Amino acids in aqueous solution

Effects of Gamma-radiation on Some Carbohydrates, Hydroxy-acids, and Amino-acids in Aqueous Solutions, S. A. Barker, P. M. Grant, M. Stacey, and R. B. Ward, Nature, 183 (1959) 376-377. 

More recent experiments using even higher shock wave pressures, up to around 40 GPa (produced by a hyper-velocity impact gun) show that the extremely short periods of time (only a few microseconds) for which the pressure is applied have a lower decomposing effect on the amino acids in aqueous solution, and in ice, than had been expected. The exact analysis of the products showed that small amounts of simple peptides were also formed. These results point to the complexity of questions on biogenesis problems. Even cannon can help us in our attempts to reveal the secret of biogenesis ... 

The effect of pKa on the stability constant of the 1 1 monodentate complexes of Me3 SnX with amino acids in aqueous solutions was studied by potentiometric methods281. 

FIGURE 7.21 Synthesis of a dipeptide by reaction of an amino-acid A-carboxyanhydride (A) with an amino-acid ester in tetrahydrofuran65 and (B) with an amino acid in aqueous solution.67... 

The oxidation of cysteine, as well as other amino acids, was studied by Mudd et a/. Individual amino acids in aqueous solution were exposed to ozone the reported order of susceptibility was cysteine, methionine, tryptophan, tyrosine, histidine, cystine, and phenylalanine. Other amino acids were not affected. This order is similar to that for the relative susceptibility of amino acrids to radiation and to lipid peroxides. Evaluation of the ozonization products revealed that cysteine was converted to cysteic acid, as well as cystine methionine to methionine sulfoxide tryptophan to a variety of pioducrts, including kynurenine and N-formylkynurenine tyrosine also to a variety of products, includiitg dihydroxyphenylalanine histidine to ammonia, proline, and other compounds and cystine in part to cysteic acid. In some cases, the rate and end products depended on the pH of the solution. 

Fig. 35. The pH dependence of Moffitt s bQ values of poly(a-amino acids) in aqueous solution 102) ...

Amino acids in aqueous solution contain weakly acidic a-carboxyl groups and weakly basic a-amino groups. In addition, each of the acidic and basic amino acids contains an ionizable group in its side chain. Thus, both free amino acids and some amino acids combined in pep tide linkages can act as buffers. The quantitative relationship between the concentration of a weak acid (HA) and its conjugate base (A-) is described by the Henderson-Hasselbalch equation. 

The charge properties of amino acids are very important in determining the reactivity of certain amino acid side chains and in the properties they confer on proteins. The charge properties of amino acids in aqueous solution may best be considered under the general treatment of acid-base ionization theory. We find this treatment useful at other points in the text as well. 

Gorner H, Nikogosyan DN (1997) Indirect 248 nm photolysis of aliphatic amino acids in aqueous solution. J Photochem Photobiol B Biol 39 84-89... 

Monig J, Chapman R, Asmus K-D (1985) Effect of the protonation state of the amino group on the OH radical induced decarboxylation of amino acids in aqueous solution. J Phys Chem 89 3139-3144... 

Although these experiments did not provide the desired systems needed to amplify chirality, they were helpful in elucidating the stereochemical mechanism of the role played by additives in the early stages of crystal nucleation. This information was instrumental to the elaboration of appropriate model systems for the amplification of chirality, such as the generation of homochiral lysine via crystals of nickel/caprolactam [131] and the auto catalytic process of the spontaneous segregation of racemic enantiomers of amino acids in aqueous solutions assisted by centrosymmetric glycine crystals grown at interfaces. 

The effects of pH on racemization rates of free amino acids in aqueous solution are complex (21). However, when bone fragments were heated at 100°C in solutions of pH 2-9, the rate constant for aspartic acid racemization was essentially independent of pH (37). It was proposed that the phosphate from the bone s inorganic phase acts as a buffer. This same buffering action would take place in a bone under natural conditions of burial as well. 

Amino acids bonded to silica and loaded with Cu ions can interact in a steroselective manner with amino acids in aqueous solution. The copper ion forms a complex with both the bound and the sample amino acids, as shown in Figure 22.1. Ligand-exchange phases are suited for the separation of amino acids as well as of some )3-amino alcohols and similar molecules because these compounds bear two polar functional groups in adequate spacing. This approach has found limited interest because the column efficiencies are rather low, the detectability of the nonderivatized sample compounds can be a problem and the mobile phase needs to contain copper. 

The compounds were synthesized at 45° from the dichlorotri-ethylenetetraamine moiety and the appropriate amino acid in aqueous solution with pH maintained at 7.0 +0.1 by pH-stat (9). 

We have investigated a large variety of cyclopeplides and analytes using quartz sensors in gas and liquid phases. Here, we report on the interaction of cyclopeptides 1 and 2 (Fig. 10.9) with proteinogenic amino acids in aqueous solution. Both peptides contain three thiol groups for the covalent and conformationally restricting attachment to the gold electrodes. They differ in only one amino acid (L-Lys in 1, L-Phe in 2). 

J.L. Burguera, M. Burguera, M. de la Guardia, A. Salvador, Pyrolysis-flow-injection analysis-spectrophotometric determination of amino acids in aqueous solutions, Anal. Chim. Acta 261 (1992) 23. 

Electron Acceptance Reduction and Adduct Formation. Acceptance of electrons at specific sites on amino acids and peptides depends on their reactivities and produces different chemical consequences. Among the sites of particular importance are the terminal amino and carboxyl groups, the ring groups, the peptide carbonyl, and the sulfur bonds. Reactivities of these are reflected in the rate constants for reaction of solvated electrons with individual amino acids in aqueous solutions, as shown in Table I and as discussed by Simic (53). More detailed information, however, regarding the stepwise progression from attachment to specific radical formation has been obtained from low temperature studies. 

The decarboxylation is more enigmatic. Although models for Schiff base formation and transamination are common, models for decarboxylation are rare. If PLP is mixed with an amino acid in aqueous solution, then Schiff base formation will occur, followed by slow transamination. No decarboxylation is observed. If an a-methyl amino acid is used, thus preventing transamination, slow decarboxylation is observed, but only at temperatures above 100°C (106). Thus, the problem in enzymic decarboxylation is to catalyze a very slow reaction and avoid a much more favorable reaction. 

Co-)/-Ray Induced Formation of Sulfur-Containing Amino Acids in Aqueous Solutions... 

A similar dimetallic Gp Rh methylcytosine complex from the same research group 137 also binds aromatic amino acids in aqueous solution at pH = 7. H NMR studies suggest that selective hydrogen bonding occurs between the amino acid NHs and G02 groups, the oxygen of the Rh(/i-OH) assemblies, and the carbonyl and amine functions of the methylcytosine ligands. No stability constants are reported. 

Polymerization of amino acids in aqueous solution has, in general, only been partially successful. An exception is the remarkable reaction of amino-acyladenylates in the presence of a suspension of montmorillonite to produce chains of up to hfty amino acid residues in length (Paecht-Horowitz, 1974). By contrast, in the absence of the clay mineral, the aminoacyladeny-lates hydrolyze rapidly and produce, at most, short chains of four or five amino acids. The mechanism of this reaction is unclear and the possible origin of these highly unstable compounds on the primitive Earth has not been demonstrated directly. Possibly such activated eunino acids could be formed under evaporating conditions from the free amino acids in the presence of adenosine cyclic-2, 3 -phosphate (Lohrmann and Orgel, 1973). 

Table 39 contains some stability constant data for chromium(II) and amino acids in aqueous solution. 

Comparison of the complex-formation constants for bofli 1 1 (57 and 58) and 1 2 (such as 59) species ° with those obtained for the respective copper(II) complexes with parent amino acids revealed that the fructosyl moiety provides for an additional chelate effect in D-fructose-a-amino acids and as a consequence, a significant increase in the complex stability. In the absence of an anchoring chelating group, such as a-carboxylate, the D-finctosamine structure is not a good copper(II) chelator, and Cu(n) expectably does not form stable complexes with the carbohydrate in A -d-Iructose-L-lysine peptides. Although it would be expected that iron(III) complexes with D-finctose-amino acids in aqueous solutions, no related thermodynamic equilibrium studies have been done so far for this important redox-active metal. 

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