Formation of disulphides

The preparation of peptides with intermolecular disulphides also requires 

Protocol 5. Pretreatment to reduce undesired disulphide bridges 

Add DTT (5 eq relative to the number of desired thiol groups), and flush the mixture with N2 and leave standing for 2-6 h at 25 C.  

Add 10% AcOH(aq) to adjust the reaction mixture to pH 2-3, and lyophilize the mixture. 

Remove the excess DTT and other low molecular weight components by gel filtration. 

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Although sodium sulphide reacts readily with haloalkanes [2] and activated aryl halides (see Chapter 2) [e.g. 3-5] in the presence of a quaternary ammonium catalyst to form symmetrical thioethers (Table 4.1), a major side reaction results in the formation of disulphides owing to the oxidation of the intermediate thiols under the basic conditions. Consequently, little use has been made of this procedure for the synthesis of thioethers [3, 6], but the corresponding reaction of the a,(0-dihaloalkanes to yield cyclic thioethers has proved to be a valuable procedure for the synthesis of thietanes [7] (Table 4.2). The ring closure with the secondary dihaloalkanes is considerably more difficult to effect than is the reaction of the primary dihaloalkanes. 1,3-Dihydrobenzo[c]thiophene (89%) is produced in the reaction of 1,2-bis(bromomethyl)benzene with sodium sulphide (Scheme 4.1) [8]. The direct... 

As indicated above, the traditional base-catalysed hydrolysis of 0,5-dialkyl thio-carbonates for the synthesis of thiols is generally unsatisfactory, as oxidation leads to the formation of disulphides. Under phase-transfer conditions, the procedure produces thioethers to the virtual exclusion of the thiols, as a result of the slow release of the thiolate anions in the presence of the electrophilic ester. However, a simple modification of the reaction conditions provides an efficient one-pot reaction [50] from haloalkanes (Table 4.15) via the intermediate formation of the thermally labile (9-/ert-butyl-5-alkyl dithiocarbonates (Scheme 4.8). 

The anodic scan section of cyclic voltammetry for pyrrhotite electrode, respectively, in pH=7, 8.8, 11, 12.1, 12.7 buffer solution with dithiocarbamate are presented in Fig. 4.28. The cyclic voltammograms curve at pH = 8.8 is also presented in Fig. 4.28. It can be seen that the anodic ciurent peak emerges at about O.IV. As pH increases, the peak moves to the left. This peak may correspond to the formation of disulphide. When the concentration of dithiocarbamate is 10 mol/L, the oxidation of dithiocarbamate forming disulphide is h = 0.22 V, which agrees with the results in Fig. 4.26. When the hydrophobic entity disulphide was formed, the flotation of pyrrhotite could be possible. 

Saccharomyces cerevisiae processing High yields (g/l in Pichia pastoris) Inexpensive Formation of disulphides Glycosylation mammalian systems... 

Insect cell expression Baculovirus High-level expression Formation of disulphides Glycosylation Glycosylated product may differ from mammalian systems Not necessarily fully functional... 

The proteins can participate in sulphydryl-disulphide interchange reactions at temperatures above about 75°C at the pH of milk, but more rapidly at or above pH 7.5. Such interactions lead to the formation of disulphide-linked complexes of / -lg with K-casein, and probably as2-casein and a-la, with profound effects on the functionality of the milk protein system, such as rennet coagulation and heat stability. 

Recently, active recombinant a-LTX has been generated using bacteria in which both thioredoxin reductase and glutathione reductase are inactivated to improve the formation of disulphide bonds in expressed proteins (Li et al. 2005). The toxin is expressed as a fusion with glutathione-S-transferase (GST), which is used for affinity purification of the recombinant toxin and can be subsequently removed by selective proteolysis. Considering the relative ease of generating recombinant proteins in bacteria, this approach will facilitate structure-function studies of a-LTX. 

Leitner,. V.M., Walker, G.F. and Bernkop-Schniirch, A. (2003b) Thiolated polymers evidence for the formation of disulphide bonds with mucus glycoproteins. Eur. 

A sustained drug release is favourable for drugs with short elimination half-life. It can be controlled by hydration and diffusion mechanisms or ionic interactions between the drug and the polymeric carrier. In the case of diffusion control the stability of the carrier system is essential, as its disintegration leads to a burst release. Therefore, the cohesiveness of the polymer network plays a crucial role in order to control the release over several hours. Due to the formation of disulphide bonds within the network thiomers offer adequate cohesive stability. Almost zero-order release kinetics could be shown for insulin embedded in thiolated polycarbophil matrices (Clausen and Bernkop-Schnurch 2001). In the case of peptide and protein drugs release can be controlled via ionic interactions. An anionic or cationic polymer has to be chosen depending... 

Mercaptans. — Bunge3 electrolyzed the alkali salts of ethyl and methyl mercaptans and observed the formation of disulphides at the positive pole. In the case of the sulpho-compounds, however, the free adds were generated. 

The formation of disulphides (143) on irradiation of S-alkyl-3-oxobutanethioates (144) in benzene or ethanol appears to involve a Type I cleavage, as shown in Scheme 6, rather than direct carbon-sulphur bond homolysis. °° In contrast, the phenacyl sulphides (145) are converted in high yield into the thiones (146) via a Type II cleavage. " Examples of the... 

When secondary arsines are treated with sulphur, excess of the latter leading to the formation of disulphides ... 

Transport through membrane Formation of disulphide bridges... 

D-penicillamine is so named because it was first isolated as an amine, from the degradation products of penicillin by Abraham et al [87]. Later studies showed the characteristic chemical behavior of D-penicillamine which involve three types of reactions, formation of disulphide links, formation of thiazolidine rings, and formation of metal complexes and chelates [67]. It was first used in 1956 in the treatment of Wilson s disease [88]. D-penicillamine has since been used in the treatment of many diseases, such as cystinuria [89], rheumatoid arthritis [90-92], systemic sclerosis [93], primary bdiary cirrhosis [94], heavy metal poisoning due to lead [95], cadmium [%], and mercury [97], and hyperviscosity syndrome [99]. In rheumatoid arthritis, D-peni-cdlamine has been widely accepted as an effective second line treatment. Despite of its effectiveness, it causes many adverse effects, such as skin rashes [99,100], taste abnormalities [100,101], hepatic dysfunction [102-104], gastrointestinal toxiciiy [99,105], proteinuria [100,106], hematuria [107, 108], thrombocytopenia [92, 109], aplastic anemia [110], lupus-like syndrome [111, 112], Goodpasture s-tike pulmonary renal syndrome [113-115], vasculitis [116,117], myasthenia gravis [118-122], polymyositis [123, 124], and dermatomyositis [125]. 

Detailed investigation of the oxidation of ethane thiol in the presence of copper-, cobalt-, and nickel-containing catalysts was also carried out [138]. The reaction was stoichiometric to disulphide, and the dependence of the rates of oxidation on the concentration of individual reactants is summarised in Table 4. It can be seen that the concentrations of added metal bear little resemblance to the concentrations of catalytically active metal. The change from initial to final rates usually occurred at about 10— 30% of total conversion and was attributed to the formation of disulphides which can compete for coordination sites on the metal ion. 

In contrast with a i- and jS-caseins, the presence of systein residues in as2- and K-caseins leads to the formation of disulphide bonds. The number of molecules that bond to each other in K-casein differs from that in as2-casein. That is to say that in K-casein not less than 10 molecules join each other via disulphide linkages, while in most of as2-caseins only 2 molecules are linked by disulphide bonds and form a dimer. As well, intermolecular disulfide bonds have been reported for as2- and K-caseins [6, 11]. The joint of two classes of caseins for instance as2-casein-K-casein, has also been reported [6] due mainly to the different charges of C-terminal regions in these two molecules [8]. 

The formation of disulphides from sulphenyl chloride occurs by a radical mechanism, and the kinetics of the homolytic dissociation have been investigated using radical traps... 

The mutations prevent the formation of disulphide bonds preventing normal folding of proinsuUn in the endoplasmic reticulum (ER) leading to ER stress, 3-cell apoptosis and neonatal diabetes mellitus (INS-NDM) Inactivating mutations cause decreased transcriptional activity influencing both pancreatic development and the transcription of key genes for insulin secretion (HNFIA-MODY)... 

Different mechanisms are proposed for this reaction depending upon the presence of added cyanide. A mechanism similar to that outlined in equations (49) and (50) is suggested for the oxidation in the presence of added cyanide, i.e. slow formation of thiyi radicals and fast formation of disulphide via dimerization of the radicals or further oxidation of them to... 

There is not, however, general agreement with this explanation. Indeed, direct oxidation of mercaptide ion to sulphonic add was proposed by Berger who considers the formation of disulphide as a side reaction. 

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