Sulfur-free amino acids

While sulfur-free amino acids are broken down to amines via decarboxylation, the sulfur-containing amino acids such as cysteine can undergo more complex reactions. Fisher and Scott suggest that, because cysteine produces a powerful reducing aminoketone, hydrogen sulfide could be produced by reducing mercaptoacetaldehyde or cysteine.2... 

The mixture is then cooled and sodium bisulfite added to decolorize. Recrystallization of the product from methanol gives about 8 g, MP 147° to 150°C. The /3-(3-amino-2,4,6-triiodophenyl)-0 -ethylpropionic acid may be purified further by precipitation of the morpholine salt from ether solution and regeneration of the free amino acid by treatment of a methanol solution of the morpholine salt with sulfur dioxide. The pure amino acid has the MP 155° to 156.5°C. 

A solution of 5.0 g of a-ethyl-(3-(aminophenyl)propionic acid in 100 ml of water containing 5 ml of concentrated hydrochloric acid was added over a period of h hour to a stirred solution of 3.2 ml of Iodine monochioride in 25 ml of water and 25 ml of concentrated hydrochloric acid heated to 60°C. After addition was complete, the heating was continued for h hour longer at 60° to 70°C. A black oil separated which gradually solidified. The mixture was then cooled and sodium bisulfite was added to decolorize. Recrystallization of the product from methanol gave about 8 g of a-ethyl-(3-(2,4,6-triiodo-3-aminophenyl-pro-pionic acid, MP 147° to 150°C. The product could be further purified by precipitation of its morpholine salt from ether solution and regeneration of the free amino acid by treatment of a methanol solution of the morpholine salt with sulfur dioxide. The pure amino acid had the MP 155° to 156.5°C (corr). 

PEC. The patent includes the production method for the biocatalyst, with a characteristic inhibitory effect on certain enzyme expression. The expression recombinant vector contains a promoter free from manifesting inhibition due to an inorganic sulfur compound or a sulfur-containing amino acid. The recombinant microorganism also contains a gene for desulfurizing the sulfur-containing heterocyclic compound. 

Therefore the problem of searching retardants for these chain reactions of free radicals is critical. For instance, it is known that sulfur-containing amino acid (cysteine) attracts unpaired electrons of protein [2,3], Similar properties are reported about selenium, the element of the same subgroup Vl-a of the System as sulfur [4],... 

Protons are mainly derived from two sources—free acids in the diet and sulfur-containing amino acids. Acids taken up with food— e.g., citric acid, ascorbic acid, and phosphoric acid—already release protons in the alkaline pH of the intestinal tract. More important for proton balance, however, are the amino acids methionine and cysteine, which arise from protein degradation in the cells. Their S atoms are oxidized in the liver to form sulfuric acid, which supplies protons by dissociation into sulfate. 

Svensson, A. Bordas, J. Hughes, E.A. Mant, G. Radiation damage induced in free nucleotides and sulfur-containing amino-acids by monoenergetic X-rays. Chance, B., Deisenhofer, J., Ebashi, S., Goodhead, D.T., Helliwell, J.R., Huxley, H.E., lizuka, T., Kirz, J., Mitsui, T., Rubenstein, E., Sakabe, N., Schmahl, G., Stuhrmann, H.B., Wuthrich, K., Zaccai, G., Eds. Synchrotron Radiation in the Biosciences, 1994 721-729. 

Stillings and Hackler (54) studied the effects of the tempeh fermentation, the action of Rhizopus oligosporus on dehulled, soaked, and sterilized soybeans. The PER of tempeh reached its peak after 24 hours fermentation and then dropped. They ascribed the changes to an increase and then a decrease in tryptophan chemically present. The PER decrease after 24 hours was also attributed to an increased quantity of mold protein, which is lower in sulfur amino acid concentration than the original soy protein. Free amino acids increased markedly, presumably due to the action of the mold proteolytic enzymes. Free methionine was present in the largest amount, and as such was highly available, thus partially accounting for the initial increase in PER. 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

That summary is based on the reports of a well-conceived and carefully executed research program carried out by Rohan. Mohr et al. (7) extended these studies and was able to draw additional conclusions. First, without exception, free amino acids are much more sensitive to destruction in this system than the peptide-bound amino acids. Second, differences in the stability of amino acids under these conditions are not great —from 25% loss for isoleucine to 68.5% for lysine, over a relatively short period of time. In this system the reducing sugars must be the limiting factor, since the glucose and fructose are completely destroyed or removed. Third, neither cystine nor cysteine are reported to be present, and the only other sulfur-containing amino acid, methionine, is present at a much lower concentration than any other amino acid. Clearly, as we shall see later, cocoa would probably have a considerably different flavor if cysteine or cystine were present in the fermented beans. 

Table 2. Gibbs free energy of the formation of equilibrium complexes of sulfur-containing amino acids with silica surface in hquid state (SM5.42R / 6-31+G).

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