Disulfide group

Reactions of the Disulfide Group. Besides the thiol end groups, the disulfide bonds also have a marked influence on both the chemical and physical properties of the polysulftde polymers. One of the key reactions of disulfides is nucleophilic attack on sulfur (eq. 4). The order of reactivity for various thiophiles has been reported as (C2H O) P > R, HS , C2H5 S- >C,H,S- >C,H,P,... 

The solubility in alkali sulfide solutions stems from the presence of disulfide groups —S—S— in ortho position to the terminal amino groups. These disulfide groups are reduced to the mercapto groups —SH, which are soluble in alkaU. 

Mechanism of Dyeing. Wool (qv) is a complex protein polymer based on amino acid building blocks, and the polymer chains are cross-linked by disulfide groups. Amino acids (qv) contain both amino and carboxyUc acid groups and therefore the wool polymer contains both —NH2... 

CH2SH + 1/2 O2 -CH2-S-S-CH2 + H2O Disulfide bonds form between the side chains of two cysfeine residues. Two SH groups from cysteine residues, which may be in different parts of the amino acid sequence but adjacent in the three-dimensional structure, are oxidized to form one S-S (disulfide) group. 

Several precautions were taken to ensure the immobilization chemistry. First, the sulfhydryl groups containing the macromolecular fraction was spectrophotometrically determined according to the literature [15]. We found that every set of 150 base pairs contained approximately one disulfide group. Since the DNA fragment used has hundreds of base pairs, each DNA strand seems to have one disulfide as its terminal group. Next, we made IR spectral measurements in a reflection-absorption (RA) mode [14b]. A freshly evaporated gold substrate was immersed into the DNA solution for 24 h at 5°C. The substrate was carefully rinsed with deionized water, dried under vacuum and was immediately used for the measurements. An Au substrate treated with unmodified, native sonicated CT DNA solution was also prepared as the control measurement. The / -polar-ized radiation was introduced on the sample at 85° off the surface normal and data were collected at a spectral resolution of 4 cm with 2025 scans. 

Sulfo-LC-SMPT is not as stable as SMPT. The sulfo-NHS ester is more susceptible to hydrolysis in aqueous solutions and the pyridyl disulfide group is more easily reduced to the free sulfhydryl. Stock solutions of sulfo-LC-SMPT may be prepared in water, but should be used immediately to prevent loss of amine coupling ability. 

Figure 1.73 The disulfide group of a cystamine-modified protein may undergo disulfide interchange reactions with another sulfhydryl-containing protein to yield a disulfide-linked conjugate.

Figure 1.74 Thiol-containing disulfide reductants reduce disulfide groups through a multi-step process producing a mixed disulfide intermediate.

Reported structures for homobifunctional aryl azides include a biphenyl derivative and a naphthalene derivative (Mikkelsen and Wallach, 1976), a biphenyl derivative containing a central, cleavable disulfide group (Guire, 1976), and a compound containing a central l,3-diamino-2-propanol bridge between phenyl azide rings that are nitrated (Guire, 1976). The only commercially available homobifunctional photoreactive crosslinker is BASED. 

Figure 5.2 SPDP can react with amine-containing molecules through its NHS ester end to form amide bonds. The pyridyl disulfide group then can be coupled to a sulfhydryl-containing molecule to create a cleavable disulfide bond.

Figure 5.3 SMPT can form crosslinks between an amine-containing molecule and a sulfhydryl-containing compound through amide and disulfide linkages, respectively. The hindered nature of the disulfide group provides better stability toward reduction and cleavage.

Figure 5.18 SASD is a photoreactive crosslinker that can be used to modify amine-containing compounds through its NHS ester end and subsequently photoactivated to initiate coupling with nucleophiles (after ring expansion to an intermediate dehydroazepine derivative). The crosslinks may be selectively cleaved at the internal disulfide group using DTT.

Figure 5.32 APDP can modify sulfhydryl-containing compounds through its pyridyl disulfide group to form disulfide bonds. Its phenyl azide end then can be photolyzed with UV light to couple with nucleophiles via a ring-expansion process. The disulfide group of the crosslink can be selectively cleaved using Dll.

With mixing, add an aliquot of the crosslinker to the dendrimer solution to provide the desired molar excess of reagent. For many applications, less than 10 pyridyl disulfide groups are needed per dendrimer molecule therefore, molar ratios in the range of 5-20 X excess of crosslinker over the amount of dendrimer present typically are used. 

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