Sulfur incorporation

Rubber used in practical applications is crosslinked through disulfide (-S-S-) bonds, and is known as vulcanized rubber. Can you name another important class of polymers which are crosslinked through disulfide bonds Examine vulcanized rubber. How many individual strands does it comprise Are these strands of natural rubber or of gutta-percha What is the percentage (by weight) of sulfur incorporated into the polymer (The molecular weight of the sample is 1701 amu.) Does this classify as a low-sulfur polymer (<3%), a high-sulfur polymer (>10%) or in between ... 

Pulse plating techniques with symmetric or asymmetric waves have been employed for improving the deposition of CdS in acidic aqueous baths of cadmium sulfate and thiosulfate precursors [46, 47], A better control of sulfur incorporation in the deposits was reported. 

In the 1970s, Kishi published a series of landmark papers [36] describing the total syntheses of ( )-dehydrogliotoxin (1973) [36b], ( )-sporidesmin A (1973) [36c], ( )-gliotoxin (1976) [36d], and ( )-hyalodendrin (1976) [36e] in which he employed a new method for epidithiodiketopiperazine synthesis (Scheme 9.4). Cognizant of the harsh conditions required in all of the sulfur incorporation methods developed at the time, it was determined that thiolation would be performed in the early stages of the syntheses. A dithiol intermediate obtained in a similar fashion to Trown s epidithiodiketopiperazine was protected as a dithioacetal, and after elaboration of this core diketopiperazine structure, the dithioacetal was unraveled under mild conditions in the final steps to afford the target epidisulfides. 

The reaction of the same bismuthonium ylide with isothiocyanates in the presence of a copper(H) catalyst afforded the sulfur-incorporated spiro-cyclic compound as the main product, in addition to a small amount of carbon-carbon bond formed products (Scheme 12) [28],... 

In some organisms sulfur incorporation involves homocysteine as an intermediate. In such cases cysteine formation occurs by a transsulfuration reaction, with the intermediate formation of L,L-cystathionine (fig. 21.86). Cystathionine is formed in a simple condensation reaction from serine and homocysteine by cystathionine-jS synthase. 

Meanwhile, complexes with more extended n systems, or with more po-larisable atoms such as sulfur incorporated into the ring, have lower oxidation potentials, and their MLCT excited states are correspondingly lower. Figure 20 illustrates the increasing red-shifts observed upon going from... 

The first approach is based on the hypothesis that C/S ratios in pre-cultural sediments represent thoroughly decomposed seston. If this is true, one can partition sediment sulfur into a "seston" component and an "excess" component representing sulfur incorporated by dissimilatory reduction ... 

The isotopic composition of organosulfur in the Rundle samples cannot be used to discriminate between assimilated sulfur and sulfur incorporated into organic matter by sulfidization. Its elevated concentration, 2.3 wt% of the organic matter, however, suggests that organosulfur was, at least in part, formed by sulfidization processes. 

In summary, these studies point toward important roles for organic matter and salinity in determining sulfur behavior, but are not focused on intermediate sediments and, with a few exceptions (3), are not petrographic in nature. A reconnaissance investigation was undertaken to ascertain possible influences on sulfur incorporation in Mississippi Delta Plain sediments and to determine forms of sulfur in these sediments. 

Figure 6. Postulated intramolecular sulfur incorporation into A7,22- and A7,24(28)-sterols. Reactions in an intermolecular fashion do not seem to...

Figure 7. Proposed intra- and intermolecular sulfur incorporation into A5-sterols.

A more critical evaluation of the above mentioned ratios and phenomena reveals the usefulness of the various palaeosalinity indicators. Distribution patterns of methylated chromans and the relative abundance of gammacerane are not influenced by sulfur incorporation reactions and may directly reflect species distributions in the palaeoenvironment. To some extent this holds for 14a(H),17a(H)/140(H),170(H)-steraneratios as well, although incorporation of sulfur may influence this ratio and original A7/A5-sterol ratios do not always correlate with hypersaline environments. The isoprenoid thiophene ratio is highly useful as a palaeosalinity indicator since the distribution of the C20 isoprenoid thiophenes directly reflects the distribution of their substrates. The other parameters (pristane/phytane ratio, odd-over-even carbon number predominance of n-alkanes, relative abundance of C35 hopanes and/or hopenes) should be used with caution because they obviously depend on the quenching by sulfur of specific lipids, a process which is not restricted to hypersaline environments. 

Figure 19. (a) Depth profile of C35 thiophene hopanoids XXVI and XXVII for the Jurf ed Darawish oil shale, (b) Formation of C35 thiophene hopanoids by sulfur incorporation into bacteriohopanetetrol. 

The resin fractions of organic sulfur-rich bitumens are for a substantial part composed of monomers with linear, isoprenoid, steroid, hopanoid and carotenoid carbon skeletons connected to each other by (poly)sulfide linkages. These structural units may contain additional intramolecular sulfur linkages. This sulfur-rich geopolymer is also formed by sulfur incorporation into functionalised lipids in an intermolecular fashion during early diagenesis. 

The tribofilms consist of amorphous, short-chain phosphates (ortho- and metaphosphates), with some evidence of sulfur incorporated into the phosphate chain structure. Tribofilm structure depends not only on the lubricant composition of the substrate material, but also on the severity of the rubbing contact. The ZDDP typically forms phosphates under antiwear conditions, whereas sulfides and sulfates are typically formed under severe or very severe conditions, li > 0.8 (Willermet et al., 1997). 

Figure 19 Model for the intramolecular and intermolecular reaction of reduced sulfur species with functionalized lipids. Proposed reaction scheme for sulfur incorporation into (17 )-13 8(J7)-malaharica-14(27),17,21-triene (I)...

Figure 20 Progress of intramolecular sulfurization of malabaricatriene (I) with depth in anoxic Cariaco basin sediments. Plot of the ratio of the concentrations of I to the sum of the concentrations of I + V (see Figure 19 for structures) as a function of sediment depth. The progress of transformation of I-V with increasing sediment depth is indicated by the steady decrease in the relative abundance of the precursor lipid (I). Inset plot of In ([ ]/([ ] + [V])) versus sediment age (= time), used to empirically determine the first-order rate constant for sulfur incorporation...

Krein E. B. and Aizenshtat Z. (1994) The formation of isoprenoid sulfur compounds during diagenesis simulated sulfur incorporation and thermal transformation. Org. Geochem. 21(10-11), 1015-1025. 

Putschew A., Scholz-Bottcher B. M., and RuIIkotter J. (1996) Early diagenesis of organic matter and related sulfur incorporation in surface sediments of meromictic Lake Cadagno in the Swiss Alps. Org. Geochem. 25, 379-390. 

Goldhaber M. B. and Kaplan 1. R. (1980) Mechanisms of sulfur incorporation and isotope fractionation during early diagenesis in sediments of the Gulf of California. Mar. Chem. 9, 95-143. 

---