Sulfur structural relationships

In each of the composition diagrams in Fig. 14.2, the numbers represent a series of reactions run at a defined composition and temperature. These are isometric sulfur slices through three-dimensional K/P/RE/S quaternary phase diagrams. As just one example of what we have studied. Table 14.1 identifies the compositions at each point and the resulting phase(s). We have rigorously studied how phase formation is dependent upon the compositions of reactions for the rare-earth elements Y, Eu, and La and we have also discovered key structural relationships between the rare-earth elements, indicating a significant dependence on rare-earth and alkali-metal size for sulfides and selenides. 

Fig. 13.9. An illustration of the copper-sulfur cores observed in [Cu28Si4(P Bu2Me)] 11 (top) and [CU50S25(P Bu2Me)i6] 12 (bottom) illustrating the structural relationships between the two clusters [39].

The structural relationships of sulfur in all three phases are exceedingly complex and there has been considerable confusion concerning new allo-tropes, which subsequently turned out to be mixtures or to be impure, and also concerning nomenclature. We shall deal only with the main, well established species. 

The structure of the membrane of green sulfur bacteria appears to be fundamentally different. Most of the BChl a is contained in a water soluble BChl a protein which is attached to, rather than imbedded in the membrane (13). About one-fourth of the BChl a, however, forms part of a core complex (Fig. 3), which contains approximately 20 BCls a per reaction center (14). The complex also contains about 15 molecules of BChl c or a closely related pigment (15). Flash spectroscopic evidence indicates that one of these molecules acts as electron acceptor in the primary charge separation (16,17). The peptide composition of the core complex may suggest a structural relationship to the core of photosystem I of plants (18). 

A relationship between the redox state of an iron—sulfur center and the conformation of the host protein was furthermore established in an X-ray crystal study on center P in Azotobacter vinelandii nitroge-nase (270). In this enzyme, the two-electron oxidation of center P was found to be accompanied by a significant displacement of about 1 A of two iron atoms. In both cases, this displacement was associated with an additional ligation provided by a serine residue and the amide nitrogen of a cysteine residue, respectively. Since these two residues are protonable, it has been suggested that this structural change might help to synchronize the transfer of electrons and protons to the Fe-Mo cofactor of the enzyme (270). 

The remarkable range of redox potentials in the iron sulfur proteins, already noted, illustrates the principle that nature, having discovered a ligand system, attempts to extract from it the maximum utility. Certainly the outstanding problem awaiting solution in these proteins is an explanation for the relationship between structure and redox potential. Thus... 

The 1 R,6R,7R,8S-as-fused structure and conformation of 102 were elucidated on the basis of their NMR spectroscopic data. The observed formation of only one sulfonium salt in this cyclization reaction was remarkable in that either sulfur atom might have been expected to participate in tosylate displacement. The H NMR spectrum of salt 102 shows a large three-bond scalar coupling of 10.6 Hz between H-6 (5 = 4.736) and H-7 (5 = 4.606) this indicates that they have an almost antiperiplanar relationship. The equatorial orientation of H-6 and the 3C6 conformation of its six-membered cycle are consistent with the strong NOEs observed between H-7 and both H-2axja and and... 

---