S Bridging

No discrete molecule containing covalent 3(8) linkage (Fig. 33) has been found as yet. The crystal structure of NH4Ag(SCN)2 is built up by AgSCN molecules, NHj ions, and NCS ions. The coordination around the S atom of a NCS ion is a distorted flat trigonal pyramid of three Ag and one C atoms. The Ag—S distances range within 2.630 to 2.742 A and these bonds may be rather ionic. 

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If the protein of interest is a heteromultimer (composed of more than one type of polypeptide chain), then the protein must be dissociated and its component polypeptide subunits must be separated from one another and sequenced individually. Subunit associations in multimeric proteins are typically maintained solely by noncovalent forces, and therefore most multimeric proteins can usually be dissociated by exposure to pEI extremes, 8 M urea, 6 M guanidinium hydrochloride, or high salt concentrations. (All of these treatments disrupt polar interactions such as hydrogen bonds both within the protein molecule and between the protein and the aqueous solvent.) Once dissociated, the individual polypeptides can be isolated from one another on the basis of differences in size and/or charge. Occasionally, heteromultimers are linked together by interchain S—S bridges. In such instances, these cross-links must be cleaved prior to dissociation and isolation of the individual chains. The methods described under step 2 are applicable for this purpose. 

An X-ray crystal-structure determination (370) revealed that mixed, nickel coordination exists in the trimeric complex, [Ni(S2CPh)2]3. The trimeric structure is similar to that found (371) in [Pd(PhCS2>]. The structure contains one molecule of type A linked centrosymmetrically through short Ni-S bridges to two molecules of type B, the three molecules being closely parallel (XXVII). Bonamico and co-workers (372)... 

S , bridging sulfurs S , terminal sulfurs (cysteine Sy) N5, coordinating imidazoles av., average distances. 

In Desulfovibrio ferredoxins a general binding motif can be outlined, taking into account the presence of only one [3Fe-4S], one or two [4Fe-4S] cores, the substitution of one cluster by a S-S bridge, or even the replacement of the second cysteinyl residue by an aspartic acid (as in D. vulgaris Miyazaki Fdl and D. africanus Fdlll), which can, in certain conditions, coordinate a fourth iron atom to build a [4Fe-4S] 88) (Fig. 6). 

C13-0124. Design a protein containing ten amino acids whose tertiary structure would be roughly spherical with a hydrophobic interior and a hydrophilic exterior. Include one S—S bridge that would help stabilize the structure. 

Fig. 3. (A) Disposition of afi unit in the membrane, based on sequence information [14,15], selective proteolytic digestion of the a subunit [5,6] and hydrophobic labelling (Table 1). The model for the (S subunit is based on sequencing of surface peptides and identification of S-S bridges [64,65]. T, T2 and C3 show location of proteolytic splits. CHO are glycosylated asparagines in the P subunit. (B) Peptide fragments remaining in the membrane after extensive tryptic digestion of membrane-bound Na,K-ATPase from outer medulla of pig kidney as described by Karlish et al. [7,58].

The intramembrane segments consist of 21-25 amino acid residues with overrepresentation of the hydrophobic residues Phe, He, Leu, Val, Trp, Tyr, but also of Pro and Cys. This may suggest that S-S bridge formation is part of stabilizing intramembrane structures. Prolines or glycines break the continuity of membrane... 

The heteronuclear cluster [Mo3CoS4(H2O)10]4+ has been reported, where the Co is formally zerova-lent.168 The core structure (18) features two edge-linked cubane units with Co—S bridging the two clusters. 

Nickel is found in thiolate/sulflde environment in the [NiFe]-hydrogenases and in CODH/ACS.33 In addition, either a mononuclear Ni-thiolate site or a dinuclear cysteine-S bridged structure are assumed plausible for the new class of Ni-containing superoxide dismutases, NiSOD (A).34 [NiFe]-hydrogenase catalyzes the two-electron redox chemistry of dihydrogen. Several crystal structures of [NiFe]-hydrogenases have demonstrated that the active site of the enzyme consists of a heterodinuclear Ni—Fe unit bound to thiolate sulfurs of cysteine residues with a Ni—Fe distance below 3 A (4) 35-39 This heterodinuclear active site has been the target of extensive model studies, which are summarized in Section 6.3.4.12.5. 

While SCN usually binds via its N atom to Ni11 (or it adopts a N,S bridging mode), rare S-coordination has also been observed.437 (107) involves both N- and S-coordinated terminal thiocyanate, where the Ni—S—C angle is 98.3(1)°, whereas the Ni—N—C angle is 168.7(2)°. S bonding seems to be stabilized by intermolecular H bonding to the uncoordinated thiocyanato-N atom. 

The [Ni(NCS)f,]4 ion is almost perfectly octahedral, with Ni—N distances of around 209.5 pm and N—Ni—N angles around 89.5°. The Ni—N—C and N—C—S entities are practically linear.438,439 In [Ni(NCS)2L2] where L is a R-substituted pyridine, stereochemistry and spin state depend on the type and positions of R.431 While for 2-Me- and 2-Et-pyridine square planar complexes are observed, other pyridins lead to coordination polymers with pseudo-octahedral Ni11 due to N,S-bridging thiocyanate. Ni11 thiocyanato complexes have been studied quite intensively as hosts for inclusion compounds.440"442... 

Ligands of the type RN[(CH2)2SH]2 (R = alkyl, aryl, (CH2)2SH, (CH2)2SR) form dimeric S-bridged complexes (465) with Ni11. In the case of tripodal ligands the third arm rests uncoordinated.1293-1 96... 

There are very few example of S,S-bridged dimers, though tetrakisdithiocarboxylatoplatinum(III) dimers were reported before their carboxylato analogues. R=Me, PhCH2 or... 

The most thoroughly studied mixed S jA-ligand is pyridine-2-thiolate because of its versatile coordination modes. The compound [Ag(SPy)] has a graphite-like array of silver(I) ions and has a semiconductor behavior. The PyS ligand acts as a fi3-N,S,S bridge. [Ag5(SPy)(HSPy)BF4] has a layered structure with the coordination modes of PyS showed in Figure 14.1 04... 

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