Sensors for Amino Acids

Amino acids constitute the second largest source of nonprotein nitrogen in serum, urea being major source. The determination of total serum amino acids can provide valuable clinical information. Single amino acids are measured to gain access to particular enzyme activities of transaminases and peptidases for instance. Amino acids are also important in the food industry and in biotechnology. Their concentration in food can be used as a measure of the nutritive value of the food. 

Two enzymes with a broad substrate specificity have been utilized in biosensors for amino acids L-amino acid oxidase (EC 1.4.3.2) and D-amino acid oxidase (EC 1.4.3.3). They catalyze the irreversible formation of the respective a-keto acids  

This reaction can be coupled to oxygen- and hydrogen peroxidesensing electrodes (Guilbault and Lubrano, 1974) as well as to poten-tiometric pH, NH3, or NH4 electrodes (Guilbault and Hrabankova, 1971). 

The following substrates can be determined with almost identical sensitivity by using D-amino acid oxidase in combination with an ammonium ion sensitive electrode D-alanine, D-leucine, D-norleucine, D-methionine, and D-phenylalanine. L-amino acid oxidase sensors have been described for L-leucine, L-cysteine, L-methionine, L-tryptophan, and L-tyrosine (Guilbault and Hrabankova, 1971), and L-histidine and L-arginine (Tran-Minh and Broun, 1975). 

Yao and Wasa (1988a) assembled modified electrodes for amino acids by crosslinking L- or D-amino acid oxidase with glutaraldehyde on silanized platinum probes. The sensors were employed as detectors in high pressure liquid chromatography. Whereas the L-amino acid oxidase electrode responded to L-tyrosine, L-leucine, L-methionine, and L-phenylalanine in amounts as low as 2 pmoles, the D-amino acid electrode measured only D-methionine and D-tyrosine. The response time in steady state measurements was only 5-10 s. 

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The sensing of organic matter has been an active area within supramolecular chemistry, where luminescent sensors for amino acids,sngars, and so on have been developed. 

A chiral zinc(ll) complex cmitaining a terpyridine fluorophore with C2 symmetry 13 (Fig. 5c) and a crown ether residue was recently reported as fluorescent sensor for amino acids, using the metal ion and the crown ether unit as binding sites... 

Liu SY, He YB, Qing GY et al (2005) Fluorescent sensors for amino acid anions based on calix[4]arenes bearing two dansyl groups. Tetrahedron Asymmetry 16 1527-1534... 

Xu KX, Qiu Z, Zhao JJ et al (2009) Enantioselective fluorescent sensors for amino acid derivatives based on BINOL bearing benzoyl unit. Tetrahedron Asymmetry 20 1690-1696... 

An amperometric sensor for amino acids based on flow injection analysis (FIA) and using microelectrodes (10 pm diameter) primarily of P(Py) doped with sulfonate dopants such as tosylate and 3-sulfobenzoate was demonstrated by Akhtar et al. [823, 824]. Linear response was demonstrated for analytes such as aspartic acid and glutamic acid over the concentration range 7.5 X 10 to lO" with sensitivities in the region of 1.5 nC-M and detection limits of ca. 10 M. These authors also showed the use of a pattern recognition technique using the responses of six detector electrodes. Fig. 17-11 shows typical response of one of their sensors. 

Figure 10.17. Comparison of sensor signals of the three cyclopeptide receptor layers sensitive for amino acids in buffer-free neutral water [97].

As it has been shown, basically all chiral MIP-based electrochemical sensors were developed up till now for amino acids or monosaccharides. However, chiral pharmaceuticals present more complex structures compared to the already mentioned molecules thus, their efficient molecular imprint is considered to be more difficult. Moreover, in the case of amino acids, the asymmetric carbon is at the molecule s extremity carrying two functional groups (-NH2 and -COOH) strongly interacting with the used common functional monomers, thus easily leading to highly enantiospecific imprinted cavities. 

The voltage sensor is the part of a channel protein responsible for detection of the membrane potential. A voltage sensor of the voltage-dependent Na+ channel was predicted by Hodgkin and Huxley in 1952. Positively charged amino acid residues in S4 of each repeat play an essential role as the voltage sensor. 

Chen, Q., Wang, J., Rayson, G., Tian, B., and Lin, Y., Sensor array for carbohydrates and amino acids based on electrocatalytic modified electrodes, Anal. Chem., 65, 251, 1993. 

Protein fragment complementation assays are based on an enzyme reassembly strategy whereby a protein-protein interaction promotes the efficient refolding and complementation of enzyme fragments to restore an active enzyme. The approach was initially developed using the reconstitution of ubiquitin as a sensor for protein-protein interactions (Johnsson and Varshavsky, 1994). Ubiquitin is a 76 amino acid protein that... 

Similar to the work described by Spohn et al. [34], a trienzyme sensor was developed recently for the determination of branched-chain amino acids (L-valine, L-leucine, and L-isoleucine). Leucine dehydrogenase, NADH oxidase, and peroxidase were coimmobilized covalently on tresylate-hydrophylic vinyl polymer beads and packed into a transparent PILL tube (20 cm X 1.0 id), which was used as flow cell. The sensor was free of interferences from protein and NH4+ and it was stable for 2 weeks. The sensor system was applied to the determination of branched-chain amino acids in plasma with recoveries ranging from 98 to 100% [36],... 

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