine was partially separated with a resolution of 0.77. The influences of type and concentration of buffer salt, the pH of buffer solution, and the type and ratio of organic modifier on the chiral separation were evaluated and optimized. For a better insight into the enantiorecognition mechanisms, molecular docking was carried out via the Autodock software. The lowest binding energy and the optimal conformations of the analytes/CSP complexes were supplied, and the mechanisms of chiral recognition were determined. According to the results, the key interactions for the chiral recognition of these six analytes on CDMPC were π-π interactions, hydrophobic interactions, hydrogen bond interactions, and some special interactions.

Separation of tetrahydrozoline enantiomers in capillary electrophoresis with cyclodextrin-type chiral selectors and investigation of chiral recognition mechanisms

The recognition power and affinity pattern of various cyclodextrins (CD) towards the enantiomers of tetrahydrozoline (THZ) were studied using capillary electrophoresis (CE). As expected, affinity of THZ enantiomers and selectivity of recognition towards CD derivatives was strongly dependent on the cavity size and substituent type and pattern on the CD rims. Not only were the affinity strength and selectivity of recognition affected by the size of the cavity and chemistry of the CDs but also the affinity pattern. Another interesting example of opposite affinity pattern of enantiomers towards α- and β-CD was observed here. In addition, opposite affinity pattern of THZ enantiomers was seen towards β-CD and its acetylated derivatives, while methylation of β-CD did not affect the affinity pattern of THZ enantiomers. In order to get more information about structural mechanisms of the multivariate dependences mentioned above, rotating frame Overhauser enhancement spectroscopy (ROESY) and computation techniques were used. Significant differences between the structure of THZ complexes with different CDs with both methods were encountered. Good correlations between experimentally determined and computed structure of complexes, as well as between computed complex stabilities and enantiomer migration order (EMO) in CE were observed.

Capillary electrophoresis and molecular modeling of the chiral separation of aromatic amino acids using α/β-cyclodextrin and 18-crown-6

In this work, chiral separation of enantiomers of three amino acids was achieved using capillary electrophoresis technique with α-cyclodextrin (αCD) as a running buffer additive. Only tryptophan has exhibited baseline separation in the presence of αCD, while the enantiomers of the other two amino acids, phenylalanine and tyrosine, were only partially separated. The addition of 18-crown-6 (18C6) as a second additive imparted only slight improvement to the separation of all enantiomers. On the other hand, all three racemic amino acid mixtures demonstrated no indication of separation when the larger cavity cyclodextrin members, β- and γCD, are used as running buffer chiral additives. However, remarkable improvements in the separation of the enantiomers of phenylalanine and tyrosine were obtained when 18C6 is used together with βCD as a running buffer additive. Surprisingly, tryptophan enantiomers were not separated by the dual additive system of cyclodextrin and crown ether. Using electrospray ionization mass spectrometry (ESI-MS), all amino acids were found to form stable binary complexes with individual hosts as well as ternary compounds involving the crown ether and the cyclodextrin. Furthermore, we used molecular dynamics (MD) simulations to build a clear picture about the interaction between the guest and the hosts. Most of these complexes remained stable throughout the simulation times, and the molecular dynamics study allowed better understanding of these supramolecular assemblies.

Immobilization of Chondroitin Sulfate A onto Monolithic Epoxy Silica Column as a New Chiral Stationary Phase for High-Performance Liquid Chromatographic Enantioseparation

Chondroitin sulfate A was covalently immobilized onto a monolithic silica epoxy column involving a Schiff base formation in the presence of ethylenediamine as a spacer and evaluated in terms of its selectivity in enantioseparation. The obtained column was utilized as a chiral stationary phase in enantioseparation of amlodipine and verapamil using a mobile phase consisting of 50 mM phosphate buffer pH 3.5 and UV detection. Sample dilution by organic solvents (preferably 25% v/v acetonitrile-aqueous solution) was applied to achieve baseline enantioresolution (Rs > 3.0) of the individual drug models within 7 min, an excellent linearity (R² = 0.999) and an interday repeatability of 1.1% to 1.8% RSD. The performance of the immobilized column for quantification of racemate in commercial tablets showed a recovery of 86–98% from tablet matrices. Computational modeling by molecular docking was employed to investigate the feasible complexes between enantiomers and the chiral selector.

Utilization of the hysteresis phenomenon for chiral high-performance liquid chromatographic method selection in polar organic mode

Polysaccharide-based chiral stationary phases (CSPs) are outstandingly suitable to play a key role in chiral HPLC method selection strategies, since they provide high success rates. One reason for this ability is that they adopt a diversity of higher order structures in various eluents, resulting in versatile chiral environments. A potential to extend this versatility further was expected and examined in the present study, based on the recently discovered hysteretic behavior of a widely used chiral selector (CS), amylose tris(3,5-dimethylphenylcarbamate). The hindered transitions of its structure, which are behind the history dependence of its separation ability, were used as a tool to identify distinct states of the chiral selector in order to exploit an extended selectivity space. The identification was carried out using a single diagnostic compound, as opposed to the common approach where testing a library of compounds is required. Eluent mixtures consisting of 2-propanol and either methanol or ethanol were scrutinized in terms of stability and robustness of the observed retentions. The solvent mixtures that were eligible for practical application in these respects were used to construct a screening sequence, including identical compositions combined with different column pretreatment. The gain achievable by using the proposed sequence was then evaluated using 15 enantiomer pairs with focus on resolution, enantiomer elution order and chemoselectivity.

Investigation of dielectric constants of water in a nano-confined pore

We report the dielectric properties of pure water confined in a silica hydrophilic nanopore determined from the computation of the density of liquid in the confined phase by the grand canonical Monte Carlo (GCMC) simulations. The silica cylindrical nanopore is divided into n concentric radial shells to get a better insight into the dielectric properties of nanoconfined water. We find that the average values of the dielectric constants are very close and almost independent of the number of concentric radial shells. The decrease in the dielectric constant of confined pure water is due to the strong orientation of water dipoles in the vicinity of the surface while water dipoles do not exhibit any preferential orientation in bulk phase.

Investigation of dielectric properties in a silica hydrophilic pore with two water reservoirs.