Mechanistic Study of Microwave-Assisted Acid Hydrolysis of Peptides on TLC Plates

Abstract

Microwave-induced acid hydrolysis was performed with isolated peptides on TLC layers. This vapor phase method was compared to solution phase techniques. A mechanistic study of peptide bond clevage was carried out by varying exposure conditions and reaction time. Acid hydrolysis products were identified by ninhydrin visualization. Also, TLC and TLC/MS were used to elucidate hydrolysis mechanisms.     

Normal Phase TLC Separation of Enantiomers Using Chiral Ion Interaction Agents

Abstract

A method for the thin layer chromatographic (TLC) separation of enantiomers and diastereomers involving the use of chiral ion interaction agents is described. Several aromatic amino alcohols were resolved by TLC on diol and/or high performance silica gel plates using a mobile phase containing (1R)-(-)- ammonium-10-camphorsulfonate or N-benzoxycarbonyl-glycyl-L-proline (ZGP). Many of these chiral aromatic amino alcohols are of pharmacological importance as α- and β-adrenergic blockers, adrenergic compounds, and anti-glaucoma agents. A comparison was made between various N-CBZ-amino acid derivatives as chiral counter ions/chiral mobile phase additives (CMAs). These separations could not be achieved on other normal phase TLC stationary phases including microcrystalline cellulose, alumina and ordinary silica gel plates.     

The Application of Cross-Scanning to Conventional Thin-Layer Chromatography

Abstract

The technique of cross-scanning, the scanning of thin-layer chromatographic (TLC) plates perpendicular to the direction of development, is applicable to conventional quantitative TLC where significant advantages over normal scanning are realized by its use. Several factors are discussed which influence the ability to cross-scan, such as application of the sample, solvent development and instrumentation.     

Atomic Absorption Detector for Liquid-Liquid Chromatography

Abstract

A detector system specific for the measurement of chelating agents and applicable to liquid-liquid chromatography has been developed. A solution containing the chelating agent is passed over a short column of chelating ion exchange resin in the copper form, and directly into the aspirator of a recording atomic absorption spectrophotometer set to measure copper. A quantity of copper proportional to the quantity of the chelating agent is stripped from the column and recorded as a peak. Precision and accuracy compare with normal atomic absorption techniques, with an average mean standard deviation of 3%, and a corresponding accuracy. The detection limit for this method is 5x10-⁷ millimoles of EDTA or NTA. Determinations are rapid and reproducible in solutions having a pH in the range of 4 to 9. The role of interfering ions is considered. The applicability of the method as a specific detector system for liquid-liquid chromatographic separations of chelating agents is discussed. Recovery of chelating agents from solutions with an ionic strength approximating natural fresh waters is demonstrated.     

Synthetic Inorganic Ion Exchangers. XIV Thin-Layer Chromatography of Metal Ions on Thorium Phosphate: Quantitative Separation of Au (III) from Several Metal Ions

Abstract

The analytical potential of thorium phosphate as an ion exchanger has been explored by thin layer chromatographic (TLC) technique. Binder-free thin layers of thorium phos -phate have been explored for several important binary and ternary separations. A TLC method has been developed for quantitative separation of microgram-quantities of Au(III) from several metal ions in pure butanol system.     

Chiral Separations: A Comparison of Hplc and TLC

Abstract

Comparisons are made for separations attained in normalphase HPLC and TLC involving N-carbobenzyloxy-glycyl-L-proline and (+/?)?10-camphorsulfonic acid as chiral mobile phase additives/chiral counter ions (CMA). Possible reasons for differences in stereoselectivity of β-cyclodextrin CMA and chiral stationary phases (CSP) are discussed. In addition, differences in solubility and proposed separation mechanisms for native cyclodextrins (CD) versus derivatized CD are discussed. Inherent differences in high performance liquid chromatographic (HPLC) and thin-layer chromatographic (TLC) methods are outlined. The pros and cons of using HPLC and TLC are considered.     

Unconventional TLC systems in lipophilicity determination: A review

ABSTRACT

This review focuses on four unusual thin-layer chromatography (TLC) approaches for the determination of lipophilicity: (1) the use of medium-polar stationary phases: CN, NH₂, and DIOL instead of RP plates, together with water-based mobile phase; (2) the use of silica gel in a typical normal-phase manner and treating extrapolated retention indices as the “reversed lipophilicity”; (3) the use of oil impregnated silica gel in the reversed-phase manner; and (4) the use of salting-out mobile phases. The chromatographic indices obtained in these systems are numerously reported as well correlated with lipophilicity and they are an interesting alternative to classical RP systems approaches.     

Dark conglomerate phases of bent-core liquid crystals

ABSTRACT

Spontaneous or induced chiral symmetry breaking in achiral systems is unusual and understanding the origin of such a phenomenon has been an important area of research for several years. The optically isotropic mesophases exhibited by unconventional liquid crystals are one of the most interesting systems to investigate spontaneous chiral symmetry breaking in liquid crystal mesophases formed by achiral moieties. The dark conglomerate (DC) phases are one such optically isotropic family of phases. In this paper, a detailed account of the tendency of bent-core mesogens to form a variety of polar smectic phases, the formation of DC phases due to layers deformations and the general optical, electrical, physical properties of the DC phases are given. An example of a DC phase which exhibit distinct electro-optic properties is described with the nature of dynamics of the response and physical reasons responsible for such behaviour. The challenges and prospects of the DC phases are discussed for their potential applications in novel devices.     

Synthesis, binding affinity, and relaxivity of target-specific MRI contrast agents

Although magnetic resonance imaging (MRI) is one of the most important imaging modalities of the central nervous system (CNS), one of the main drawbacks of MRI is its limited specificity. This can potentially be partially alleviated by target-specific contrast agents. In the present paper we describe a simple high yield synthesis of two such gadolinium-based spiperone targeted MRI contrast agents, 1a and 1b. The R₁ relaxivities of 1a and 1b were evaluated and found to be 5.94 and 8.31 mM⁻¹ s⁻¹, respectively at 9.4T, while their R₂ relaxivities at the same magnetic field were found to be 18.05 and 22.60 mM⁻¹ s⁻¹, respectively. In addition and very importantly compound 1a, which is a gadolinium-based, spiperone-targeted MRI contrast agent, was found to preserve some of the spiperone affinity toward the dopamine D2 receptor. Compounds 1a and 1b thus represent potential agents for in vitro dopamine receptor imaging using MRI in experimental models. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Design of Therapeutic Chelating Agents

Abstract

The factors involved in the design of therapeutic chelating agents are outlined on the basis of the theoretical analyses of ligand design and experimental data obtained in animal studies. The starting point in such design must always be those factors which assure that a sufficiently high stability constant be achieved, and here the analyses presented by Martell and his co-workers furnish a general approach. If the removal of intracellular metal deposits is to be achieved, additional factors need to be considered to incorporate variables which govern the interaction of the chelating agent with the membrane systems of those organs within which the toxic metal is concentrated. For these, the QSAR (quantitative structure activity relationship) procedure of Hansch furnishes a useful guide. This allows the development of direct structure-efficacy correlations (DSEC) involving molecular parameters in addition to those which are directly involved in the determination of the stability constant. In several cases data are available which indicate how the relative efficacy of two chelating agents with essentially identical stability constant expectations is dependent upon structural features which govern the relative ease with which such molecules can gain access to intracellular deposits. The combination of these approaches allows the joint use of in vitro and in vivo data to design improved therapeutic chelating agents with an increased probability of success when tested in vivo.     

Determination of Triazine and Chlorophenoxy Acid Herbicides in Natural Water Samples By Solid Phase Extraction and Quantitative Thin Layer Chromatography

Abstract

Atrazine, simazine, 2, 4-D, silvex, and 2, 4, 5-T, were determined in natural water samples at 10 ppb levels by solid phase extraction on disposable C₁₈ columns and TLC on preadsorbent silica gel layers impregnated with AgNO₃, exposure to UV light, and densitometric scanning. Recoveries ranged from 70 to 88% for the triazines and 93 to 100% for chlorphenoxy acid herbicides, with average CV values of 7 to 8%. Solid phase extraction proved to be an advantageous alternative to classical liquid-liquid partition for the analysis of water for these compounds by quantitative TLC.     

Determination of Solvation Effects in Liquid Adsorption Chromatography with Mixed Mobile Phases

Abstract

A model of chromatographic process, involving formation of multimolecular solute-solvent solvates in the mobile phase, is discussed. This model leads to an equation describing dependence of the capacity ratio upon mobile phase composition. The TLC data for some solutes chromatographed in n-heptane+chloroform on silica gel are interpreted by means of the above equation.     

Quantification of Niacin and Niacinamide in Vitamin Preparations by Densitometric Thin Layer Chromatography

Abstract

A densitometric TLC method was developed for the determination of the B-3 vitamins niacin and niacinamide in commercial vitamin preparations. The method involves removal of excipients by ethanol precipitation, separation of the vitamins by high performance TLC on phosphor-containing silica gel layers, and in situ scanning of sample and standard zones. The per cent recoveries for niacinamide in vitamin products were 96 to 104% and for niacinamide added to products 99 and 102%. Recoveries of added niacin were 99 to 102%, but recoveries of niacin in products ranged from 0 to 74% of the label values. Additional ingredients did not interfere with the determination.     

Thin Layer Chromatographic Method for Rapid Identification and Quantification of Corticosteroid Sodium Phosphates in Pharmaceutical Preparations

Abstract

A rapid and inexpensive thin layer chromatography (TLC) procedure for the assay of dexamethasone sodium phosphate (DSP) and betamethasone sodium phosphate (BSP) persent in pharmaceutical preparations is described. Free steroids liberated after alkaline phosphatase reaction is isolated by TLC on silica gel layer and estimated after elution with ethanol. Assay of unesterified steroids and identification of the preservatives like methyl or propyl parabenzoates, phenol, benzyl alcohol etc. may also be carried out by this method without any additional cost. Possibility of using this method for other corticosteroid sodium phosphates (CSP) are discussed.     

Synthetic Inorganic Ion-Exchangers XV. Thin-Layer Chromatography of Metal Ions on Thorium Tungstate: Quantitative Separation of Hg(II) from Several Other Metal Ions

Abstract

The analytical application of Thorium tungstate as an ion exchanger has been explored by thin-layer chromatographic (TLC) technique. Binder-free thin layers of thorium tungstate have been employed for some important binary and ternary separations. Quantitative separation of Hg(II) from the mixture of several other metal ions has been achieved.     

Thin-layer adsorption chromatography with mixed mobile phases. 1. Characterization of chromatographic systems showing energetic heterogeneity of adsorbent surfaces with regard to admolecules of solvents and solutes

Thin-layer adsorption chromatography with a multicomponent mobile phase is discussed. A new equation for predicting R_M-values in TLC with mixed mobile phases using the R_M-ValUeS obtained for pure solvents is proposed. This equation takes into account effects of energetic heterogeneity of the adsorbent surface with regard to adsorbed molecules of solvents and solutes. Experimental verification of this equation is presented for R_M-data obtained by TLC using a binary mobile phase.     

Surface-Mediated Synthesis of O-Alkyl 2-Methoxyethyl Alkylphosphonates Under Solvent Free Conditions: Potential Marker of Nerve Agents

This article describes rapid and efficient surface mediated synthesis of O-alkyl 2-methoxyethyl alkylphosphonates from alkylphosphonic acids and alcohols using dicyclohexyl carbodiimide (DCC)-Celite as a solid support. These compounds are markers of nerve agents. 2-Methoxyethyl methylphosphonic acids (2a–d) were reacted with various alcohols to yield O-alkyl 2-methoxyethyl alkylphosphonates (AMEAPs, 3a–m). This synthetic method has advantages over other methods in terms of selectivity, shorter reaction time, high yield, and easy work-up.     

Thin-layer chromatographic and high-performance thin-layer chromatographic ready-for-use layers with lipophilic modifications

Stable silica gel sorbents with aliphatic or aromatic groups are formed by chemical modifications of the silanol groups with special reactive silanes. Various lipophilic surface modifications on silica gels with varying pore structures are tested with regard to their chemical and physico-chemical characteristics, their wettability and their chromatographic retention data. The main problem in TLC is the preparation of abrasion-resistant layers on glass or on foils which meet the usual high standard of quality and are also suited for quantitative determinations. Thin-layer chromatography on reversed-phase layers can only be performed if the complete wettability of the lipophilic stationary phase in dry form is guaranteed by the mobile phase. Adsorption-chromatographic separations with lipophilic eluents and reversed-phase partition-chromatographic separations with hydrophilic eluents are performed, for example, with dyes, with polycyclic aromatic hydrocarbons and with lipids. The great differences in selectivity caused by the various modifications of the sorbent and the varying eluent composition are remarkable. Ready-for-use layers with lipophilic surface modifications complement the existing range of pre-coated layers and thus widen the application of TLC and HPTLC considerably.     

NaSn5: An Intermetallic Compound with Covalent α-Tin and Metallic β-Tin Structure Motifs

A novel and unusual three-dimensional network of tin atoms is present in NaSn₅, in which metallic layers analogous to those in β-Sn alternate with tetravalent units analogous to α-Sn. The compound shows the emergence of pentagonal-dodecahedral units from the metallic β-Sn modification (see structure on the right; all unlabeled spheres are Sn atoms). Quantum-mechanical investigations indicate the simultaneous presence of structural regions with localized and delocalized bonds.     

Determination of Captan,Folpet, and Captafol in Water,Apples, and Lettuce By Quantitative Thin-Layer Chromatography

Abstract

Captan, folpet, and captafol were determined in water, lettuce, and apples by TLC of extracts on preadsorbent silica gel layers, detection with silver nitrate reagent, and densitometric scanning. The fungicides were extracted from water on Chromosorb 102 raicrocolumns. Cleanup on a Florisil column was required for the food extracts. Recoveries from distilled and tap water ranged from 76–98% at 0.02 ppm and 81–94% at 0.007 ppm. Recoveries from lettuce ranged from 88–94% and from apples 84–90%, both at 0.25 ppm. The selectivity, sensitivity, and precision of the method are adequate for routine residue analysis.