Use of water-soluble 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide for the fluorescent determination of uronic acids and carboxylic acids

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

L-Carnitine moiety assay: an up-to-date reappraisal covering the commonest methods for various applications

L-Carnitine and its esters are typical endogenous substances. Their homeostatic equilibria are effectively controlled by various mechanisms which include rate-limiting enteral absorption, a multicomponent endogenous pool which is regulated according to a mammillary metabolism, an asymmetric body distribution and a saturable tubular reabsorption process leading to renal thresholds. In formal pharmacokinetic and metabolic investigations, the whole L-carnitine pool should be investigated, owing to the rapid interchange process between the various components of the pool. Free L-carnitine, as well as its acyl esters, must therefore be considered from an analytical viewpoint. L-Carnitine, acetyl-L-carnitine and total L-carnitine (the latter as an expression of the whole pool) can easily be assayed by enzyme or radioenzyme methods. Propionyl-L-carnitine and other esters containing fatty acids with more than three carbon atoms can be assayed using various HPLC approaches. Tandem mass spectrometry is another excellent approach to the assay of carnitine and its short-chain, medium-chain and long-chain esters. As L-carnitine contains a chiral carbon atom, the enantioselectivity of the assays is also considered in this review. Metabolites produced by enteral bacteria, namely tri-, di- and mono-methylamine, gamma-butyrobetaine, along with other systemic metabolites, namely trimethylamine N-oxide and N-nitroso dimethylamine, are very important in quantitative and toxicokinetic terms and require specific assay methods. This review covers the commonest methods of assaying carnitine and its esters, their impurities and pre-systemic and systemic metabolites and gives analytical details and information on their applications in pharmaceutics, biochemistry, pharmacokinetics and toxicokinetics.

Determination of L-carnitine, acetyl-L-carnitine and propionyl-L-carnitine in human plasma by high-performance liquid chromatography after pre-column derivatization with 1-aminoanthracene

A new sensitive high-performance liquid chromatographic procedure for the determination of L-carnitine (LC), acetyl-L-carnitine (ALC) and propionyl-L-carnitine (PLC) in human plasma has been developed. Precolumn derivatization with 1-aminoanthracene (1AA), performed in phosphate buffer in the presence of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) as catalyst, is involved. The fluorescent derivatives were isocratically separated on a reversed-phase column (C18). The eluate was monitored with a fluorimetric detector set at 248 nm (excitation wavelength) and 418 nm (emission wavelength). Because of the presence of endogenous carnitines, the validation was performed using dialyzed plasma. The identity of the derivatized compounds was assessed by mass spectrometry and the purity of the chromatographic peaks was confirmed by HPLC-tandem mass spectrometry. The limits of quantitation were 5 nmol/ml for LC, 1 nmol/ml for ALC and 0.25 nmol/ml for PLC. The recovery of the extraction procedure was in the range 82.6%-95.4% for all 3 compounds. Good linearity (R approximately 0.99) was observed within the calibration ranges studied: 5-160 nmol/ml for LC, 1-32 nmol/ml for ALC and 0.25-8 nmol/ml for PLC. Precision was in the range 0.3-16.8% and accuracy was always lower than 10.6%.

Method for the quantification of alginate in microcapsules

Alginate is a key reagent in the preparation of microcapsules for cell transplantation. To address the question of the intracapsular alginate concentration, a sensitive assay has been developed to quantify the alginate content of microcapsules. The method is based on the metachromatic change induced by alginate binding to the dye, 1,9-dimethyl methylene blue (DMMB). The assay has a high sensitivity and precision. It covers a wide concentration range enabling the measurement of alginate in dilute supernatants as well as in microcapsules. For the latter, the membrane is initially dissolved by incubating the microcapsules in an alkaline medium. The effect of potentially interfering substances (poly-L-lysine (PLL), citrate, chloride, sodium) and of pH has been studied. Poly-L-lysine interfered with the assay at pH 6.5 but not at pH 13. Interference by sodium augmented with increasing sodium concentration and reached a plateau at 200 mM. This problem was overcome by routinely adjusting all samples to 500 mM sodium. The other substances tested had a negligible effect on the assay. The reliable measurement of alginate with this new assay will allow the optimization of the intracapsular alginate concentration.

Quantitative carbohydrate analyses of the tectorial and otoconial membranes of the guinea pig

Carbohydrate composition of the tectorial membrane (TM) and the otoconial membrane (OM) of the guinea pig was analyzed after hydrolysis, using high-performance anion-exchange chromatography and pulsed amperometric detection. Both of the tissues were highly glycosylated; the carbohydrate content being 24-42% of protein. GlcN, Gal, Glc and Man were found to be the major component sugars of TM, whereas little GalN was found. Fuc and NANA were also present, but NGNA was not detectable. After digestion with thermolysin for solubilization, OM was separated into two fractions: insoluble mineral particles of the otoconia (OM-ppt) and a soluble fraction from the gelatinous layer (OM-sup). These two fractions showed distinct carbohydrate composition from each other. Further analyses using glycosidases revealed that TM contained asialyl and monosialyl but little di-, tri- and tetrasialyl N-glycosides, and OM-sup did not seem to be susceptible to endo-beta-galactosidase, which is known to cleave some N-acetyl-polylactosamine and keratan sulfate. Based on these analyses, it can be suggested that most of the carbohydrates in TM are likely to be asialyl and monosialyl N-glycosides. N-Glycosides may be predominant in the otoconia as well, and a polymer structure consisting of GlcN(Ac) and Gal other than N-acetyl-polylactosamine may exist in the gelatinous layer of OM. O-Glycosylation of the usual type appeared to be minor in all the fractions.

Automated precolumn fluorescence labelling by carbodiimide activation of N-acetylaspartate and N-acetylaspartylglutamate applied to an HPLC brain tissue analysis

An automated method is described to couple carboxyl-containing metabolites to the fluorophore 2-aminoanthracene in aqueous solution (containing 75% methanol) in the presence of N,N-dicyclohexylcarbodiimide. The reaction was optimized for N-acetylaspartate (N-Ac-Asp) and N-acetylaspartylglutamate (N-Ac-Asp-Glu). The reactions occurred within 5 min at room temperature in the presence of 0.5-2 mM HCl. At concentrations of electrolytes exceeding 10 mM the coupling reaction became suboptimal. Derivatization was performed in a commercial precolumn derivatization unit. Additional tubing was needed to provide the reagents prior to reversed-phase HPLC and fluorescence detection. The assay is linear over at least three orders of magnitude; as little as 1 pmol could reproducibly be assayed in 100 micrograms wet weight brain tissue extracted with a mixture of methanol and 4 mM HCl (9:1, v/v). N-Ac-Asp and N-Ac-Asp-Glu levels in several brain regions and spinal cord were similar to those so far reported. The compounds could not be detected in peripheral tissue. The advantages, prospects and limitations of the present approach over existing methods to estimate water-soluble carboxylic acids is discussed.