Ian L. Johnson

The oxidation of glutathione by nitroprusside: Changes in glutathione in intact erythrocytes during incubation with sodium nitroprusside as detected by 1H spin echo NMR spectroscopy

Spin echo 1H NMR spectroscopy showed that when the hypotensive agent sodium nitroprusside, Na2[Fe(CN)5NO]·2H2O, was incubated with intact erythrocytes in 2H2O saline, glutathione in the erythrocytes was oxidised to diglutathione. This was confirmed by 1H FT NMR for the in vitro reaction. 13C FT NMR showed that the stoichiometry of the glutathione nitroprusside reaction was 1:1; the inorganic products were nitric oxide and hexacyanoferrate(II), [Fe(CN)6]−. At no stage was free cyanide liberated. The reaction of nitroprusside with glutathione, which occurs after the nitroprusside has crossed the erythrocyte membrane, is compared with the reaction of nitroprusside with haemoglobin. In neither of these reactions with major erythrocyte components was any free cyanide liberated by sodium nitroprusside.

Enzyme inhibition by sodium nitroprusside

Abstract The enzyme which mediates relaxation of arterial muscle, and thus controls blood pressure, is guanylate cyclase, an enzyme with a thiol group at the active site and others elsewhere and a prosthetic haem group. Reaction of nitroprusside, a potent vasodilator, with other enzymes (papain and GAPDH) possessing a thiol group at an active site leads only slowly to enzyme inhibition by formation of disulphide bonds. Computer graphics show that the significant thiol group is accessible to the bulky nitroprusside. With enzymes containing a haem group (lactoperoxidase and catalase) inhibition by, or reaction with, nitroprusside is much faster. This suggests that the rapid vasodilator action of nitroprusside is due to its reaction with the haem group of guanylate cyclase.

The solution constitution of the roussin ester Fe2(SBu-t)2(NO)4

Abstract The1H and15N NMR spectra of the Roussin ester Fe2(SBu-t)2(NO)4 show that in solution it exists as a mixture of two isomeric forms (I andII), of C2h- and C2v- symmetry, respectively. Unlike other similar esters, Fe2(SR)2(NO)4, the isomers are present in non-equal proportions: the equilibrium constant K = [II]/[I] is unchanged in the temperature range 220–298 K, indicating that entropy factors are primarily responsible for the unequal abundance ofI andII.

Electron paramagnetic resonance study of the reactions of Roussin esters Fe2(SR)2(NO)4 with a range of nucleophilic anions

Abstract The Roussin methyl ester Fe 2 (SMe 2 )(NO) 4 reacted with bromide or iodide to yield the known paramagnetic mononuclear complexes [Fe(NO) 2 X 2 ] − (X = Br or I). With thiocyanate the reactions of Fe 2 (SR) 2 (NO) 4 (R = Me or Bu t ) proceeded via the S-bonded intermediates [Fe(NO) 2 (SR)(SCN)] − to the N-bonded product [Fe(NO) 2 (NCS) 2 ] − : azide and cyanate gave a similar product [Fe(NO) 2 (NXY) 2 ] − , with no detectable intermediates. Nitrite yielded the known [Fe(NO) 2 (NO 2 ) 2 ] − with a range of Fe 2 (SR) 2 (NO) 4 (R = Me, Et, Pr i or Ph) in a range of solvents (CH 2 Cl 2 , acetone or DMF): Fe 4 S 4 (NO) 4 with nitrite in CH 2 Cl 2 gave a mixture of [Fe(NO) 2 (NO 2 ) 2 ] − and [Fe(NO) 2 (SH) 2 ] − . With Fe 2 (SMe) 2 (NO) 4 , sulphate and chromate yielded complexes of the type, [Fe(NO) 2 (SMe)(OR)] 2− (R = CrO 3 or SO 3 ). The bis-chelating ligand piperazinobis(dithiocarbamate) with Fe 2 (SMe) 2 (NO) 4 gave both a five-coordinate square pyramidal mononitrosyl and a tetrahedral dinitrosyl complex, based on Fe(NO)S 4 and Fe(NO) 2 S 2 chromophores, respectively. The two complexes [Fe(NO) 2 (S 2 MoS 2 )] − and [Fe(NO)(S 2 MoS 2 ) 2 ] 2− which are formed by reaction of tetrathiomolybdate with a range of esters Fe 2 (SR) 2 (NO) 4 have been synthesized also by redox reactions from the known complexes [Fe(NO) 2 (S 2 MoS 2 )] 2− and [Fe(S 2 MoS 2 ) 2 ] 3− , respectively.

Electron paramagnetic resonance study of the reactions of tetrathiomolybdate with roussin salts and esters, and with some related iron nitrosyls

Abstract The tetrathiomolybdate ion [MoS4]2− reacts in DMF solution with Roussin esters Fe2(SR)2(NO)4 (R = Me, Et, n-Pr, i-Pr, n-Bu,t-Bu, n-C5H11) to yield the paramagnetic iron nitrosyls [Fe(NO)2(SR)2]− (1), [Fe(NO)2(S2MoS2]− (2) and [Fe(NO)(S2MOS2)2]− (3). The new complexes (2) and (3) have been characterized by EPR spectroscopy and the assignment to them of constitutions based respectively upon tetrahedral and square pyramidal iron is supported by EHMO calculations. Fe2(SPh)2(NO)4 with [MoS4]2− yields only [Fe(NO)2(SPh)2]−, and preformed (3) reacts with PhS− to give firstly EPR-silent species, and then [Fe(NO)2(SPh)2]−. The mononitrosyl (3) can also be formed by reaction of [MoS4]2− with [Fe4S3(NO)7]−, Fe4S4(NO)4, or Fe2I2(NO)4.

Study by carbon-13 NMR and by EPR of the reactions between the nitroprusside ion and haemoglobins

High frequency 13C NMR spectroscopy using nitroprusside 90% enriched in 13C shows that the nitroprusside ion forms a 1:1 complex with deoxyhaemoglobin, but does not interact with either oxyhaemoglobin or methaemoglobin: additionally hexacyanoferrate(II) is formed in the deoxyhaemoglobin reaction only, but does not bind to haemoglobins. EPR spectroscopy shows that deoxyhaemoglobin, but not oxyhaemoglobin or methaemoglobin, reduces nitroprusside to the intermediate [Fe(CN)4NO]2−. A reaction scheme is suggested to rationalise the con- version of [Fe(CN)5NO]2− to [Fe(CN)6]4− in these reactions.

Isotopic exchange reactions between iron-sulphur-nitrosyl clusters and nitrite

Abstract The anion [Fe 4 S 3 (NO) 7 ] − undergoes slow exchange with labelled nitrite [ 15 NO 2 ] − to yield a product [Fe 4 S 3 ( 14 NO)( 15 NO) 6 ] − in which complete isotopic exchange has occurred at the basal Fe(NO) 2 groups, but with no exchange at the apical Fe(NO) group. The neutral Fe 4 S 4 (NO) 4 reacts rapidly with [ 15 NO 2 − to give fully exchanged [Fe 4 S 3 ( 15 NO) 7 ] − , and it is proposed that the conversion proceeds by fragmentation, followed by complete isotopic exchange and rapid reassembly. The binuclear anion [Fe 2 S 2 (NO) 4 ] 2− also yields, with [ 15 NO 2 ] 2− in CD 2 Cl 2 solution, the fully exchanged [Fe 4 S 3 ( 15 NO) 7 ] − , and a mechanism involving successive fragmentation, exchange and reassembly steps is proposed; however in aqueous solution, a clean exchange reaction occurs to give [Fe 2 S 2 ( 15 NO) 4 ] 2− . Neutral binuclear esters Fe 2 (SR) 2 (NO) 4 (R = Me, Et, or Ph) with [ 14 NO 2 ] − yield the mononuclear paramagnetic [Fe( 14 NO) 2 ( 14 NO 2 ) 2 ] − , and with [ 15 NO 2 ] − the analogous [Fe( 15 NO) 2 ( 15 NO 2 ) 2 ] − .

Spectroscopic and reactivity comparison of pentacyanonitrosylruthenate(2-) with pentacyanonitrosylferrate(2-), nitroprusside

Lithium penta(cyano-13C)nitrosylruthenate (2-), Li2[Ru(13CN)5NO], in which the anion is the ruthenium analogue of the nitroprusside ion, has been synthesized at 90% isotopic enrichment, and characterized spectroscopically. Despite the very high level of 13C enrichment, no two-bond coupling 2J(13Cax-Ru13Ceq) was detected in the high-frequency 13C NMR spectrum of Li2[Ru(13CN)5NO], nor was any such coupling observed in Li4[Ru(13CN)5(15NO2)] although both two-bond couplings to 15N, 2J(13Cax-Ru15NO2) and 2J(13CeqRu15N) were observed. Li2[Ru(13CN)5(14NO)] reacted with excess of Li[15NO2] to yield Li4[Ru(13CN)5(15NO2)] only: no Li2[Ru(13CN)5(15NO)] was observed. Li4[Ru(13CN)5(14NO2)] however showed no exchange with Li[15NO2]. While [Ru(CN)5NO]2− reacted with both OH− and SH− in reactions similar to those of [Fe(CN)5NO]2−, no reactions were detected between [Ru(CN)5NO]2− and piperidine, [CH(CN)2]−, [CH(COCH3)2]−, MeS−, or [S2O4]2−, all of which are known to react readily with [Fe(CN)5NO]2−