Disulfide intermolecular

Polyacrylamide gel electrophoresis results suggest that p-LG undergoes a greater conformational loss as a fimction of extrusion temperature than a-LA, presumably due to intermolecular disulfide bond formation. Atomic force microscopy indicates that texturization results in a loss of secondary structure of aroimd 15%, total loss of globular structure at 78 °C, and conversion to a random coil at 100 °C (Qi and Onwulata, 2011). Moisture has a small effect on whey protein texturization, whereas temperature has the largest effect. Extrusion at or above 75 °C leads to a uniform densely packed polymeric product with no secondary structural elements (mostly a-helix) remaining (Qi and Onwulata, 2011). 

King, P., Li, Y., and Kochoumian, L. (1978) Preparation of protein conjugates via intermolecular disulfide bond formation. Biochemistry 17, 1499. 

Disulfide exchange can also sometimes occur, and prompt a reduction in biological activity (Figure 6.21). Intermolecular disulfide exchange can result in aggregation of individual polypeptide molecules. 

The X-ray crystal structure of 6-chloro-2,3-dihydro-7-methyl-5-methyIene-2//, 3H, 5H- l,4-dithiepin-l,l,4,4-tetraoxide has been published and a short intermolecular contact across an inversion centre noted <00AX(Qel09>. An experimentally direct and efficient approach to 1,3-dithiepins has been reported using 1 qi-alkyldihalides and carbon disulfide and sodium borohydride, to generate the sulfide nucleophile . 

The principle of oxidation of bis-cysteine peptides on resin consists of deprotection and oxidation of a cysteine pair in a single step or in two separate steps to form the disulfide bond while the peptide chain remains attached to the solid support. The strategy is especially effective for the formation of intramolecular disulfides since the solid support simulates high dilution via the pseudo-dilution effect, thus, largely preventing intermolecular reactions. 163 98 10° ... 

For the synthesis of double-stranded symmetrical and unsymmetrical monocystine peptides the formation of an intermolecular disulfide bridge is required. For homodimerization of cysteine peptides all the methods discussed in Section 6.1.1 can be applied taking into account the reactivity of the different oxidative agents toward sensitive amino acid residues present in the peptide sequences. Synthetic approaches based on the direct use of suitable cystine derivatives can be envisaged, at least for small-size peptides since disproportionation would in all cases retain the homodimeric structure 241... 

This method is based on the activation of one cysteine peptide as a sulfenohydrazide derivative by the reaction of the free thiol group of the cysteine peptide with azodicarboxylic acid as a dialkyl ester or dimorpholide depending on the solubility requirements.156 Following the isolation and characterization of the S-activated intermediate, reaction with a second free thiol cysteine component via thiolysis leads to the desired intermolecular disulfide bond, as shown in Scheme 4. 

A further development of the DMSO/H+ method for oxidation of cysteine peptides led to the cysteine-sulfoxide acid-catalyzed intermolecular disulfide formation with a second S-unprotected or acid-labile protected cysteine component as shown in Scheme 19. 1471 The protonation of the sulfoxide by TfOH in the case of 5(0)-Mob or TFA in the case of 5(0)-Acm derivatives provides electrophilicity to the sulfur atom to allow attack by the second S-unprotected cysteine component (formed by the fast deprotection of the 5-Mob group with TfOH in presence of dimethylsulfide) to generate in a site-directed manner the interchain disulfide bond. Although extensive experience with this method has not been accumulated for interchain disulfide bridging, it has been successfully applied for intrachain site-directed disulfide bond formation in chicken calcitonin-gene-related peptide.1 79 ... 

Although it is a general rule to operate at high dilution (10-4-10 5M) in oxidative folding reactions to avoid formation of intermolecular disulfide-bonded species, p.-conotoxin GIIIB offers an opposite example (Scheme 10). This toxin belongs to a family of conotoxins from marine snails consisting of 22 amino add residues with three intramolecular disulfide bonds.185 ... 

Oxidation of mono-cysteine peptides to the dimer is a straightforward reaction that can produce only the desired product. In the case of bis-cysteine peptides statistically the oxidation leads to the homodimers in parallel and antiparallel orientation as well as to the disulfide-bridged monomer and oligomers. When the two cysteine residues are placed in the adjacent position formation of homodimers is highly favored over the cyclic monomer (Section 6.1.5.1) and the product distribution depends strongly on the peptide concentration. Such a type of intermolecular disulfide bridging is present in bovine seminal ribonuclease, where an antiparallel alignment occurs at the interface of the dimer. 97 ... 

---