RSNO Structure

The S-N=0 bond is labile and as a result energetic parameters related to the S-N bond have relied on computational investigations (Barberger et al., 2000 2001 Fou et al., 2002 de Oliveira et al., 2002 Lu et al., 2001). The predicted S-N and N-O bond lengths from one such investigation (Baciu and Gauld, 2003) are presented in Fig. 4.1. 

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This chapter summarizes the current state of knowledge on the role of S-nitroso-thiols in mammalian systems under the following headings Structure and cellular reactivity of RSNOs formation of RSNOs in the biological milieu and physiological role of RSNOs. 

Hbp C93 S-nitrosation could also be accomplished by exposure to RSNOs (GSNO or CysNO). The rates RSNO-dependent Hb-S-nitrosation was 10-fold larger in oxy-Hb than in deoxy-Hb. Conversely, the rate of spontaneous decay of deoxy-Hb-SNO was -20-fold larger than oxy-Hb-SNO. An explanation for this differential reactivity was presented in a subsequent study (Stamler et al, 1997) where protein modeling data based on the X-ray structures of Hb in T and R states indicated that in OxyHb the SNO of Cys (1 93 is protected from solvent. In contrast, in deoxyHb the SNO is highly exposed to solvent. The implication was that the NO+ on Cys (193-S-NO could be transferred to thiols in RBC and eventually effluxed to induce vasodilation under conditions of low 02 saturation. 

DNICs are spontaneously [128] formed in aqueous media using a simple Fe(II) salt, S-nitrosothiol and thiol, with a ratio of Fe2+/RSH of 1 20. NO is transferred quantitatively from the sulfur atom in the RSNO to the iron. The complete mechanism is yet to be fully determined. A 1 2 ratio results in the formation of an EPR silent yellow dinuclear iron complex ([Fe2(RS)2(NO)4]. At the higherer ratio, the green paramagnetic, mononuclear dinitrosyl predominates. The reaction is very straightforward at pH 7.8, under an inert atmosphere and in water. Under anaerobic conditions the stability of this compound is enhanced, however, in the presence of air and hydrogen peroxide, it readily decomposes to give the dinuclear complex [126] which is similar in structure to the Roussin red salt, as shown in Scheme 5.5. 

The triorganotin oxides and hydroxides are relatively straightforward. The complete structure of trimethyltin hydroxide has not yet been determined, but exposure of a solution of trimethyltin chloride to moisture gives the half-hydrolysis product Me3SnCbMe3Sn0H H20, in which trigonal-planar Me3Sn units are alternately bridged by Cl and OH, with rSnO = 212.5(2) and 212.1(2) pm, and rSnCl = 290.7(3) and 289.2(3) pm (Equation (132)).353... 

Bulky groups R and R might be expected to favour the existence of monomeric molecules RsSnC CR in the crystal. The only examples which appear to be known are where R = phenyl, cyclohexyl, or neophyl.21 The closest to the ideal structure 13-4 is tricyclohexyltin trifluoroacetate, with rSn-0 208(4) and 311(4) pm, though this might be regarded as a weakly associated polymer.13 Triphenyltin (o-2-hydroxy-5-methylphenyl-azo)benzoate approaches the structure 13-5 with rSnO 207.0(5) and 246.3(7) pm.22... 

Hexaphenyldistannane reacts with 9,10-phenanthraquinone (OPQO) to give first the expected monoradical Ph3SnOPQO which decomposes on standing to give the crystalline diradical Ph2Sn(OPQO )2, which has a distorted octahedral structure in which each semiquinone unit is unsymmetrically bonded to the tin, with rSnO ca. 213 and 221 pm.76... 

Not until X-ray diffraction studies were performed was their structure finally clarified. Now, two types of hexanuclear organotin oxo-carboxylates are known -hexamers of composition [RSnO(OOCR )]6, which have a hexagonal prismatic cage structure, 165, (or drum structure) and ladder type compounds, 166, of composition [ RSnO(OOCR ) 2 RSn(OOCR )2X ] (X = R COO or Cl) both contain Sn202 aggregates. The compounds have been reviewed (510). 

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