Aqueous reactions cysteine

Schieberle P. and Hofmann T. (1998) Characterization of key odorants in dry-heated cysteine-carbohydrate mixtures comparison with aqueous reaction systems. Am. Chem. Soc. Symp. Ser. 705, 320-30. 

Reactions between A -(l-chloroalkyl)pyridinium chlorides 33 and amino acids in organic solvents have a low synthetic value because of the low solubility of the amine partner. A special protocol has been designed and tested in order to circumvent this drawback. Soon after the preparation of the salt, an aqueous solution of the amino acid was introduced in the reaction medium and the two-phase system obtained was heated under reflux for several hours. However, this was not too successful because sulfur dioxide, evolved during the preparation of the salt, was converted into sulfite that acted as an 5-nucleophile. As a result, A -(l-sulfonatoalkyl)pyridinium betaines such as 53 were obtained (Section IV,B,3) (97BSB383). To avoid the formation of such betaines, the salts 33 were isolated and reacted with an aqueous solution of L-cysteine (80) to afford thiazolidine-4-carboxylic acids hydrochlorides 81 (60-80% yields). 

To a suspension of 35.2 grams (0.2 mol) of L-cysteine hydrochloride monohydrate stirred in a reaction vessel containing 87 ml of 91% aqueous tetrahydrofuran under a nitrogen... 

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

Besada [12] described a spectrophotometric method for determination of penicillamine by reaction with nitrite and Co(II). Penicillamine is first treated with 1 M NaN02 (to convert the amino-group into a hydroxy-group), then with 0.1 M CoCl2, and finally the absorbance of the brownish-yellow complex obtained is measured at 250 nm. The process is carried out in 50% aqueous ethanol, and the pH is adjusted to 5.4— 6.5 for maximum absorbance. The calibration graph is linear over the concentration range of 0.25-2.5 mg per 50 mL, and the mean recovery (n = 3) of added drug is 99.7%. Cystine, cysteine, methionine, and other amino adds do not interfere. 

Another advantage to the use of a thiol additive is that the abundance of free thiol groups in the reaction environment will prevent the oxidation of the cysteine thiol at the N-terminal of the other peptide. Without added thiol transesterification catalysts, disulfide formation resulting in dimerization of the Cys-peptide would be a dominant side reaction in aqueous, oxygenated buffer conditions. 

The two examples from our work we are going to describe below are the design and study of liposomal diepitope constructs combining either (i) B and T-helper (Th) peptide epitopes, which induced particularly powerful humoral responses (21) (Fig. 3) or (ii) CTL and Th epitopes, which provided a powerful antitumor vaccine (74) (Fig. 4). For the production of these constructs we have conjugated peptides that contain a cysteine residue either at the N- or C-terminus, to the surface of preformed liposomes by reaction with thiol reactive functionalized phospholipids and/or PamaCys lipopeptide anchors (Fig. 2). To that end, we have developed strategies that give, in aqueous media, high... 

There is some information concerning the reaction of ozone with chemicals under aqueous conditions. The information available suggests that double-bond cleavage takes place, just as it does under nonaqueous conditions, except that ozonides are not formed. Instead, the zwitterionk intermediate reacts with water, producing an aldehyde and hydrogen peroxide. In addition to double-bond cleavage, a number of other oxidations are possible. Mudd et showed that the susceptibility of amino acids is in the order cysteine, tryptophan, methionine. 

Early studies have shown that tryptophan, tyrosine, histidine, methionine and cysteine, either as free amino acids or as components of peptides, are excellent substrates for O2 oxidation reactions. Usually, reaction of O2 with amino acids is mostly described in terms of chemical quenching with the exception of tryptophan, for which collisional deactivation as the result of physical quenching is not neghgible. The rate constants of O2 toward the main reactive amino acids that show a strong solvent dependence are reported in Table 2 for neutral aqueous solutions with values within the range 0.8-3.7... 

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