Valine characterisation

Valine.—This amino acid is contained mixed with leucine in the fractions of the esters which boil between 6o° and 90° C. Its isolation and separation from leucine is of extreme difficulty, since these compounds, as well as their copper salts into which they are converted by boiling with freshly precipitated cupric oxide, tend to form mixed crystals. Its isolation was only effected by these means in certain cases, and its amount is really much more than the figures represent from its yield. It is best characterised by conversion into its phenylhy-dantoine derivative by treatment with phenyl isocyanate in alkaline solution. The phenylureido acid is first formed, and this loses a molecule of water, as shown by Mouneyrat, and is changed into its anhydride or phenylhydantoine by treatment with hydrochloric acid. The following reactions occur —... 

Ru(PPh3)(H2O)j(SB "0 constimtes a series of complexes, made from RuClj(PPh3)3 and a chiral Schiff base SB obtained from salicylaldehyde and the L-forms of alanine, valine, serine, arginine, cysteine and aspartic acid. They were characterised by IR, circular dichroism, H and C[ H] NMR spectroscopies and by cyclic voltammetry. The supposed structure of one is shown in Fig. 1.41 [917]. 

T Schwecke, Y Aharonowitz, H Palissa, H von Dohren, H Kleinkauf, H van Liempt. Enzymatic characterisation of the multifunctional enzyme 5-(L-a-aminoadipyl)-L-cysteinyl-D-valine synthetase from Streptomyces clavuligerus. Eur J Biochem 205 687-694, 1992. 

MA Tavanlar. Molecular characterisation of the 8-(L-a-aminoadipyl)-L-cysteinyl-D-valine synthetase (ACVS) from filamentous fungi determination of the domain arrangement, cloning and expression of the third domain. Ph.D. thesis, University of Los Banos/Technical University of Berlin, 1996. 

Van Valin, R. Morse, J.W. (1982) An investigation of methods normally used for selective removal and characterisation of trace metals in sediments. Marine Chemistry 11, 535-564. 

Previously, we have examined the formation of amino acid hydroperoxides following exposure to different radical species [100]. We observed that valine was most easily oxidised, but leucine and lysine are also prone to this modification in free solution. Scheme 12 illustrates the mechanism for formation of valine hydroperoxide. However, tertiary structure becomes an important predictor in proteins, where the hydrophobic residues are protected from bulk aqueous radicals, and lysine hydroperoxides are most readily oxidised. Hydroperoxide yield is poor from Fenton-derived oxidants as they are rapidly broken down in the presence of metal ions [101]. Like methionine sulphoxide, hydroperoxides are also subject to repair, in this case via glutathione peroxidase. They can also be effectively reduced to hydroxides, a reaction supported by the addition of hydroxyl radical in the presence of oxygen. Extensive characterisation of the three isomeric forms of valine and leucine hydroxides has been undertaken by Fu et al. [102,103], and therefore will not be discussed further here. 

Andrew Hamilton (Princeton) reported [late submission] the recent synthesis and characterisation of a macrocycle (7), designed specifically to control the environment around a complexed metal in order to influence its catalytic properties. The macrocycle was synthesised from a bipyridyl unit and two valine residues. It was designed to provide (a) ligands capable of stabilising a range of metal oxidation states, (b) chiral groups which are close enough to influence the approach to the metal, (c) a cavity in order to effect substrate specificity and (d) sufficient stability to withstand oxidation. Complexation with a number of transition metals has been successful and preliminary studies on the Fe complex show that in the presence of iodosylbenzene in acetonitrile, it will epoxidise cyclooctene in 55% yield. 

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