Iron-sulfur-nitrosyl complexes

Iron-sulfur-nitrosyl complexes (continued) from tetranuclear precursors, 32 343 tellurium analogs, 32 350 tetrairon complexes, N NMR spectroscopy, 32 365 x-ray crystallography [Fe(NO)(S2CNR2)2 and related clusters, 32 359-361... 

The recent upsurge of interest in iron-sulfur-nitrosyl complexes has been stimulated in part by the reported isolation of [Fe2(SMe)2(NO)4] from natural sources (12), by the obvious resemblances between these complexes and the naturally occurring [2Fe-2S] and [4Fe-4S] clusters of iron sulfur proteins (23, 14), and by the connections between tetrairon-sulfur-nitrosyls and cubane-type clusters (15). Most of the work in this area has been published in the past 5 years or so, and no review has previously been made. However, a number of excellent reviews of the wider aspects of metal-nitrosyl chemistry have appeared (16 19). 

The principal objective of the earliest X-ray studies to be carried out on iron-sulfur-nitrosyl complexes, those on [Fe2(SEt)2(NO)4] (10) and Cs[Fe4S3(NO)7] (11), was the establishment of their gross chemical constitution. More recent X-ray studies have been concerned not only with gross structure, but additionally with detailed comparisons within series of similar species as a possible probe of electronic structure. 

The diamagnetic behavior of the diiron and tetrairon complexes, despite the presence of formally d1 and/or d9 iron centers, indicates very strong coupling between the individual paramagnetic centers all theoretical treatments of polynuclear iron-sulfur-nitrosyl complexes to date have been based on the assumption of diamagnetism in even-electron species and have employed molecular orbital methods at various levels of approximation. 

Although many of the details remain to be clarified, transfer of nitrosyl groups onto preformed iron-sulfur frameworks of the natural type is well established (107,108). The transfer of nitrosyl groups from iron-sulfur-nitrosyl complexes to atoms other than iron is more problematical, as reported work on the ability of these complexes to nitrosate secondary amines RR NH, with formation of secondary nitrosamines RR NNO, has provided conflicting conclusions. 

The central role of the complexes [Fe(NO)2(SR)2]- in the reaction chemistry of iron-sulfur-nitrosyl complexes and their very ready formation both in in vitro and in vivo (108,123-125,128,129) suggest that the antimicrobial activity of nitrite depends not only upon the disruption of respiration [by destruction of the natural iron-sulfur clusters of redox proteins (70S)] but also specifically upon the formation... 

The isolation of [Fe2(SMe)2(NO)4] from certain preserved vegetables and its implication in diet-related carcinogenesis, underlines the similarities between the iron- sulfur nitrosyl systems and the complexes discussed above. The proto-typic iron-sulfur nitrosyl complexes are Roussin s salts (Section Nitric Oxide Complexes with Sulfur Donors ). [Fe2 (SR)2 (N0)4] complexes are known with a wide range of R groups. They can be synthesized directly from iron(ll) salts, as shown in equation (20), or a variety of other mononuclear iron complex precursors including [Fe(NO)(S2CNMe2)2] (with excess RS ), [Fe(CO)3NO], and [Fe(NO)2(SR)2] . 

There is interest in the possible use of other metal nitrosyl complexes as vasodilators, but from the series KJM(CN)6NO] where M = V, Cr, Mn, and Co (n = 3) or M = Mo (n = 4) neither the Cr nor the Mn complexes exhibit any hypotensive action (504). Iron-sulfur-nitrosyl clusters such as [Fe4S4(NO)4] are active, and their effects can be potentiated by visible light (505). 

The number of iron-selenium-nitrosyl complexes is substantially smaller than the iron-sulfur-nitrosyl species, as considerably less work in this area has been reported. However there are a number of differences between the sulfur systems and their analogs containing selenium or tellurium. For selenium it is convenient to divide the complexes into three classes, dependent upon the stoichiometry of the metal-chalcogen framework. 

The reactions of the anion [Fe2S2(NO)4]2 with electrophiles have already been described (Section II,B,2). Apart from these reactions, most of the chemistry so far reported for iron-sulfur-nitrosyl systems involves the dinuclear complexes [Fe2(SR)2(NO)4], the tetranuclear [Fe4S3(NO)7] and [Fe4S4(N0)4], and the paramagnetic mononuclear species [Fe(NO)2(SR)2] and [Fe(NO)(SR)3] , which prove to be important reactive intermediates in a wide range of reactions. 

S. M. Aldoshin (Director). This session included four presentations. Professor N.P. Konovalova reported on antioxidants and donors of nitrogen monoxide (antitumor effects of nitroxides and NO-donors) the report of N.A. Sanina and S.M. Aldoshin was concerned with a new class of NO-donors (synthesis, structure, properties, and practical use of sulfur-nitrosyl complexes of iron). Free-radical mechanisms of induction and development of secondary necrosis after gun wounds were the subject of the lecture by G.N. Bogdanov L.D. Smirnov reported on pharmacological properties and promising clinical application of antioxidants of the heteroaromatic array. 

The most complex interactions are observed in ternary iron-sulfur-nitrosyl systems. Depending on the bond nature, the ternary iron-sulfur-nitrosyl species may be classified into two groups (i) iron complexes with the S-nitrosothiol ligand containing the [Fe—N(SR)0] moiety and (ii) RS—Fe—NO compounds, where both NO and sulfur are coordinated to the Fe-center. Irrespective of the structural details, the mutual interactions of all components are very strong due to considerable bond delocalization within both ternary systems. 

Roussin s red and black salts were among the earhest synthesized nitrosyl complexes. These iron-sulfur nitrosyls were discovered by Roussin while studying the action of sulfur on solutions of sodium nitroprusside and described in 1858 [42]. 

The low reactivity of both Cyt111 and Cyt11 toward NO can be attributed to occupation of the heme iron axial coordination sites by an imidazole nitrogen and by a methionine sulfur of the protein (28). Thus, unlike other heme proteins where one axial site is empty or occupied by H20, formation of the nitrosyl complex not only involves ligand displacement but also significant protein conformational changes which inhibit the reaction with NO. However, the protein does not always inhibit reactivity given that Cat and nNOS are more reactive toward NO than is the model complex Fem(TPPS)(H20)2 (Table II). Conversely, the koS values... 

Quantitative conversion of the iron in succinate dehydrogenase to this form is possible if additional cysteine is added to the reaction mixture. It is probable that not enough cysteinyl sulfur ligands are available for complex formation without addition of the extra cysteine some of the nitrosyl complex does form without any cysteine addition in these systems. 

A large number of iron-containing proteins form nitrosyl complexes. Heme proteins, iron-sulfur proteins, and other iron proteins such as nonheme iron dioxygenases all form characteristic nitrosyl complexes. In enzymes in which the metal center has an open coordination position, NO often can be bound without severe disruption of the site. This introduces the possibility of reversibility of inhibition. 

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