Disulphides, cyclic

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)...

AVP and OT are cyclic nonapeptides with a disulphide bridge between the cysteine residues 1 and 6, resulting in a six-amino acid ring and a COOH-terminal a-amidated three-residue tail. OT differs only in two amino acids from AVP lie in position 3, which is essential for OT recqrtor (OTR) stimulation and Leu in position 8. AVP has a Phe in position 3 and an Arg in position 8. Arg 8 is essential for acting upon vasopressin receptors (Fig. 1). Lysipressin, found in pigs and some marsupials, has a Lys in position 8 [1]. 

Vasopressin is closely related to oxytocin and both peptides are cyclic in that they contain a disulphide bridge. Although much is known about the peripheral actions of the peptides the extent of our current knowledge of their possible CNS function is that vasopressin appears to act as a cognitive enhancer and has positive effects on learning processes in animals. Vasopressin acts on three receptors, Via and b and a V2 receptor. 

Reaction of perthiophosphonic anhydrides (64) with amines leads first to (105) and then, by further attack, to (106). With ammonia itself the second addition proceeds at the same phosphorus atom as the initial attack, giving (107) and (108). The anhydride (64) is also reported to react with 1,3-dioIs to give cyclic phosphonyl disulphides (109). Thermal decomposition of phenylphosphinic anhydride (110) may lead to the formation of PhP since in the presence of benzil the formation of the phosphorane (111) was observed. ... 

Still another related method consists in reacting dienes with dimethyl disulphide in the presence of iodine, to produce the corresponding bis(o ,/J-dmiethylmercapto) derivative241-243. Again, characteristic fragmentation is obtained in the standard El spectra. However, this method involves complications if the double bonds are separated by less than four methylene groups due to the formation of cyclic thioethers. 

Cyclic thionocarbonates are formed under soliddiquid phase-transfer catalysed conditions from the reaction of diols with carbon disulphide [63]. The reaction has been specifically described for the reaction of carbohydrates, but should be generally applicable to all diols. 

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... 

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. 

Polarography has been successfully applied to the investigation of structural problems involving sulphur compounds. The presence of a disulphide bond has been established by means of the polarographic reduction waves of cytochrome C (156) and lipoic acid (757), and in cyclic disulphides of the oxytocine and vasopressine type (158). The elucidation of the process responsible for the reduction wave of lipoic acid was carried out by comparison with reduction waves of cyclic disulphides, where the disulphide bond was incorporated into rings of various size. The similarity indicated that in lipoic acid an S—S bond which is a part of a larger cyclic system is reduced. 

As a weak base (pKa = 3.9), pantoprazole is highly ionized at low pH and readily accumulates in the highly acidic canalicular lumen of the stimulated parietal cell. In this acidic environment, pantoprazole is rapidly converted to the active species, a cationic cyclic sulfonamide, which binds covalently to cysteine residues on the luminal (acidic) surface of H+, K+-ATPase to form a mixed disulphide, thereby causing irreversible inhibition of gastric proton pump function. As H+, K+-ATPase represents the final step in the secretary process, inhibition of this enzyme suppresses gastric acid secretion regardless of the primary stimulus [1],... 

In order to clarify the mechanism, the reaction of carbon disulphide with mercury bis(n-butanethiolate) was studied. On the basis of results obtained, it was suggested that this reaction involved the formation of a coordination complex, followed by the formation of active species containing the Hg-SC(S) bond. Moreover, the cyclic trithiocarbonate, ethylene trithiocarbonate, found to be present in trace amounts in copolymerisation products, was excluded as a possible intermediate for the copolymer formation, since it did not undergo any polymerisation under the given conditions [249],... 

SFTI-1 (6, 5), an acyclic permutant of the proteinase inhibitor SFTI-1, cis-trans-trans conformer (ct-a) Disulphide analogue of the cyclic sunflower trypsin inhibitor SFTI-1... 

Figure 4 Structural features and diversity of bacteriocins. Solution NMR structures are shown for (a) the lantibiotic actagardine (PDB code 1AJ1) the pedocin-like bacteriocins (b) curvacin A (PDB code 2A2B) and (c) leucocin A (PDB code 1CW6) (d) the A and B chain of the two-chain bacteriocin lactococcin G in DPC micelles (PDB codes 2JPJ and 2JPK) and the cyclic bacteriocins (e) subtilosin (PDB code 1AJ1) and (f) AS-48 (PDB code 1E68). Disulphide bonds and lanthionine linkages are shown in ball-and-stick. Chain termini are labelled N and C and the cyclisation points on the cyclic peptides are labelled with residue numbers.

Plants express a variety of typically very stable proteinase inhibitors in a range of tissues, from seeds to leaves to flowers, which are toxic to microbes and predatory insects. These peptide-based inhibitors come in a range of sizes, from the smallest gene-encoded cyclic peptide known to date, sunflower trypsin inhibitor 1 (SFTI-1),1/92 a 14-residue cyclic peptide with a single disulphide bond, to squash inhibitors93 that are approximately 30 residues in size and feature a cystine-knot motif, to 50-residue proteinase inhibitors found in the stigmas of the ornamental... 

In addition to the reactions discussed in the previous section, the free radical produced photochemically from PhI(02CAd)2 reacted with alkyl or aryl or cyclic disulphides to afford the corresponding adamantyl sulphides in good yield [79] ... 

Under photochemical conditions several 1,4-dipoles reacted with alkenes, alkynes, phenyl isothiocyanate and carbon disulphide to afford, after iodobenzene expulsion, cyclic adducts in moderate to low yields. This drawback is offset by the fact that it is difficult to obtain some of these compounds through alternative methods. Table 10.6 are listed selected examples of cyclic compounds formed by this methodology. Formally, these reactions can be considered as 1,3-dipolar cycloadditions, in which the 1,3-dipole comes from the iodonium precursor after elimination of iodobenzene actually, they arise most probably through 6-membered intermediate iodanes. 

Various non-legume, non-Poaceae Bowman-Birk protease inhibitors have been isolated that have sequence homology to the other BBPIPs [137-140, 285-288] (Table 10). The Ananas BBPIPs are disulphide-linked heterodimers [137-140]. The Helianthus BBPIP SFTI-1 is a small cyclic peptide (cyclotide) [285, 286] whereas the Solanum tuberosum BBPIP is similar to the single-headed Poaceae BBPIPs [287, 288]. 

As previously stated, these cyclic peptides have shown significant bioactivity in different screening regimes and a strong structure-activity relationship has been noted by several workers, with the disulphide bridge in the ulithiacyclamides e.g. 17,18) making these the most cytotoxic of the compounds isolated from L. Patella. It is thought that the... 

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