Detection of amino groups

Carbon paste and graphite epoxy electrodes modified with RUO2 can be used for detection of amino acids and peptides in F1A systems. Optimal conditions are in strongly alkaline solutions at +0.45 V vs Ag/AgCl electrode, with a fast and linear response. Carbon paste electrodes can be modified also with C03O4371. Colorimetric methods for the determination of amino groups attached to a solid support may give erroneous values... 

Since the early detection of amino acids with ninhydrin, many derivatization procedures have investigated these solutes. 4-Dimethylaminoazobenzene-4 -sulfonyl chloride (DABS-C1) is performing well, but the best seems to be FMOC (9-fluorenyl chloroformate) (63). More generally, each class of reactions replaces the active hydrogens of the OH, NH, and SH groups. 

In this part we will describe recent achievements in the development of biosensors based on DNA/RNA aptamers. These biosensors are usually prepared by immobilization of aptamer onto a solid support by various methods using chemisorption (aptamer is modified by thiol group) or by avidin-biotin technology (aptamer is modified by biotin) or by covalent attachment of amino group-labeled aptamer to a surface of self-assembly monolayer of 11-mercaptoundecanoic acid (11-MUA). Apart from the method of aptamer immobilization, the biosensors differ in the signal generation. To date, most extensively studied were the biosensors based on optical methods (fluorescence, SPR) and acoustic sensors based mostly on thickness shear mode (TSM) method. However, recently several investigators reported electrochemical sensors based on enzyme-labeled aptamers, electrochemical indicators and impedance spectroscopy methods of detection. 

Amino acids are chemically reactive at their a-amino groups and their side chains. The carboxyl groups are relatively unreactive unless activated. A very useful reaction is the oxidative deamination of amino acids with ninhydrin. The reaction produces a blue pigment, which can be used for the detection of amino acids both qualitatively and quantitatively. The series of reactions producing the blue complex is given in Equation (4.2). 

Because amino groups act autocatalytically (15-17) in the presence of water, for acid catalysis an excess of HC1 was used to overcompensate the formation of -NH3+C1 . In these cases, the gels were washed with methanol and water until no Cl" could be detected in the filtrate. How far the incorporation of amino groups into silica could affect the adsorption of acid components was of interest. Lactic acid and a sulfonic acid (a commercially available dye named Telon Light Yellow) were chosen as test components (18). In Figure 7 the adsorption isotherm of lactic acid is shown. Unmodified Si02 does not have remarkable adsorption in aqueous solution under these circumstances. The result shows the effect of the amino modification quite clearly, because the lactic acid load of the adsorbent is remarkable, and it is difficult to adsorb small water-soluble molecules in an aqueous environment. 

For the qualitative and semiquantitative detection of amino acids on thin layer plates, one may use either the blue condensation product with ninhydrine or the more sensitive dye fluorescamine, which becomes fluorescent only upon condensation with amine groups. The chromophore presumably becomes much more rigid by electron donation from the vinylogous lactam nitrogen to its carboxyl oxygen atom (Scheme 9.4.4). 

Analytical procedures involving reduction and determination of mercaptan are not accurate determinations of cystine in permanent-waved hair or in hair treated with mercaptan because mixed disulfide is reduced to mercaptan during analysis adsorbed mercaptan can also interfere in the determination. Procedures that do not involve reduction of hair such as ninhydrin detection (alpha-amino group) or dinitrofluorobenzene (DNFB) reaction followed by chromatographic separation [1, 58] discriminate between mercaptans and, therefore, should be better analytical procedures for detecting the different types of mercaptans and disulfides actually present in permanent-waved hair. 

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