Evaluation and comparison of n‐alkyl chain and polar ligand bonded stationary phases for protein separation in reversed‐phase liquid chromatography

Protein retention is very sensitive to the change of solvent composition in reversed‐phase liquid chromatography for so called “on–off” mechanism, leading to difficulty in mobile phase optimization. In this study, a novel 3‐chloropropyl trichlorosilane ligand bonded column was prepared for protein separation. The differences in retention characteristics between the 3‐chloropropyl trichlorosilane ligand bonded column and n‐alkyl chain modified (C₂, C₄, C₈) stationary phases were elucidated by the retention equation . Retention parameters (a and c) of nine standard proteins with different molecular weights were calculated by using homemade software. Results showed that retention times of nine proteins were similar on four columns, but the 3‐chloropropyl trichlorosilane ligand bonded column obtained the lowest retention parameter values of larger proteins. It meant that their retention behavior affected by acetonitrile concentration would be different due to lower |c| values. More specifically, protein elution windows were broader, and retentions were less sensitive to the change of acetonitrile concentration on the 3‐chloropropyl trichlorosilane ligand bonded column than that on other columns. Meanwhile, the 3‐chloropropyl trichlorosilane ligand bonded column displayed distinctive selectivity for some proteins. Our results indicated that stationary phase with polar ligand provided potential solutions to the “on–off” problem and optimization in protein separation.     

Comparative fingerprint analyses of extracts from the root bark of wild Hippocratea excelsa and “Cancerina” by high‐performance liquid chromatography

A simple high‐performance liquid chromatography method was developed for the identification and comparison of quinone‐methide triterpenes in wild Hippocratea excelsa and “cancerina” to establish the chromatographic profile of these compounds in root bark. The essential chromatographic conditions for this method are based on a gradient system with a reversed‐phase column (C₁₈) using proportions of water, methanol, and tetrahydrofuran as mobile phases to correctly separate the signals at 254 and 420 nm and compare the signals to those reported in the literature. The chromatograms exhibit good resolution and precision. Statistical analysis showed that the chromatographic profiles of wild H. excelsa and cancerina do not exhibit significant differences (p0.05) in their area proportions or relative retention times. The method developed in this study is suitable for the identification of the major chemotaxonomic markers of the Celastraceae family and can be used for quality control of this herbal root bark, which has uses today in Mexican folk medicine.     

Simultaneous determination of two‐component isotherm parameters and lumped mass transfer coefficients in RPLC with the 0‐1 model‐inverse method

The 0‐1 model‐inverse method of nonequilibrium nonlinear chromatography was developed to simultaneously determine the isotherm parameters and the lumped mass transfer coefficients of the two‐component systems in RPLC. By comparing the simulated elution curves with experimental curves with regard to profiles and areas, the suitable isotherm parameters and the lumped mass transfer coefficients were obtained with the 0‐1 model‐inverse method. With a solute cell unit width of cm, the average errors of the peak areas were 0.178% for one component and ?0.40% for two components, and the numerical diffusions of the 0‐1 model for the contribution to band broadening may be negligible. In addition, the results showed that the lumped mass transfer coefficients decrease as the solute concentration increases. The 0‐1 model‐inverse method has not only the advantages of high calculation speed (less than 10 min for one‐component systems or approximately 3 h for two‐component systems using an ordinary computer) and high accuracy in simultaneously obtaining thermodynamic parameters and kinetic parameters of two‐component systems, but this method also possesses the potential to optimally design and control the time‐variant preparative chromatographic system due to the thermodynamic state recursion and the Lagrangian‐Eulerian presentation of the 0‐1 model.     

Evaluation of the quality consistency of powdered poppy capsule extractive by an averagely linear‐quantified fingerprint method in combination with antioxidant activities and two compounds analyses

A novel averagely linear‐quantified fingerprint method was proposed and successfully applied to monitor the quality consistency of alkaloids in powdered poppy capsule extractive. Averagely linear‐quantified fingerprint method provided accurate qualitative and quantitative similarities for chromatographic fingerprints of Chinese herbal medicines. The stability and operability of the averagely linear‐quantified fingerprint method were verified by the parameter r . The average linear qualitative similarity (improved based on conventional qualitative “Similarity”) was used as a qualitative criterion in the averagely linear‐quantified fingerprint method, and the average linear quantitative similarity was introduced as a quantitative one. was able to identify the difference in the content of all the chemical components. In addition, was found to be highly correlated to the contents of two alkaloid compounds (morphine and codeine). A simple flow injection analysis was developed for the determination of antioxidant capacity in Chinese Herbal Medicines, which was based on the scavenging of 2,2‐diphenyl‐1‐picrylhydrazyl radical by antioxidants. The fingerprint–efficacy relationship linking chromatographic fingerprints and antioxidant activities was investigated utilizing orthogonal projection to latent structures method, which provided important pharmacodynamic information for Chinese herbal medicines quality control. In summary, quantitative fingerprinting based on averagely linear‐quantified fingerprint method can be applied for monitoring the quality consistency of Chinese herbal medicines, and the constructed orthogonal projection to latent structures model is particularly suitable for investigating the fingerprint–efficacy relationship.     

Theoretical and experimental investigations on migration behavior of weak monoprotic acids in continuous two‐dimensional ion exchange/reversed phase CEC

A continuous 2D ion exchange/RP CEC system was constructed in this report and retention characteristics of weak monoprotic acids in the column were investigated theoretically. The transport equations were deduced for predicting migration behavior of weak monoportic acids based on mixing model combining ion exchange, RP, and electrophoretic separation mechanism. The influences of separation voltage, length of capillary, pH value, and ionic strength of buffer and concentration of organic modifier in mobile phase on the separation were well described by the equations. It was verified that the migration rate of solute was linearly related to voltage in the case of lower voltage, decreased with the increase of the first‐dimensional length, and . The retention time of solute increased with enhancing ionic strength and concentration of organic modifier. These theoretical results were all demonstrated by the relevant experiments. The relevant application was performed for the aqueous extract of Rhizoma gastrodiae, showing the obvious potential in method development for the analysis of complex samples.     

Combined column–mobile phase mixture statistical design optimization of high-performance liquid chromatographic analysis of multicomponent systems

A statistical approach for the simultaneous optimization of the mobile and stationary phases used in reversed-phase liquid chromatography is presented. Mixture designs using aqueous mixtures of acetonitrile (ACN), methanol (MeOH) and tetrahydrofuran (THF) organic modifiers were performed simultaneously with column type optimization, according to a split-plot design, to achieve the best separation of compounds in two sample sets: one containing 10 neutral compounds with similar retention factors and another containing 11 pesticides. Combined models were obtained by multiplying a linear model for column type, C8 or C18, by quadratic or special cubic mixture models. Instead of using an objective response function, combined models were built for elementary chromatographic criteria (retention factors, resolution and relative retention) of each solute or pair of solutes and, after their validation, the global separation was accomplished by means of Derringer's desirability functions. For neutral compounds a 37:12:8:43 (v/v/v/v) percentage mixture of ACN:MeOH:THF:H₂O with the C18 column and for pesticides a 15:15:70 (v/v/v) ACN:THF:H₂O mixture with the C8 column provide excellent resolution of all peaks.     

Liquid chromatography with tandem mass spectrometry for the simultaneous identification and quantification of cardiovascular drugs applied to the detection of substandard and falsified drugs

The counterfeiting of pharmaceuticals has been detected since about 1990 and has alarmingly continued to pick up steam. We have been recently involved in an evaluation program of some of the most commonly prescribed cardiovascular drugs in Africa, for analysing an important number of tablets or capsules obtained from different places in seven African countries. A reversed‐phase high‐performance liquid chromatography with tandem mass spectrometry method was developed and validated to simultaneously control the identity and the quantity of acenocoumarol, amlodipine, atenolol, captopril, furosemide, hydrochlorothiazide and simvastatin in tablets. Their separation was performed on a Kinetex® C₁₈ (100 mm × 2.1 mm inside diameter, 2.6 μm) column using a gradient elution of 20 mM ammonium formate buffer and acetonitrile (90:10 10:90 v/v) at a flow rate of 0.5 mL/min. The analytes were detected using electrospray ionisation tandem mass spectrometry in both positive and negative modes with multiple reaction monitoring. Tandem mass spectrometry fragmentation patterns of captopril, furosemide and acenocoumarol, up to now not detailed in the literature, were also studied to assist in the selection of the most relevant transitions towards the objectives. The developed method was validated as per International Conference on Harmonisation guidelines with respect to specificity, linearity, trueness, precision, limits of detection and quantification. It has been successfully applied to the control of oral forms of seven cardiovascular drugs collected in African countries.     

Multiobjective optimization strategy based on desirability functions used for the microemulsion liquid chromatographic separation and quantification of norfloxacin and tinidazole in plasma and formulations

The aim of the present study was to optimize a microemulsion liquid chromatography method for the simultaneous determination of norfloxacin and tinidazole binary mixture using a chemometric protocol. Optimization experiments were conducted through a process of screening and optimization. A 2^7‐4 fractional factorial design was used as screening design. While the location of optimum conditions was established by applying Derringer's desirability function. The optimal mobile phase composition was predicted to be: 3.5% w/v SDS, 10.03% v/v 1‐propanol, 0.5% v/v 1‐octanol, and 0.3% triethylamine in 0.02 M phosphoric acid at pH 6.5. The mobile phase was delivered isocratically at a flow rate of 1 mL/min with UV detection at 290 nm. Tinidazole and norfloxacin were eluted with retention times of 1.8 and 5.8 min, respectively. The calibration plots displayed good linear relationships in the concentration ranges of 0.5–50 and 0.75–75 μg/mL for norfloxacin and tinidazole, respectively. The method was successfully applied for determination of both drugs in pharmaceutical dosage forms and real human plasma. Where the accuracy was proved by the low values of % error and high values of recovery, also the relative standard deviation for the results did not exceed 1.5%, proving the precision of the method.     

Influence of solvent effects on retention in reversed-phase liquid chromatography and solid-phase extraction using a cyanopropylsiloxane-bonded,silica-based sorbent

Summary The solvation parameter model is used to characterize the retention properties of a cyanopropylsiloxane-bonded, silica-based sorbent with methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as mobile phases. The system constants over the composition range 1 to 50% (v/v) organic solvent indicate that retention occurs because of the relative ease of cavity formation in the solvated stationary phase compared to the same process in the predominantly aqueous mobile phase as well as from more favorable stationary phase interactions with solutes containing - and n-electrons. The capacity of the solute for dipole-type interactions is not important whereas all hydrogen-bond-type interactions result in reduced retention. Graphing the system constants as a function of mobile phase composition provides a simple mechanism for interpreting the change in capacity of the chromatographic system for retention in terms of changes in the relative weighting of fundamental intermolecular interactions. A comparison is also made with the retention properties of an octadecylsiloxane-bonded, silica-based sorbent with 30% (v/v) methanol in water as the mobile phase and the extraction characteristics of a porous polymer sorbent with 1% (v/v) methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as the sample processing solvent. Changes in sorbent selectivity due to selective uptake of the processing solvent are much smaller for the cyanopropylsiloxane-bonded sorbent than the results found for a porous polymer sorbent.     

Simultaneous determination of seven phenylethanoid glycosides in Cistanches Herba by a single marker using a new calculation of relative correction factor

Current quantitative analysis of multi‐components by a single marker is usually performed by using liquid chromatography methods coupled with ultraviolet or mass spectrometry detection to afford the relative correction factors between reference standard and other components. However, low durability of the relative correction factors caused by different chromatographic system leading this approach lacking a high accuracy. In the present study, a simple but effective method was established by employing the absorption coefficient () to calculate the relative correction factors instead of peak area or height. The absorption coefficient, a fundamental constant of physics, has been widely used for qualitative and quantitative analysis in Pharmacopoeia all over the world. According to the absorbance coefficient ratio between echinacoside and other compounds, the content of seven phenylethanoid glycosides in Cistanche deserticola and Cistanches tubulosa were determined simultaneously. The low standard method difference on quantitative measurement of seven compounds in Cistanches Herba between our method and the external standard method proved the consistency of the two methods. Using an ultra high performance liquid system, these seven bioactive phenylethanoid glycosides were baseline separated in 8 min. All the data suggested that the method was accurate and reliable for the determination of multi‐components when authentic standard substances were unavailable.     

Possibility of predicting separations in supercritical fluid chromatography with the solvation parameter model

In order to obtain a selection of optimal chromatographic columns for the separation of chlorotriazine pesticides in packed column supercritical fluid chromatography (pSFC), a multi-criteria approach is applied. For this purpose, prediction of the separations is carried out, based on quantitative structure–retention relationships, then Derringer's desirability function is proposed to determine the stationary phase that will result in the most desirable separation. The best SFC separation obtained was then optimized using a mobile phase gradient. Besides, the accuracy of the solvation parameter model as SFC retention predictive model is assessed.     

Reversed phase liquid chromatographic separation of some mono-substituted phenols with calix[6]arene-ρ-sulfonate-modified eluents

Summary The utility of water-soluble calix[6]arene--sulfonate as a mobile phase additive has been investigated for the reversed phase liquid chromatographic separation of some monosubstituted phenol isomers. Retention factors and separation factors for regioisomers of methoxyphenol, aminophenol, and nitrophenol were measured using methanol — water and acetonitrile —water mobile phases of varying composition containing calix[6]arene--sulfonate, and the values were compared with those obtained using mobile phases containing no additive. It was observed that addition of calix[6]arene--sulfonate to both acetonitrile — water and methanol —water caused a reduction in the retention of the phenol isomers but generally increased the separation between them, thereby improving the overall separation efficiency.     

Determination of phenolic acids in plant extracts using CZE with on‐line transient isotachophoretic preconcentration

A novel transient ITP–CZE for preconcentration and determination of seven phenolic acids (caffeic acid, cinnamic acid, p‐coumaric acid, ferulic acid, protocatechuic acid, syringic acid, and vanilic acid) was developed and validated. Effects of several factors such as control of EOF, pH and buffer concentration, addition of organic solvents and CDs, and conditions for sample injection were investigated. Sample self‐stacking was applied by means of induction of transient ITP, which was realized by adding sodium chloride into the sample. The CZE was realized in 200 mM borate buffer ( 9.2) containing 37.5% methanol, 0.001% hexadimethrine bromide, and 15 mM 2‐hydroxypropyl‐β‐CD. Under the optimal conditions for analysis, analytes were separated within 20 min. Linearity was tested for each compound in the concentration range of 0.1–10 μg/mL (R = 0.9906–0.9968) and the detection limits (S/N = 3) ranged from 11 ng/mL (protocatechuic acid) to 31 μg/mL (syringic acid). The validated method was applied to the ethanolic extract of Epilobium parviflorum, Onagraceae. The method of SPE was used for the precleaning of the sample.     

Solvent Effects on the Monomer/Hydrogen‐Bonded Dimer Equilibrium in Carboxylic Acids: (+)‐(S)‐Ketopinic Acid as a Case Study

The hydrogen‐bond‐assisted self‐association process of a chiral semirigid carboxylic acid, namely, (+)‐(S)‐ketopinic acid, has been studied. The multiconformational monomer/dimer equilibrium has been evaluated by means of a concentration‐dependent FTIR study that enabled the experimental equilibrium constants of the dimer formation reaction (K_dim) to be determined in two solvents of different polarity. In CDCl₃, dimeric forms predominate, even in diluted solutions ( =5.074), whereas in CD₃CN the self‐association process is hindered and monomers are always the main species, irrespective of solute concentration ( =0.194). The reliability of the dimerization constants and the derived mono‐ and dimeric experimental fractions have been proven by means of accurate matching between the experimental vibrational circular dichroism spectra of the species and the theoretical spectra generated by considering the simultaneous weighted contributions of the concomitant monomers and dimers.     

Accurate,robust, and reliable calculations of Poisson–Boltzmann binding energies

Poisson–Boltzmann (PB) model is one of the most popular implicit solvent models in biophysical modeling and computation. The ability of providing accurate and reliable PB estimation of electrostatic solvation free energy, , and binding free energy, , is important to computational biophysics and biochemistry. In this work, we investigate the grid dependence of our PB solver (MIBPB) with solvent excluded surfaces for estimating both electrostatic solvation free energies and electrostatic binding free energies. It is found that the relative absolute error of obtained at the grid spacing of 1.0 Å compared to at 0.2 Å averaged over 153 molecules is less than 0.2%. Our results indicate that the use of grid spacing 0.6 Å ensures accuracy and reliability in calculation. In fact, the grid spacing of 1.1 Å appears to deliver adequate accuracy for high throughput screening. © 2017 Wiley Periodicals, Inc.     

Real‐space grid representation of momentum and kinetic energy operators for electronic structure calculations

We show that the central finite difference formula for the first and the second derivative of a function can be derived, in the context of quantum mechanics, as matrix elements of the momentum and kinetic energy operators on discrete coordinate eigenkets defined on a uniform grid. Starting from the discretization of integrals involving canonical commutations, simple closed‐form expressions of the matrix elements are obtained. A detailed analysis of the convergence toward the continuum limit with respect to both the grid spacing and the derivative approximation order is presented. It is shown that the convergence from below of the eigenvalues in electronic structure calculations is an intrinsic feature of the finite difference method. © 2018 Wiley Periodicals, Inc.     

反相离子对高效液相色谱法分离金属配合物｛Fe［3-(2-吡啶基)-5,6-二苯基-1,2,4-三嗪］3｝2+几何异构体

采用反相离子对高效液相色谱法分离测定了金属配合物｛Fe［3-(2-吡啶基)-5,6-二苯基-1,2,4-三嗪］3｝2+ (［Fe(PDT)3］2+)的两种几何异构体,研究了流动相中有机改性剂(乙腈、甲醇)的含量、不同种类和浓度的离子对试剂(高氯酸钠和十二烷基硫酸钠(SDS))对色谱分离的影响。并在不同的试验条件下,对所获得的色谱参数(保留因子(k)、分离度、选择性因子等)进行了探讨。在不同种类及浓度的离子对试剂条件下,二元流动相中乙腈的含量与两种几何异构体的ln k之间均呈显著的线性关系。研究进一步发现SDS的浓度变化对异构体的保留因子影响程度更为显著。在上述实验的基础上,引入更能灵活调节洗脱强度和分离度的三元流动相(乙腈/甲醇/水),优化选择了三元流动相中有机改性剂的比例以及离子对试剂的种类和浓度,使得异构体的色谱分离得到了满意的结果。实验结果表明,异构体的峰面积(A)和浓度(C)之间的线性关系良好,面式和经式异构体的检测限分别为4.28和3.44 ng/mL (S/N=3)。     

Transition‐Metal‐Free Synthesis of 1,3‐Butadiene‐Containing π‐Conjugated Polymers

This work describes the synthesis of π‐conjugated polymers possessing arylene and 1,3‐butadiene alternating units in the main chain by the reaction of α,β‐unsaturated ester/nitrile containing γ‐H with aromatic/heteroaromatic aldehyde compound. By using 4‐(4‐formylphenyl)‐2‐butylene acid ethyl ester as a model monomer, the different polymerization conditions, including catalyst, catalyst amount, and solvent, are optimized. The polymerization of 4‐(4‐formylphenyl)‐2‐butylene acid ethyl ester is carried out by refluxing in ethanol for 72 h with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) as a catalyst to give a 1,3‐butadiene‐containing π‐conjugated polymer, poly(phenylene‐1,3‐butadiene), in 84.3% yield with and / (PDI) estimated as 6172 and 1.65, respectively. Based on this new methodology, a series of π‐conjugated polymers containing 1,3‐butadiene units with different substituents are obtained in high yields. A possible mechanism is proposed for the polymerization through a six‐membered ring transition state and then a 1,5‐H shift intermediate.

     

Photochemical Reaction Between 1,2‐Naphthoquinone and Adenine in Binary Water‐Acetonitrile Solutions

The photochemical reaction between 1,2‐naphthoquinone (NQ ) and adenine was investigated using nanosecond time‐resolved laser flash photolysis. With photolysis at 355 nm, the lowest triplet state T₁ of NQ was produced via intersystem crossing from its singlet excited state. The triplet‐triplet absorption of the state contributes three bands of transient spectra at 374, 596 and 650 nm, respectively, in pure acetonitrile and binary water‐acetonitrile solutions. In the presence of adenine, the observation of ⁺ (at 363 nm) and radical (at 343 and 485 nm) indicates a multistep mechanism of electron transfer process followed by a proton transfer between ³NQ * and adenine. By fitting with the Stern‐Volmer relationship, the quenching rate constant k _q of ³NQ * by adenine in binary water‐acetonitrile solutions (4/1, volume ratio, v/v) is determined as 1.66 × 10⁹ m ⁻¹ s⁻¹. Additionally, no spectral evidence confirms the existence of electron transfer between ³NQ * with thymine, cytosine and uracil.     

An Efficient N‐Heterocyclic Carbene‐Ruthenium Complex: Application Towards the Synthesis of Polyesters and Polyamides

The ruthenium benzimidazolylidene‐based N‐heterocyclic carbene (NHC) complex 4 catalyzes the direct dehydrogenative condensation of primary alcohols into esters and primary alcohols in the presence of amines to the corresponding amides in high yields. This efficient new catalytic system shows a high selectivity towards the conversion of diols to polyesters and of a mixture of diols and diamines to polyamides. The only side product formed in this reaction is molecular hydrogen. Remarkable is the conversion of hydroxytelechelic polytetrahydrofuran ( = 1000 g mol⁻¹)—a polydispers starting material—into a hydrolytically degradable polyether with ester linkages ( = 32 600 g mol⁻¹) and, in the presence of aliphatic diamines, into a polyether with amide linkages in the back bone ( = 16 000 g mol⁻¹).