Ultraviolet and fluorescence derivatization reagents for carboxylic acids suitable for high performance liquid chromatography: a review

Capturing Protein-Protein Interactions with Acidic Amino Acids Reactive Cross-Linkers

Acidic residues (Asp and Glu) have a high prevalence on protein surfaces, but cross-linking reactions targeting these residues are limited. Existing methods either require high-concentration coupling reagents or have low structural compatibility. Here a previously reported "plant-and-cast" strategy is extended to develop heterobifunctional cross-linkers. These cross-linkers first react rapidly with Lys sidechains and then react with Asp and Glu sidechains, in a proximity-enhanced fashion. The cross-linking reaction proceeds at neutral pH and room temperature without coupling reagents. The efficiency and robustness of cross-linking using model proteins, ranging from small monomeric proteins to large protein complexes are demonstrated. Importantly, it is shown that this type of cross-linkers are efficient at identifying protein-protein interactions involving acidic domains. The Cross-linking mass spectrometry (XL-MS) study with p53 identified 87 putative binders of the C-terminal domain of p53. Among them, SARNP, ZRAB2, and WBP11 are shown to regulate the expression and alternative splicing of p53 target genes. Thus, these carboxylate-reactive cross-linkers will further expand the power of XL-MS in the analysis of protein structures and protein-protein interactions.

Validation and Application of a Derivatization-Free RP-HPLC-DAD Method for the Determination of Low Molecular Weight Salivary Metabolites

Saliva is an interesting, non-conventional, valuable diagnostic fluid. It can be collected using standardized sampling device; thus, its sampling is easy and non-invasive, it contains a variety of organic metabolites that reflect blood composition. The aim of this study was to validate a user-friendly method for the simultaneous determination of low molecular weight metabolites in saliva. We have optimized and validated a high throughput, direct, low-cost reversed phase liquid chromatographic method with diode array detection method without any pre- or post-column derivatization. We indexed salivary biomolecules in 35 whole non-stimulated saliva samples collected in 8 individuals in different days, including organic acids and amino acids and other carbonyl compounds. Among these, 16 whole saliva samples were collected by a single individual over three weeks before, during and after treatment with antibiotic in order to investigate the dynamics of metabolites. The concentrations of the metabolites were compared with the literature data. The multianalyte method here proposed requires a minimal sample handling and it is cost-effectiveness as it makes possible to analyze a high number of samples with basic instrumentation. The identification and quantitation of salivary metabolites may allow the definition of potential biomarkers for non-invasive "personal monitoring" during drug treatments, work out, or life habits over time.

A Copper(II) Macrocycle Complex for Sensing Biologically Relevant Organic Anions in a Competitive Fluorescence Assay: Oxalate Sensor or Urate Sensor?

Fluorescence sensing of oxalate has garnered some attention in the past two decades as a result of this anion's prominence and impact on society. Previous work on oxalate sensors and other divalent anion sensors has led to the conclusion that the sensors are selective for the anion under investigation. However, sensor selectivity is often determined by testing against a relatively small array of "guest" molecules or analytes and studies often exclude potentially interfering compounds. For example, studies on oxalate sensors have excluded compounds such as citrate and urate, which are anions in the biological matrices where oxalate is measured (e.g., urine, blood, and bacterial lysate). In the present study, we reassessed the selectivity of a dinuclear copper(II) macrocycle (Cu₂L) in an eosin Y displacement assay using biologically relevant anions. Although previously reported as selective for oxalate, we found greater indicator displacement (fluorescence response) for urate and oxaloacetate and a significant response to citrate. These anions are larger than oxalate and do not appear to fit into the putative binding pocket of Cu₂L. Consistent with previous reports, Cu₂L did not release eosin Y in the presence of several other dicarboxylates, including adipate, glutarate, malate (except at 10 mM), fumarate, succinate, or malonate (except at 10 mM), and the monocarboxylate acetate. This was demonstrated by the failure of the anions to reverse eosin Y quenching by Cu₂L. We also assessed, for the first time, other monocarboxylates, including butyrate, pyruvate, lactate, propionate, and formate. None of these anions were able to displace eosin Y, indicating no interaction with Cu₂L that interfered with the eosin Y binding site. Single-crystal X-ray crystallography revealed that nonselective binding of the anions is likely partly caused by readily accessible copper(II) ions on the external surface of Cu₂L. In addition, π-π stacking of urate with the aromatic groups of Cu₂L cannot be ruled out as a contributor to binding. We conclude that Cu₂L is not suitable for oxalate sensing in a biological matrix unless interfering compounds are selectively removed or masked.

Determination of insoluble soap in agricultural soil and sewage sludge samples by liquid chromatography with ultraviolet detection

We have developed a new analytical procedure for determining insoluble Ca and Mg fatty acid salts (soaps) in agricultural soil and sewage sludge samples. The number of analytical methodologies that focus in the determination of insoluble soap salts in different environmental compartments is very limited. In this work, we propose a methodology that involves a sample clean-up step with petroleum ether to remove soluble salts and a conversion of Ca and Mg insoluble salts into soluble potassium salts using tripotassium ethylenediaminetetraacetate salt and potassium carbonate, followed by the extraction of analytes from the samples using microwave-assisted extraction with methanol. An improved esterification procedure using 2,4-dibromoacetophenone before the liquid chromatography with ultraviolet detection analysis also has been developed. The absence of matrix effect was demonstrated with two fatty acid Ca salts that are not commercial and are never detected in natural samples (C₁₃:₀ and C₁₇:₀). Therefore, it was possible to evaluate the matrix effect because both standards have similar environmental behavior (adsorption and precipitation) to commercial soaps (C₁₀:₀) to C₁₈:₀). We also studied the effect of the different variables on the clean-up, the conversion of Ca soap, and the extraction and derivatization procedures. The quantification limits found ranged from 0.4 to 0.8 mg/kg. The proposed method was satisfactorily applied for the development of a study on soap behavior in agricultural soil and sewage sludge samples.

High-throughput metabolic engineering: advances in small-molecule screening and selection

Metabolic engineering for the overproduction of high-value small molecules is dependent upon techniques in directed evolution to improve production titers. The majority of small molecules targeted for overproduction are inconspicuous and cannot be readily obtained by screening. We provide a review on the development of high-throughput colorimetric, fluorescent, and growth-coupled screening techniques, enabling inconspicuous small-molecule detection. We first outline constraints on throughput imposed during the standard directed evolution workflow (library construction, transformation, and screening) and establish a screening and selection ladder on the basis of small-molecule assay throughput and sensitivity. An in-depth analysis of demonstrated screening and selection approaches for small-molecule detection is provided. Particular focus is placed on in vivo biosensor-based detection methods that reduce or eliminate in vitro assay manipulations and increase throughput. We conclude by providing our prospectus for the future, focusing on transcription factor-based detection systems as a natural microbial mode of small-molecule detection.

Recent trends in the advanced analysis of bioactive fatty acids

The consumption of dietary fats have been long associated to chronic diseases such as obesity, diabetes, cancer, arthritis, asthma, and cardiovascular disease; although some controversy still exists in the role of dietary fats in human health, certain fats have demonstrated their positive effect in the modulation of abnormal fatty acid and eicosanoid metabolism, both of them associated to chronic diseases. Among the different fats, some fatty acids can be used as functional ingredients such as alpha-linolenic acid (ALA), arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), gamma-linolenic acid (GLA), stearidonic acid (STA) and conjugated linoleic acid (CLA), among others. The present review is focused on recent developments in FAs analysis, covering sample preparation methods such as extraction, fractionation and derivatization as well as new advances in chromatographic methods such as GC and HPLC. Special attention is paid to trans fatty acids due its increasing interest for the food industry.

Rapid preparation of fluorescent 9-anthrylmethyl esters for fatty acid analysis of small amount of triacylglycerols

This paper proposes a one-step method for preparation of fluorescent 9-anthrylmethyl esters from triacylglycerols (TAG) ranging in amount from 0.1 to 5 microg. It involves base-catalyzed transesterification using potassium 9-anthracenemethoxide, prepared by proton exchange between 9-anthracenemethanol and potassium tert-butoxide. Transesterification for 10 min at room temperature gave the fatty acid 9-anthrylmethyl esters in nearly maximal yields (82-85%). The products could be analyzed by reversed-phase HPLC without purification. Excellent linear relationships were observed for standard curves of 10-250 pmol of TAG standards (16:0, 19:0, 18:2 and 22:6), and differences in the slopes were less than 5% among the standards. Almost consistent compositions of the esters were observed for the products formed from 0.5 to 5 microg or less of fish oils TAG, and they were similar to those obtained by HPLC of ordinary multi-step synthesis products and by GLC of methyl esters. The present method is a great improvement of derivatization time, and is powerful for fatty acid analysis of small amounts of natural TAG.

Development of a semi-automated chemical stability system to analyze solution based formulations in support of discovery candidate selection

A semi-automated chemical stability system was developed, validated, and implemented to assess the chemical and physical stability (24 h) of intravenous and oral solution based formulations in support of preliminary in vivo drug discovery studies. The system utilizes a single Agilent 1100 LC and Xterra column with multiple UV wavelength monitoring. Mobile phase selection, either basic or acidic, is selected base upon on the physico-chemical properties of the test compound. The system was validated against 14 new chemical entities across multiple therapeutic areas. The results indicated that drug discovery compounds could be accurately quantified (<2% R.S.D.) in a wide range of formulation vehicles in greater than 90% of the test cases. This method can be used as a quantitative tool for triaging formulation variables and packaging configurations to quickly develop stable solutions for dosing.

Simultaneous Determination of Carbohydrates, Carboxylic Acids, Alcohols, and Metals in Foods by High-Performance Liquid Chromatography Inductively Coupled Plasma Atomic Emission Spectrometry

The applicability of the HPLC-ICP-AES coupling for the simultaneous determination of carbohydrates, carboxylic acids, alcohols, and metals in a single chromatographic run has been demonstrated in the present work. Five saccharides, glucose, fructose, sucrose, sorbitol, and lactose; five carboxylic acids, citric, tartaric, malic, lactic, and acetic; and three alcohols, glycerol, ethanol, and methanol, have been determined. A H+ cation exchange column has been used to separate these compounds. The chromatograms have been obtained by monitoring the carbon emission signal at 193.09 nm. The results obtained by HPLC-ICP-AES have been compared against those found with conventional detection systems (i.e., refractive index, UV, and photodyode array detectors). The HPLC-ICP-AES method has shown the following features: (i) organic compounds and metals can be simultaneously determined; (ii) the detection method is universal; (iii) for nonvolatile organic compounds, a complete calibration line can be obtained from a single injection; and (iv) it provides absolute limits of detection similar to or lower than those found with conventional detection systems (i.e., on the order of several tens of nanograms of organic compound). The methodology has been validated through the analysis of food samples such as juices, isotonic beverages, wines, and a certified nonfat milk powder sample.

Electrogenerated Chemiluminescence Detection of Amino Acids Based on Precolumn Derivatization Coupled with Capillary Electrophoresis Separation

A novel method for highly sensitive detection of primary and secondary amino acids with selective derivatization using acetaldehyde as a new derivatization reagent was proposed by capillary electrophoresis (CE) coupled with electrogenerated chemiluminescence (ECL) of tris(2,2'-bipyridine)ruthenium(II). The precolumn derivatization of these amino acids with acetaldehyde was performed in aqueous solution at room temperature for 1 h. Upon optimized derivatization, the ECL intensities and detection sensitivities of the amino acids were significantly enhanced by 20-70 times. Using four amino acids, arginine, proline, valine, and leucine, as model compounds, their derivatives could be completely separated by CE and sensitively detected by ECL within 22 min. The linear ranges were 0.5-100 microM for arginine and proline and 5-1000 microM for valine and leucine with the detection limits of 1 x 10(-7) (0.5 fmol, arginine), 8 x 10(-8) (0.4 fmol, proline), 1 x 10(-6) (5 fmol, valine), and 1.6 x 10(-6) M (8 fmol, leucine) at a signal-to-noise ratio of 3. The derivatization reactions and ECL process of amino acids were also proposed based on in situ Fourier transform infrared and ultraviolet spectrometric analyses.

Determination of bioactive eicosanoids in brain tissue by a sensitive reversed-phase liquid chromatographic method with fluorescence detection

Arachidonic acid (AA) is metabolized to prostaglandins (PGs) via cyclooxygenases (COX) catalysis, and to epoxyeicosatrienoic acids (EETs), dihydroxyeicosatrienoic acids (DiHETrEs), and hydroxyeicosatetraenoic acids (HETEs) via cytochrome P450 (CYP450) enzymes. A reliable and robust fluorescence based HPLC method for these eicosanoids was developed. A new selective reverse-phase solid phase extraction (SPE) procedure was developed for PG, DiHETrEs, HETE, and EETs of interest from rat cortical brain tissue. The eicosanoids were derivatized with 2-(2,3-naphthalimino)ethyl-trifluoromethanesulphonate (NE-OTf), followed by separation and quantification at high sensitivity using reverse-phase HPLC with fluorescent detection, and further identified via LC/MS. The derivatization was studied and optimized to obtain reproducible reactions. Various PGs, DiHETrEs, HETEs, EETs, and AA were sensitively detected and baseline resolved simultaneously. LC/MS under positive electrospray ionization selected ion monitoring (SIM) mode was developed to further identify the peaks of these eicosanoids in cortical brain tissue. The method was applied in the traumatic brain injured rat brain.

Determination of lipid profile in meningococcal polysaccharide using reversed-phase liquid chromatography

A fast and sensitive HPLC method using fluorescence detection is developed to quantitate 1-pyrenyldiazomethane (PDAM) derivatized fatty acids derived from the lipid components of both the capsular meningococcal polysaccharide and other impurities such as endotoxin in various meningococcal vaccine samples. The HPLC method is capable of well resolving 13 relevant fatty acids within 40min by using a multi-stage acetonitrile/water gradient. Endotoxin values measured by HPLC well correlated with results from the standard Limulus amebocyte lysate (LAL) assay. Furthermore, the fatty acid profiles of various process intermediate samples as well as final purified polysaccharide products were determined to better understand and characterize the purification process.

Recent progress in derivatization methods for LC and CE analysis

The derivatization procedure with a suitable fluorescence or chemiluminescence reagent is performed for the purpose of increasing the detection sensitivity and selectivity, in high-performance liquid chromatography (HPLC) and/or capillary electrophoresis (CE). In this article, recent derivatization methods and their applications to biosamples are described. In HPLC, femto mol order of mass detection limits are obtained by derivatization. Regarding the fluorescence reagents, the use of water-soluble reagents has been effective to avoid an undesired adsorption in the process of determination of peptides. In CE, the advantages of having extremely low mass detection limits (ranging from atto to yocto mol level) and requiring only a very short analysis time (less than a few minutes) are made possible by using laser-induced fluorescence or near infra-red detections.

Methods for homocysteine analysis and biological relevance of the results

It is now widely accepted that increased total plasma homocysteine (tHcy) is a risk factor for cardiovascular disease. Hyperhomocysteinemia can be caused by impaired enzyme function as a result of genetic mutation or vitamin B (B(2), B(6), B(9), B(12)) deficiency. A lot of methods are now available for tHcy determination. High-pressure liquid chromatography (HPLC) with fluorescence detection are at present the most widely used methods but immunoassays, easier to use, begin to supplant in-house laboratory methods. In order to help with the choice of a main relevant homocysteine analytical method, the characteristics, performances and limits of the main current methods are reviewed. One major drawback among all these available methods is the transferability which is not clearly established to date. The impact of both inter-method and inter-laboratory variations on the interpretation of the tHcy results are discussed.

Design and synthesis of a hydrophilic fluorescent derivatization reagent for carboxylic acids, 4-N-(4-N-aminoethyl)piperazino-7-nitro-2,1,3-benzoxadiazole (NBD-PZ-NH2), and its application to capillary electrophoresis with laser-induced fluorescence detection

A hydrophilic fluorescent derivatization reagent for fatty acids, 4-N-(4-N-aminoethyl)piperazino-7-nitro-2,1,3-benzoxadiazole (NBD-PZ-NH(2)), was designed and synthesized. NBD-PZ-NH(2) possesses not only a fluorophore and a reacting group but also a positive charge group and, thus, was hydrophilic and suitable for application to capillary electrophoresis. NBD-PZ-NH(2) reacted with fatty acids in the presence of triphenylphosphine (TPP) and 2,2'-dipyridyl disulfide (DPDS) at room temperature within 10 min. The derivatives were strongly fluoresced and were positively charged at pH below 3. The derivatives of C4-C20 fatty acids were separated within 10 min in 50% acetonitrile in water containing 30 mM ammonium acetate and 1.0 M acetic acid by capillary electrophoresis with laser-induced fluorescence (CE-LIF) detection. The detection limits attained were 6.5 nM (signal-to-noise ratio of 3). It is proposed that NBD-PZ-NH(2) is a prominent derivatization reagent for fatty acids which is suitable for CE-LIF application.

Analysis of aminofluorescein-fatty acid derivatives by capillary electrophoresis with laser-induced fluorescence detection at the attomole level: application to mycobacterial fatty acids

A method based on capillary electrophoresis coupled to laser-induced fluorescence detection was developed for the characterization of fatty acids including palmitic, stearic, oleic and tuberculostearic acids. The fatty acids were tagged by 4-aminofluorescein (AF) via a carboxylic acid-amine condensation promoted by N'-(3-dimethylaminopropyl)-N-ethylcarbodiimide (EDC) in non-aqueous solution. Using the optimized derivatization conditions, the fluorophore labeling of the fatty acids was achieved at the nanomolar level. The separation of palmitic, stearic, oleic and tuberculostearic-AF derivatives was achieved in less than 10 min, using 25 mM sodium borate buffer containing 30% of acetonitrile as running electrolyte. The concentration detection limit was found to be 5 nM while the minimum mass limit detection is around 30 attomol. This method was successfully applied to identification of mycobacteria via the characterization of tuberculostearic acid and found to be suitable for the detection of a minimum of 10(6) mycobacteria.

Electrogenerated chemiluminescence derivatization reagents for carboxylic acids and amines in high-performance liquid chromatography using tris(2,2'-bipyridine)ruthenium(II)

2-(2-Aminoethyl)-1-methylpyrrolidine and N-(3-aminopropyl)pyrrolidine (NAPP) were found to be selective and sensitive derivatization reagents for carboxylic acid by high-performance liquid chromatography (HPLC) with electrogenerated chemiluminescence detection using tris(2,2'-bipyridine)ruthenium(II). Free fatty acids and ibuprofen were used as model compounds of carboxylic acids, and the derivatization conditions were optimized with myristic acid as a representative of free fatty acids. All the fatty acids tested were reacted with NAPP to produce highly sensitive derivatives under the mild reaction conditions of room temperature for 30 min in acetonitrile containing 2-bromo-1-ethylpyridinium tetrafluoroborate and 9-methyl-3,4-dihydro-2H-pyrido[1,2-a]pyrimidin-2-one. The chemiluminescence intensities were similar for all fatty acids. The derivatives obtained from 10 free fatty acids were completely separated by reversed-phase chromatography under isocratic elution conditions. The on-column detection limits (signal-to-noise ratio of 3) with proposed HPLC separation and chemiluminescence detection were 70 and 45 fmol for myristic acid and ibuprofen, respectively. The free fatty acids in human plasma were successfully determined using the present method. Histamine, a model compound of primary amines, was also determined after precolumn derivatization with 3-(diethylamino)propionic acid at room temperature for 60 min in acetonitrile containing N,N'-dicyclohexylcarbodiimide and 3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine with the detection limit of 70 fmol.

High-sensitivity determination of the degradation products of chemical warfare agents by capillary electrophoresis-indirect UV absorbance detection

Capillary electrophoresis coupled with indirect UV absorbance detection was employed for the determination of the chemical warfare agent degradation products: methylphosphonic acid, ethyl methylphosphonate, isopropyl methylphosphonate, and pinacolyl methylphosphonate. Glutamic acid was used as a buffering agent at its isoelectric point (pH 3.22). In its zwitterionic form, glutamic acid does not act as a competing co-anion in the system, thus providing buffering capacity while maintaining high sensitivity. The indirect probe (phenylphosphonic acid) concentration was lowered to 1 mM from the 10 mM in previous literature studies, further enhancing sensitivity. Detection limits of 2 microM were achieved with hydrodynamic injection and up to 100-fold lower using electrokinetic injection. The increased buffering capacity of this system over previous methods led to migration time reproducibility RSD values of 0.18 to 0.22%. This represents a 10-fold improvement in reproducibility over previous studies with comparable or improved sensitivity.

Potentiometric detection of organic acids in liquid chromatography using polymeric liquid membrane electrodes incorporating macrocyclic hexaamines

Potentiometric detection employing coated-wire electrodes was applied to the determination of organic acids in liquid chromatography (LC). Poly(vinyl chloride)-based liquid membranes, incorporating lipophilic macrocyclic hexaamines as neutral ionophores were used as electrode coatings. The selectivity and sensitivity of the macrocycle-based electrodes were found to be superior to an electrode based on a lipophilic anion exchanger (a quaternary ammonium salt). Sensitive detection was obtained for the di- and tricarboxylic acids tartaric, malonic, malic, citric, fumaric, succinic, pyruvic, 2-oxoglutaric and maleic acids after separation in reversed-phase LC. Detection limits (signal/4sigmanoise=3) of 6 pmol for malonic acid and 2 pmol for maleic acid were attained. The detection was explained using a molecular recognition model. The hexaamine-based potentiometric electrodes had a 1-s response time at 1 ml min(-1) flow-rates. They were stable for at least 4 months, with an intra-electrode variation of 3.2% (n=5).

Systematic analysis of oxidative degradation of polysaccharides using PAGE and HPLC--MS

Oxidation of polysaccharides yields hydroxyaldehydes and hydroxycarboxylic acids. Aldehydes and carboxylic acids were separately conjugated to 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS) or tyrosine t-butyl ester (TBT). The ANTS-labeled derivatives were separated by molecular size on PAGE gels and detected by fluorescence. TBT-labeled derivatives were separated by reverse phase chromatography on a C18-HPLC column and analyzed by positive ion electrospray mass spectroscopy (HPLC--MS). This combination of procedures allowed a systematic analysis of carbohydrate oxidation products.

Determining sulfur-containing amino acids by capillary electrophoresis: a fast novel method for total homocyst(e)ine human plasma

A high-performance capillary electrophoresis (HPCE) method based on laser-induced fluorescence detection is presented here. It enables the determination of sulfur-containing amino acids within 15 min. Fluorescence of sulfur-containing amino acids in plasma is linear over a range of 50-150 micromol/L for L-methionine, 5-100 micromol/L for L-homocysteine, and 50-200 micromol/L for L-cysteine. For homocysteine, we were able to detect 1 fmol injected, equivalent to a plasma concentration of 10 nmol/L. A similar sensitivity is present for cysteine, an even lower one being found for methionine. The intra- and interassay relative standard deviations are < 1%. High-performance liquid chromatography (HPLC) methods are commonly employed for quantifying blood concentrations of sulfur-containing amino acids. A comparative analysis of HPCE and HPLC quantitation of homocysteine has been carried out in 61 blood samples. Plasma concentrations measured by HPCE were in good agreement with those obtained employing an HPLC-based method, a satisfactory correlation being observed between the concentrations obtained by the two methods (r= 0.9972). Thus, the HPCE-based procedure presented here for the measurement of sulfur-containing amino acids in plasma is a simple, fast, accurate, and very sensitive method, suitable for routine determinations in clinical studies.