Stable amino acids

As already mentioned, RNR is the metalloenzyme in which the first definitively characterized stable amino acid radical (1), later identified as a tyrosyl radical, was found in 1972. The RNR enzymes catalyse the reduction of ribonucleotides to their corresponding deoxyribonucleotides utilized in DNA biosynthesis. There are three unique classes of this enzyme, differing in composition and cofactor requirements all of them, however, make use of metal ions and free radical chemistry. Excellent reviews on RNRs are available (60, 61, 70, 89-97). 

The most persistent and stable amino acids appear to be glycine, aspartic acid, and glutamic acid, which were reported in Paleozoic anthracite from Great Britain by Heijhenskjold and Mollerberg (8). Among the most stable amino acid chelates are those formed with copper however, the stability constants for the a-amino acids of copper do not differ to the point where they indicate that the above mentioned three acids would be preferentially preserved. 

Lysine is an essential amino acid. Since lysine is a fairly acid-stable amino acid, its analysis as total lysine by the traditional hydrochloric acid hydrolysis is straightforward. Fairly recent examples for the successful determination of total lysine employing either ion-exchange (82) or reversed-phase (101) HPLC are available. 

YH Tuan, RD Phillips. Optimized determination of cystine/cysteine and acid-stable amino acids from a single hydrolysate of casein- and sorghum-based diet and digesta samples. J Agric Food Chem 45 3535-3540, 1997. 

In the pioneering work on the occurrence and stability of proteins and amino acids in fossils, Abelson determined that thermally unstable amino acids such as threonine and serine were either much reduced or absent in fossils, whereas more stable amino acids, such as glycine and alanine, were still present (see ref. 5). The total concentration of amino acids decreases dramatically with time, von Endt and Erhardt reported their results concerning the differential chemical disintegration of amino acids in compact... 

Of the various amino acids, aspartic acid has been the most widely used in fossil bone dating. This amino acid has one of the fastest racemization rates of the stable amino acids (21,22). At 20 C in bone, the half-life for aspartic acid racemization is 15,000 years. Thus, for most mid- or low-latitude sites the racemization rate of aspartic acid is... 

There has been no report on the spectrum of methionine sulfone, which is found in hydrolyzates of performate-oxidized proteins. It will probably be not very different from cysteic acid at wavelengths > 2000 A, since simple sulfones are generally transparent to 1800 A (Koch, 1950). Methionine sulfoxide is a stable amino acid found in human urine, in a variety of plant tissues, and as an intermediate in the oxidation of methionine to the sulfone. Its absorption spectrum has not been recorded. The sulfoxide chromophore is usually a broad band, located around 2100 A and of about the same intensity as alkyl sulfides (Koch, 1950). An interesting... 

In the Arctic and Antarctic Ocean amino acids were also found in humic substances isolated from DOM by XAD-2 resins (Hubberten et ai, 1995). The concentration of THAA in humic substances was between 233—246 nM, with aU hydrolysable amino acids in the deep ocean and 60% of amino acids in the surface ocean residing in this fraction. Glycine was by far the most abundant amino acid detected in the humic fraction. These authors concluded that amino acids in the XAD-2 extracts represent a refractory protein background that is present throughout the ocean. The dominance of this refractory protein background in the surface and deep ocean could explain the relatively stable amino acid distribution observed by Yamashita and Tanoue (2003) at their open ocean sites. 

The instability and chemical conversion of some OPA derivatives imply that a denvatized compound may, in fact, result in one fluorescent and two radioactive peaks (Simson and Johnson, 1976, Fig. 1). The chemical rearrangement of the derivatives may, however, be a minor factor with respect to retention and the fluorescent and nonfluorescent derivatives may coelute. The use of more chemically stable amino acid derivatives, i.e. those formed by reaction with FMOC chloride, eliminates this problem. When the radioactivity of an amino acid is measured, it is often desirable and necessary to inject larger concentrations of amino acids than in a routine expenment. With the OPA method it is then critical to (a) make sure that OPA is present in the required molar excess (Lindroth and Mopper, 1979), (b) lower the pH of the reagent mixture to spare the top of the column, and (c) use the same or lower proportion of organic solvent in the sample as in the beginning of the gradient in order to obtain a concentration of the derivatives on the column top. 

Arginine, aspartic acid, cystine, histidine, lysine, serine, threonine and methionine, being especially reactive amino acids, are somewhat decreased in roasted coffee, while the stable amino acids, particularly alanine, glutamic acid and leucine, are relatively increased. Free amino acids occur only in traces in roasted coffee. 

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