Iron -nitroprusside

These, and related, iron nitrosyl compounds have excited considerable interest because of their biological activity.Nitroprusside induces muscle relaxation and is therefore used to control high blood pressure. Roussin s black salt has antibacterial activity under conditions relevant to... 

For a comparison of experimental Mossbauer isomer shifts, the values have to be referenced to a common standard. According to (4.23), the results of a measurement depend on the type of source material, for example, Co diffused into rhodium, palladium, platinum, or other metals. For Fe Mossbauer spectroscopy, the spectrometer is usually calibrated by using the known absorption spectrum of metallic iron (a-phase). Therefore, Fe isomer shifts are commonly reported relative to the centroid of the magnetically split spectrum of a-iron (Sect. 3.1.3). Conversion factors for sodium nitroprusside dihydrate, Na2[Fe(CN)5N0]-2H20, or sodium ferrocyanide, Na4[Fe(CN)]6, which have also been used as reference materials, are found in Table 3.1. Reference materials for other isotopes are given in Table 1.3 of [18] in Chap. 1. 

From the individual contributions of the modes to the msd along the c-axis ( 6 pm ) and along the a-axis ( 8 pm ), the corresponding calculated molecular Lamb-Mossbauer factors for the c-cut crystal (/Lm,c = 0.90) and for the a-cut crystal = 0.87) were derived. Comparison with the experimental /-factor, i.e., / P = 0.20(1) and/ N> = 0.12(1) [45], indicates that by far the largest part of the iron msd must be due to intermolecular vibrations (acoustic modes) of the nitroprusside anions and its counter ions. This behavior is reflected in the NIS spectrum of GNP by the considerable onset of absorption probability density below 30 meV in Fig. 9.36a. 

Sodium nitroprusside (SNP), which is also known as Nipruss or Nipride to medical practitioners, was the first iron nitrosyl complex, prepared as far back as 1850 by Playfair [40]. The hypotensive property of SNP was first demonstrated by Johnson [41] in 1929. It was shown that application of a moderate dose of SNP reduces the blood pressure of a severely hypertensive patient without any side effect [42]. Since that time considerable research has been carried out to understand the mode of action of nitroprusside and its metabolic fate. SNP is now regarded as a potent vasodilator that causes muscle relaxation by releasing NO which activates the cytosolic isozyme of guanylyl cyclase [43-46]. 

The therapeutic effects of sodium nitroprusside depend on release of nitric oxide which relaxes vascular muscle. Sodium nitroprusside is best formulated as a nitrosonium (NO+) complex. Its in vivo activation is probably achieved by reduction to [Fe(CN)5NO]3, which then releases cyanide to give [Fe(CN)4NO]2, which in turn releases nitric oxide and additional CN to yield aquated Fe(II) species and [Fe(CN)6]4 (502). There are problems associated with its use, namely reduced activity due to photolysis (501) and its oxidative breakdown due to the action of an activated immune system (503), both of which release cyanide from the low-spin d6 iron complex. 

All shifts are at room temperature in mm./sec. Zero velocity is defined with respect to the center of a natural iron foil absorber. Sodium nitroprusside has a shift of —0.26 mm./sec. on this scale. In Ref. IS a typographical error inadvertently gave this shift as —0.361 mm./sec. 

We close with a plea to investigators to report shift data with respect to a standard absorber, either clean iron foil or sodium nitroprusside or both. The logic behind this request is obvious since anyone who has tried to intercompare results between different papers, knows how frustrating it can be when no common substances were studied. We also suggest that inherently narrow line sources be used—i.e., Cu, Pt, Pd, or Cr. Stainless steel sources are usually so broad that they can often mask important features of the spectrum. Of the host matrices mentioned we prefer Cu since it doesn t give rise to any interfering x-rays and when prepared properly doesn t show any broadening at low temperatures. 

Nitric oxide coordinated to iron modifies, in a striking manner, the properties and reactivity of free NO (Sec. 6.2). Probably the most famous such coordinated entity is the nitroprusside ion, Fe(CN)5NO . An incisive review of its reactions particularly related to its hypertensive action (it reduces blood pressure of severely hypertensive patients) is available. Nitroprusside ion reacts with a variety of bases... 

The Fe(II)-NO complexes of porphyrins 66-68) and heme proteins 24, 49, 53, 69-76) have been studied in detail by EPR spectroscopy, which allows facile differentiation between five-coordinate heme—NO and six-coordinate heme—NO(L) centers. However, only a few reports of the Mossbauer spectra of such complexes have been published 68, 77-82), and the only Fe(III)-NO species that have been studied by Mossbauer spectroscopy include the isoelectronic nitroprusside ion, [FeCCNlsCNO)] (7S), the five-coordinate complexes [TPPFe(NO)]+ 68) and [OEPFe(NO)]+ 82), and two reports of the nitro, nitrosyl complexes of iron(III) tetraphenylporphjrrins, where the ligand L is NO2 82, 83). 

Fe(CN)5(NO)] makes several appearances in a recent symposium volume dedicated to nitric oxide in biosystems, for example in cormection with iron(II) dtrate-induced oxidative stress." " Nitroprusside is the only metal nitrosyl complex in clinical use, where it is important as a rapidly acting agent for the lowering of exceptionally high blood pressure. ... 

Reaction of the manganese tropocoronand complex [Mn(tc-5,5)(NO)] with [Fe(tc-5,5)] results in complete transfer of the NO to the [Fe(tc-5,5)]. Other nitric oxide complexes appear in the sections on nitroprusside (Section S.4.2.2.6 above), on phthalocyanines (Section 5.4.3.7.4 above), and on polynuclear iron-sulfide complexes (Roussin s salts Section 5.4.5.9.2 below) Fe-por-phyrin-NO redox chemistry has been mentioned in Section 5.4.3.7.2 above. 

Nitroprusside is a complex of iron, cyanide groups, and a nitroso moiety. It is rapidly metabolized by uptake into red blood cells with liberation of cyanide. Cyanide in turn is metabolized by the mitochondrial enzyme rhodanase, in the presence of a sulfur donor, to the less toxic thiocyanate. Thiocyanate is distributed in extracellular fluid and slowly eliminated by the kidney. 

Sodium nitroprusside was first prepared and investigated in the middle of the nineteenth century, and a comprehensive summary of the earlier chemical investigations has been published (17). Up to 1910-1930, the addition reactions of bases to NP were explored, involving the characterization of colored intermediates (e.g., with SH-, SR-, and SO3 ), useful for analytical purposes. The hypotensive action of NP was first demonstrated in 1929, and a considerable research effort has attempted to establish the mode of action of NP and its metabolic fate. Questions still arise on the mechanism of NO release from NP in the biological fluids, and we refer to them below. New accounts dealing with modern structural and reactivity issues associated with the coordination of nitrosyl in NP and other complexes have appeared (18-20). From the bioinorganic and environmental viewpoint, nitrosyl iron complexes have been studied with... 

Detection.—Sulphur in the free state is readily recognisable by its general appearance and characteristics, and especially by its combustion to sulphur dioxide. Both in mixtures and compounds the presence of the element can be demonstrated by heating with charcoal and an alkali carbonate,2 or even better, on a small scale, by heating with an equal bulk of sodium or potassium,3 or with powdered iron 4 in each case some of the sulphur is converted into sulphide, which may be detected by the action of an aqueous extract on mercury or silver, or on sodium nitroprusside the metals are blackened, whilst the nitro-prusside is very sensitive in giving a purple coloration (see p. 62). Alternatively, the solution of the alkali sulphide may be acidified and tests applied for hydrogen sulphide to the vapours evolved on warming. 

The absolute velocity imparted to the drive shaft can be determined either directly or indirectly (30, 32, 87, 88). In the latter technique, the spectrum of a compound with well-established Mossbauer parameters is collected, and to the positions in the spectrum where resonances appear, specific absolute velocities can be assigned. The velocities at other positions in the spectrum are then inferred by interpolation between these known velocities. This indirect calibration is then used in the interpretation of other spectra obtained with the same drive unit. Unfortunately, compounds with well-established Mossbauer parameters may not be available for the Mossbauer isotope of interest. For 57Fe, however, this is not a problem, and metallic iron foils and sodium nitroprusside are often used for calibration purposes. Thus, the 57Fe resonance may be used to calibrate the drive unit, and this unit can then be used to study other Mossbauer isotopes if the drive unit is operated under identical conditions. 

Figure 3 shows a typical spectrum of the unreduced nitro-prusside samples where iron is introduced only as the anionic Na2Fe(CN)5NO complex. Zinc has been cation exchanged into NH Y before addition of the anionic complex. Figure 4 shows an example of samples similar to the zinc nitroprusside Y zeolite, shown in Figure 3, that are completely reduced to metallic iron by hydrogen. 

Chemical shifts relative to sodium nitroprusside not reported c5,Co—Pd source 6natural iron... 

The characteristic colours and solubilities of many metallic sulphides have already been discussed in connection with the reactions of the cations in Chapter III. The sulphides of iron, manganese, zinc, and the alkali metals are decomposed by dilute hydrochloric acid with the evolution of hydrogen sulphide those of lead, cadmium, nickel, cobalt, antimony, and tin(IV) require concentrated hydrochloric acid for decomposition others, such as mercury(II) sulphide, are insoluble in concentrated hydrochloric acid, but dissolve in aqua regia with the separation of sulphur. The presence of sulphide in insoluble sulphides may be detected by reduction with nascent hydrogen (derived from zinc or tin and hydrochloric acid) to the metal and hydrogen sulphide, the latter being identified with lead acetate paper (see reaction 1 below). An alternative method is to fuse the sulphide with anhydrous sodium carbonate, extract the mass with water, and to treat the filtered solution with freshly prepared sodium nitroprusside solution, when a purple colour will be obtained the sodium carbonate solution may also be treated with lead nitrate solution when black lead sulphide is precipitated. 

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