Shahab A. Shamsi

Cationic β-cyclodextrin derivative for chiral separations

Abstract A novel hepta-substituted β-cyclodextrin bearing the methoxyethylamine group linked to the upper cyclodextrin rim was successfully used as a chiral selector for enantiomeric separation of non-steroidal anti-inflammatory drugs (NSAIDs) and phenoxypropionic acid herbicides (PPAHs). Separation parameters such as pH and concentration were found to have major influences on enantiomeric resolution of the NSAIDs and PPAHs. Results indicate that heptakis(6-methoxyethylamine-6-deoxy)-β-cyclodextrin [β-CD-OMe (VII)] performs exceptionally well for the enantiomeric resolution of NSAIDs: indoprofen and fenoprofen ( R s =11 and 14, respectively). In addition, baseline enantiomeric separation of a mixture of six pairs of PPAHs was achieved in under 30 min. Compared to other cationic β-cyclodextrins reported in the literature, the β-CD-OMe (VII) showed improved selectivity for both classes of the aforementioned anionic racemates.

Combined use of chiral ionic liquid and cyclodextrin for MEKC: Part I. Simultaneous enantioseparation of anionic profens.

The enantiomers of five profen drugs were simultaneously separated by MEKC with the combined use of 2,3,6‐tri‐O‐methyl‐β‐cyclodextrin and chiral cationic ionic liquid, N‐undecenoxy‐carbonyl‐L‐leucinol bromide, which formed micelles in aqueous buffers. Enantioseparations of these profen drugs were optimized by varying the chain length and concentration of the IL surfactant using a standard recipe containing 35 mM 2,3,6‐tri‐O‐methyl‐β‐cyclodextrin, 5 mM sodium acetate at pH 5.0. The batch‐to‐batch reproducibility of N‐undecenoxy‐carbonyl‐L‐leucinol bromide was tested and found to have no significant impact in terms of enantiomeric resolution, efficiency, and migration time. Finally, this method was successfully applied for the quantitative determination of ibuprofen in pharmaceutical tablets.

Ginger Phytochemicals Exhibit Synergy to Inhibit Prostate Cancer Cell Proliferation

Dietary phytochemicals offer nontoxic therapeutic management as well as chemopreventive intervention for slow-growing prostate cancers. However, the limited success of several single-agent clinical trials suggest a paradigm shift that the health benefits of fruits and vegetables are not ascribable to individual phytochemicals, rather may be ascribed to synergistic interactions among them. We recently reported growth-inhibiting and apoptosis-inducing properties of ginger extract (GE) in in vitro and in vivo prostate cancer models. Nevertheless, the nature of interactions among the constituent ginger biophenolics, viz. 6-gingerol, 8-gingerol, 10-gingerol, and 6-shogoal, remains elusive. Here we show antiproliferative efficacy of the most-active GE biophenolics as single-agents and in binary combinations, and investigate the nature of their interactions using the Chou-Talalay combination index (CI) method. Our data demonstrate that binary combinations of ginger phytochemicals synergistically inhibit proliferation of PC-3 cells with CI values ranging from 0.03 to 0.88. To appreciate synergy among phytochemicals present in GE, the natural abundance of ginger biophenolics was quantitated using LC-UV/MS. Interestingly, combining GE with its constituents (in particular, 6-gingerol) resulted in significant augmentation of GEs antiproliferative activity. These data generate compelling grounds for further preclinical evaluation of GE alone and in combination with individual ginger biophenols for prostate cancer management.

Evaluation of a methacrylate bonded cyclodextrins as a monolithic chiral stationary phase for capillary electrochromatography (CEC)-UV and CEC coupled to mass spectrometry

Glycidyl methacrylate‐bonded β‐cyclodextrin (GMA‐β‐CD) is synthesized as a new chiral monomer by direct chemical bonding with GMA using a fast and simple alternative procedure. Next, rigid and homogenous monolithic columns were prepared by polymerization of GMA‐β‐CD monomer with ethylene dimethacrylate (EDMA), in the presence of commonly used porogens and a charged achiral monomer to form a versatile chiral monolith. This is the first report in which a preparation procedure for a methacrylate‐bonded CD is introduced for chiral separations in CEC. The degree of substitution of GMA‐β‐CD monomer and mobile‐phase parameters were optimized to achieve the highest enantioselectivity and plate number. To evaluate the GMA‐β‐CD monolithic column, different classes of chiral compounds were screened. Under the optimized β‐CD monolith phase and the optimum mobile‐phase conditions, 30 neutral and basic chiral compounds and two acidic compounds could be separated. The high chemical and mechanical stability, homogenous microflow and no loss of material at the interface allows for the first time the feasibility of applying this polymer‐based monolithic column for CEC coupled to ESI‐MS. Compared with CEC‐UV, CEC‐ESI‐MS showed higher sensitivity and lower resolution. However, resolution greater than 1.0 can still be obtained for majority of the select tested compound in CEC‐ESI‐MS with at least three out of seven compound providing Rs≥1.5. The results reinforce the potential of GMA‐β‐CD monolithic columns for chiral separations with high sensitivity in CEC‐ESI‐MS. Finally, using hexobarbital as the model chiral analyte, the monolithic column demonstrated excellent stability and reproducibility of retention time and enantioselectivity.

Optimized separation of β-blockers with multiple chiral centers using capillary electrochromatography–mass spectrometry☆

This work focuses on the simultaneous analysis of beta-blockers with multiple stereogenic centers using capillary electrochromatography-mass spectrometry (CEC-MS) with a vancomycin stationary phase. The critical mobile phase variables (composition of organic solvents, acid/base ratios) as well as column temperature and electric field strength, effecting enantioresolution and analysis time were first optimized sequentially. Next, to achieve global optimum a multivariate D-optimal design was used. Although multivariate approach did not improve enantioresolution any further, analysis time was significantly reduced. Under optimum CEC-MS conditions, all stereoisomers were resolved with resolution in the range 1.0-3.1 in less than 60 min with an average signal-to-noise (S/N) greater than 1000. The developed CEC-MS method has the potential to emerge as a screening method for analysis of multiple chiral compounds using a single protocol using the same column and mobile phase conditions, thus reducing the operation time and cost.

Combined use of chiral ionic liquid and CD for MEKC: Part II. Determination of binding constants

A competitive inhibition mechanism is proposed to investigate the interactions among 2,3,6‐tri‐O‐methyl‐β‐CD (TM‐β‐CD), cationic ionic liquid type surfactants, N‐undecenoxy‐carbonyl‐L‐leucinol bromide (L‐UCLB) and profens using affinity CE. The apparent binding constant of TM‐β‐CD to L‐UCLB was estimated by nonlinear and linear plotting methods. The binding constants of one representative profen (e.g. fenoprofen) to TM‐β‐CD and L‐UCLB were estimated by a secondary plotting approach. The R‐ and S‐fenoprofens have different binding constant values, resulting in the enantioseparation due to the synergistic effect of the two chiral selectors, TM‐β‐CD and L‐UCLB.

A novel positively charged achiral co-monomer for β-cyclodextrin monolithic stationary phase: Improved chiral separation of acidic compounds using capillary electrochromatography coupled to mass spectrometry

The work presented here demonstrates the incorporation of vinylbenzyl trimethylammonium (VBTA) as a novel positively charged achiral co-monomer to a glycidyl methacrylate-beta cyclodextrin (GMA/β-CD) based monolith, providing anion exchange sites with reversed electroosmotic flow (EOF) for capillary electrochromatography (CEC). The monolithic phases, GMA/β-CD-VBTA and GMA/β-CD (without co-monomer) were characterized by scanning electron microscopy, optical microscopy, pressure drop/flow-rate curves and nitrogen adsorption analysis. After optimizing the stationary phase and mobile phase parameters, chiral separations of 41 pairs of structurally diverse anionic chiral analytes were compared individually using the GMA/β-CD-VBTA and GMA/β-CD monolithic columns. The GMA/β-CD-VBTA monolith chiral stationary phase separated significantly more acidic compounds compared to the GMA/β-CD column. To-date there has been limited work in the development of chiral monolithic column for CEC-mass spectrometry (MS). Because of good electrodriven flow characteristics, which allow the column to maintain a stable current in the absence of outlet vial, GMA/β-CD-VBTA column was successfully coupled to single quadrupole mass spectrometer for CEC-MS of several chiral test compounds. In addition, the same monolithic CEC column when coupled to a triple quadrupole MS instrument, two orders of magnitude higher sensitivity was observed compared to a single quadrupole MS instrument.

Multivariate approach for the enantioselective analysis in micellar electrokinetic chromatography-mass spectrometry: I. Simultaneous optimization of binaphthyl derivatives in negative ion mode

A mixture of two molecular micelles polysodium N-undecenoxy carbonyl-L-leucinate, (poly-L-SUCL) and polysodium N-undecanoyl leucylvalinate, (poly-L-SULV) was utilized in micellar electrokinetic chromatography-electrospray ionization-mass spectrometry (MEKC-ESI-MS) to simultaneously separate and detect enantiomers of binaphthyl derivatives. Separation parameters such as background buffer composition, voltage, temperature, and nebulizer pressure were optimized using a multivariate central composite design (CCD). Baseline enantioseparation for both analytes was achieved. The CCD was also used in the optimization of sheath liquid and spray chamber parameters to achieve optimum ESI-MS response. The results demonstrate that CCD is a powerful tool for the optimization of MEKC-MS parameters and the response surface model analysis can provide in-depth statistical understandings of the significant factors required to achieve maximum enantioresolution and ESI-MS sensitivity.

Sulfated and sulfonated polysaccharide as chiral stationary phases for capillary electrochromatography and capillary electrochromatography–mass spectrometry

The applications of polysaccharide phenyl carbamate derivatives as chiral stationary phases (CSPs) for capillary electrochromatography (CEC) are often hindered by longer retention times, especially using a normal-phase (NP) eluent due to very low electroosmotic flow (EOF). Therefore, in this study, we propose an approach for the aforementioned problems by introducing two new types of negatively charged sulfate and sulfonated groups for polysaccharide CSPs. These CSPs were utilized to pack CEC columns for enantioseparation with a NP eluent. Compared to conventional cellulose tris(3,5-dimethylphenyl carbamate) or CDMPC CSPs, the sulfated CDMPC CSP (sulfur content 4.25%, w/w) shortened the analysis time up to 50% but with a significant loss of enantiomeric resolution (approximately 60%). On the other hand, the sulfonated CDMPC CSP (sulfur content 1.76%, w/w) not only provided fast throughput but also maintained excellent resolving power. In addition, its synthesis is much more straightforward than the sulfated one. Furthermore, we studied several stationary phase parameters (CSP loading and silica gel pore size) and mobile phase parameters (including type of mobile phase and its composition) to evaluate the throughput and enantioselectivity. Using the optimized conditions, a chiral pool containing 66 analytes was screened to evaluate the enantioselectivity under three different mobile phase modes (i.e., NP, polar organic phase (POP) and reversed-phase (RP) eluents). Among these mobile phase modes, the RP mode showed the highest success rate, whereas some degree of complementary enantioselectivity was observed with NP and POP. Finally, the feasibility of applying this CSP for CEC-MS enantioseparation using internal tapered column was evaluated with NP, POP and RP eluents. In particular, the NP-CEC-MS provided significantly enhanced sensitivity when methanol was replaced with isopropanol in the sheath liquid. Using aminoglutethimide as model chiral analyte, all three modes of CEC-MS demonstrated excellent durability as well as excellent reproducibility of retention time and enantioselectivity.

Chiral separation with dipeptide-terminated polymeric surfactants: the effect of an extra heteroatom on the polar head group.

Chiral recognition of two binaphthyl derivatives and three benzodiazepines were studied by use of polymeric surfactants in electrokinetic chromatography. Four specific dipeptide terminated (multichiral) micelle polymers were synthesized for this study. These include poly (sodium‐N‐undecanoyl‐L‐alanyl‐leucinate)‐(poly L‐SUAL), poly (sodium‐N‐undecanoyl‐L‐valyl‐leucinate) (poly L‐SUVL), poly (sodium‐N‐undecanoyl‐L‐seryl‐leucinate) (poly L‐SUSL), and poly(sodium‐N‐undecanoyl‐L‐threonyl‐leucinate) (poly L‐SUTL). In addition to the chiral separation study, the physicochemical properties (critical micelle concentration and specific rotation) of each polymer were investigated. The molecular weights of the various dipeptide‐terminated micelle polymers were determined using analytical ultracentrifugation. These dipeptide‐terminated micelle polymers were designed to study the effect of the extra heteroatom at the polar head group of the micelle polymer (i.e., poly L‐SUSL compared to poly L‐SUAL and poly L‐SUTL compared to poly L‐SUVL) on the enantiomeric separation of the binaphthyl derivatives and benzodiazepines. The synergistic effect of three chiral centers (poly L‐SUTL) provided improved resolution over that of two chiral centered dipeptide‐terminated micelle polymer in the case of (±)‐temazepam, (±)‐oxazepam, (±)‐binaphthol, and (±)‐binaphthol phosphate. The chiral recognition mechanisms in these cases were additionally controlled by the presence of the extra heteroatom located on the polar head group of the micelle polymers.