Bridging sulfur

An interesting bridged-sulfur compound, which is a natural constituent of Iranian oil, has been synthesized 478) by the reaction of a bicyclic bis-enamine with sulfur dichloride and subsequent Wolff-Kishner reduction of an initial sulfur-bridged diketone. Sulfur dichloride has also been added to a number of vinylogous amides 479). 

The Mulliken charges of the iron and sulfur atoms, and especially the bond index between the bridging sulfurs (see Tables 1 and 2), favor a bonding description of a single S - S bond that stems from the [Fe - - Fe] core... 

Figure 4. The calculated spectrum of the complex after a Lorentzian band convolution. Region I is dominated by bridging-sulfur-to-iron CT transitions, while region II is mostly due to organic-sulfur-to-iron electron transitions. Regions I and II are explained in a MO diagram. The vertical lines correspond to the experimental bands observed in the absorption spectrum of the [Fe2 (J. - S2) P o - CqH4S) ) ] complex, from Reference 1.

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

The lower redox potential in menaquinone-oxidizing be complexes can be attributed to the absence of the hydrogen bond from the Oy of Ser 163 (ISF)/Ser 130 (RFS) to the bridging sulfur S-1 (see Section III,B,2). This serine is completely conserved in Rieske proteins of bci and bef complexes, but in all sequences of Rieske proteins from menaquinone-oxidizing be complexes, alanine or glycine is found in... 

Earlier studies (162) on synthetic iron-sulfur-based clusters showed that protons bind to the bridging sulfur atoms and increase the rate of substitution at the metal atoms. FeMoco exhibits similar behavior, with protonation causing considerable acceleration in the... 

In this review we will deal with iron-sulfur proteins where the iron atoms are coordinated only by cysteine ligands and bridging sulfurs, as well as rubredoxin (Rd hereafter), which is the initial building block in all subsequent discussions. 

Fig. 4. Representation of the ligand sphere of the [2Fe-2S] cluster of the Rieske protein from spinach and the attribution of g-tensor to moleculEir axes as discussed in the text. Ser 130 has been observed to influence the redox potentiEd of the cluster via hydrogen interactions with the acid-labile bridging sulfur.

Copper(II) complexes of 2,6-lutidylphenylketone thiosemicarbazone, 38, have been prepared from copper(II) chloride and copper(II) bromide [186]. Similar to 2-pyridyl thiosemicarbazones, 38-H coordinates via the ring nitrogen, the azomethine nitrogen and the thiol sulfur based on infrared spectral assignments. Magnetic susceptibilities and electron spin resonance spectra indicate dimeric complexes and both are formulated as [Cu(38-H)A]2 with bridging sulfur atoms. The electronic spectra of both halide complexes show band maxima at 14500-14200 cm with shoulders at 12100 cm S which is consistent with a square pyramidal stereochemistry for a dimeric copper(II) center. 

Mammalian metallothioneins typically bind seven metal ions in cluster structures, with bridging sulfur groups, as seen in the x-ray structure of the Cd5Zn2MT complex (86). It is therefore difficult to develop a simple formation-constant description for the binding of metal ions to MT (87), considering that protonation-deprotonation equilibria of the free protein itself should also be taken into account. However, the usefulness of Table VIII as a guide to the affinity of metal ions for mercapto donor ligands is seen in that the ability of metal ions to... 

The geometry of the Fe4S3 core is composed of a (open base) trigonal pyramid of iron atoms with a triply bridging sulfur atom apically positioned above each face. 

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