Electrochemical activity of 6-aminoquinolyl urea derivatives of amino acids and peptides. Application to high-performance liquid chromatography with electrochemical detection

Electrochemistry in sensing of molecular interactions of proteins and their behavior in an electric field

Electrochemical methods can be used not only for the sensitive analysis of proteins but also for deeper research into their structure, transport functions (transfer of electrons and protons), and sensing their interactions with soft and solid surfaces. Last but not least, electrochemical tools are useful for investigating the effect of an electric field on protein structure, the direct application of electrochemical methods for controlling protein function, or the micromanipulation of supramolecular protein structures. There are many experimental arrangements (modalities), from the classic configuration that works with an electrochemical cell to miniaturized electrochemical sensors and microchip platforms. The support of computational chemistry methods which appropriately complement the interpretation framework of experimental results is also important. This text describes recent directions in electrochemical methods for the determination of proteins and briefly summarizes available methodologies for the selective labeling of proteins using redox-active probes. Attention is also paid to the theoretical aspects of electron transport and the effect of an external electric field on the structure of selected proteins. Instead of providing a comprehensive overview, we aim to highlight areas of interest that have not been summarized recently, but, at the same time, represent current trends in the field.

3,4-Dihydroxy-l-phenylalanine as a biomarker of oxidative damage in proteins: improved detection using cloud-point extraction and HPLC

Oxidized protein adducts are formed under conditions of oxidative stress and may represent a valuable biomarker for a variety of diseases which share this common aetiology. A suitable candidate biomarker for oxidized proteins is protein-bound 3,4-dihydroxyl-l-phenylalanine (l-DOPA), which is formed on 3'-hydroxylation of tyrosine residues by hydroxyl radicals. Existing methodologies to measure protein-bound l-DOPA employ lengthy acid hydrolysis steps (ca. 16h) which may cause artifactual protein oxidation, followed by HPLC with detection based on the intrinsic fluorescence of l-DOPA. We report a novel method for the measurement of protein-bound l-DOPA which involves rapid hydrolysis followed by pre-column concentration of 6-aminoquinolyl-derivatives using cloud-point extraction. The derivatized material is resolved by reversed-phase HPLC in less than 30min and has derivatization chemistry compatible with both UV and fluorescent detection, providing detection down to the femtomole level. The method provides identical results to those found with highly specific ELISA-based techniques and requires only basic instrumentation. The stability of the 6-aminoquinolyl-derivatives together with the fast and sensitive nature of the assay will be appealing to those who require large sample throughput.

Determination of hemocoagulase agkistrodon in a pharmaceutical preparation by high-performance liquid chromatography with pre-column derivatization and fluorescence detection

Currently, there is no analytical method for the quantification of hemocoagulase agkistrodon (HCA) in pharmaceutical preparations. This study presents a pre-column derivatization method for the quantification of HCA, a compound extracted from the venom of Agkistrodon acutus, in a pharmaceutical preparation (trade name Suling). In the proposed method, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate was used to tag the HCA substrate, and the derivatives were analyzed by high-performance liquid chromatography with fluorescence detection. Complete and homogeneous derivatization of HCA was confirmed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry analysis. The specificity of the method was validated by forced degradation, and interference was assessed using a placebo. Under the optimum chromatographic conditions, the calibration curve was linear over a range of 10 to 500 ng/mL, featuring a correlation coefficient of 0.9999. The limits of detection and quantification of the method were 0.57 and 1.6 ng/mL, respectively. The percentage recovery of HCA in quality control samples ranged from 97.49 to 99.15%. Overall, this novel method can be applied to the quantitative determination of HCA in pharmaceutical preparations.

Spectrofluorimetric determination of human serum albumin using terbium-danofloxacin probe

A spectrofluorimetric method is proposed for the determination of human serum albumin (HSA) and bovine serum albumin (BSA) using terbium-danofloxacin (Tb³⁺-Dano) as a fluorescent probe. These proteins remarkably enhance the fluorescence intensity of the Tb³⁺-Dano complex at 545 nm, and the enhanced fluorescence intensity of Tb³⁺-Dano is proportional to the concentration of proteins (HSA and BSA). Optimum conditions for the determination of HSA were investigated and found that the maximum response was observed at: pH = 7.8, [Tb³⁺] = 8.5 × 10⁻⁵ mol L⁻¹, [Dano] = 1.5 × 10⁻⁴ mol L⁻¹. The calibration graphs for standard solutions of BSA, HSA, and plasma samples of HSA were linear in the range of 0.2 × 10⁻⁶ − 1.3 × 10⁻⁶ mol L⁻¹, 0.2 × 10⁻⁶ − 1.4 × 10⁻⁶ mol L⁻¹, and 0.2 × 10⁻⁶ − 1 × 10⁻⁶ mol L⁻¹, respectively. The detection limits (S/N = 3) for BSA, HSA, and plasma sample of HSA were 8.7 × 10⁻⁸ mol L⁻¹, 6.2 × 10⁻⁸ mol L⁻¹, and 8.1 × 10⁻⁸ mol L⁻¹, respectively. The applicability of the method was checked using a number of real biological plasma samples and was compared with the UV spectrometric reference method. The results was showed that the method could be regarded as a simple, practical, and sensitive alternative method for determination of albumin in biological samples.

Optimization of separation and detection of 6-aminoquinolyl derivatives of amino acids by using reversed-phase liquid chromatography with on line UV, fluorescence and electrochemical detection

The combined use of UV-absorbance, fluorescence and electrochemical detection was proposed for the analysis of a set of thirteen amino acids by reversed-phase liquid chromatography (RP-HPLC) using 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate as a precolumn derivatization reagent. The utility of using three detectors in series was demonstrated. The separation of all derivatized amino acids was optimized with the aid of a computer optimization program from only four simple linear gradient measurements. The effectiveness of a reliable retention prediction of solutes under any gradient profile using other gradient or isocratic data was also examined.

Advances in the study of luminescence probes for proteins

Spectral probes (or labels) have been widely used for the investigation and determination of proteins and have made considerable progress. Traditional luminescence probes include fluorescent derivatizing reagents, fluorescent probes and chemiluminescence probes which continue to develop. Of them, near infrared (NIR) fluorescent probes are especially suitable for the determination of biomolecules including proteins, so their development has been rapid. Novel luminescence probes (such as nanoparticle probes and molecular beacons) and resonance light scattering probes recently appeared in the literature. Preliminary results indicate that they possess great potential for ultrasensitive protein detection. This review summarizes recent developments of the above-mentioned probes for proteins and 195 references are cited.

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.

Homogeneous fluorescent derivatization of large proteins

A method of homogeneously derivatizing large proteins for highly sensitive analysis is described. Homogeneity of the derivative was realized by tagging all the free amino groups of proteins. With this method, alpha-chymotrypsinogen A, ovalbumin and bovine serum albumin were derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). Prior to the derivatization, all the proteins were reduced and alkylated. After reacting the resulting unfolded proteins with excessive amounts of AQC, the samples were analyzed with matrix assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) to determine the derivatization degree. The results indicated that all three proteins had been, or had almost been, fully derivatized. HPLC and CE were used for characterizing these protein derivatives. Under the optimized fluorescence detection conditions, the detectability of the tagged proteins was 2400-6200 times better than that detected at UV 280 nm, 170-300 times better than detected at UV 214 nm, and 150-420 times better than measured with their native fluorescence.

Labeling reactions applicable to chromatography and electrophoresis of minute amounts of proteins

Chromatography and electrophoresis have become extremely valuable and important methods for the separation, purification, detection and analysis of biopolymers and HPLC/HPCE may become the premier, preferable approaches for both qualitative and quantitative analyses of most proteins, especially from recombinant materials. This includes smaller peptides, polypeptides, proteins, antibodies and all types of protein or antibody-conjugates (antibody-enzyme, protein-fluorescent probe, antibody-drug and so forth). This entire Topical Issue of Journal of Chromatography emphasizes the application of chromatography and electrophoresis to protein analysis. This particular review deals with approaches to the selective tagging or labeling of proteins at trace (minute) levels, again using either chromatography or electrophoresis, with the emphasis on modern HPLC/HPCE methods and approaches. We discuss here both pre- and post-column labeling methods and reagents, techniques for realizing selective labeling of proteins or antibodies, applicable approaches to protein preconcentration in both HPLC and HPCE areas and in general, methods for improving (lowering) detection limits for proteins utilizing chemical or physical derivatization and/or preconcentration techniques. There are really two major goals or emphases in that which follows: (1) methods for selective labeling of proteins prior to or after HPLC/HPCE and (2) labeling of proteins at trace levels for improved separation-detection and lowered detection limits. We discuss here a large number of specific references related to both pre- and post-column/capillary derivatizations for proteins, as well as methods for improved detectability in both HPLC and HPCE by, for example, analyte preconcentration on a solid-phase extractor or membrane support, capillary isotachophoresis and other methods. Selective reactions or derivatizations on proteins refers to the ability to tag the protein at specific (e.g. reactive amino sites) in a controlled manner, with the products having the same number of tags all at the very same site or sites. The products are all the same species, having the same number of tags at the same locations on the protein. Selective reactions can also refer to the idea of tagging all of the protein sample at only a single, same site or at all available sites, homogeneously.