Intramolecular electron transfer and binding constants in iron hexacyanide-cytochrome c complexes as studied by pulse radiolysis.

Internal oxidation and reduction rates of horse cytochrome c in the complexes CII . Fe(III)(CN)6(3)- and CIII . Fe(II)(CN)6(4)-, are 4.6 . 10(4)s-1 and 3.3 . 10(2)s-1, respectively. The binding site of the iron hexacyanide ions on either CII or CIII are kinetically almost indistinguishable; binding constants range from 0.87 . 10(3) to 2 . 10(3)M-1. The present pulse radiolytic kinetic data is compared with that from NMR, T-jump and equilibrium dialysis studies.     

Preferred sites for electron transfer between cytochrome c and iron and cobalt complexes

The kinetics of oxidation of eight different singly substituted 4-carboxy-2,6-dinitrophenyl (CDNP) horse ferrocytochromes c, modified at lysine 7, 13, 25, 27, 60, 72, 86, or 87, and of one trinitrophenyl horse ferrocytochrome c, modified at lysine 13, by the 3- and 3+ inorganic complexes hexacyanoferrate(III) (Fe(CN)6(3-) ) and tris(1,10-phenanthroline)cobalt(III) (Co(phen)3(3+) ) have been characterized. The influence of the modified residues on the bimolecular rate constants for these reactions define the protein molecular surface involved. The site of electron exchange for both oxidants appears to be the solvent accessible edge of the heme prosthetic group or a closely related structure on the "front" surface of the molecule. The reaction with Fe(CN)6(3-) is most strongly influenced by modification of lysine 72, a residue to the left of the exposed heme edge. (CDNP lysine 72 cytochrome c yields a 3.6-fold decrease in the bimolecular rate constant, as compared to that for the native protein.) However, it is the region around lysine 27, to the right of the heme edge, that is most influential in the reaction with Co(phen)3(3+). (CDNP-lysine 27 cytochrome c exhibits a 7.3-fold increase in the rate constant, as compared to that for the native protein.) The kinetics of reaction of the CDNP-lysine 13, 60, 72, and 87 modified cytochromes c with Fe(CN)5(4-aminopyridine)2- as oxidant and Fe(CN)5(4-aminopyridine)3- and Fe(CN)5-(imidazole)3- as reductants have also been determined and further illustrate the influence of electrostatics on the kinetics of such protein-small molecule electron exchanges.     

Ferrocyanide - a novel catalyst for oxymyoglobin oxidation by molecular oxygen

A comparative study of the rates of ferrocyanide-catalyzed oxidation of several oxymyoglobins by molecular oxygen is reported. Oxidation of the native oxymyoglobins from sperm whale, horse and pig, as well as the chemically modified (MbO(2)) sperm whale oxymyoglobin, with all accessible His residues alkylated by sodium bromoacetate (CM-MbO(2)), and the mutant sperm whale oxymyoglobin [MbO(2)(His119-->Asp)], was studied. The effect of pH, ionic strength and the concentration of anionic catalyst ferrocyanide, [Fe(CN)(6)](4-), on the oxidation rate is investigated, as well as the effect of MbO(2) complexing with redox-inactive Zn(2+), which forms the stable chelate complex with functional groups of His119, Lys16 and Asp122, all located nearby. The catalytic mechanism was demonstrated to involve specific [Fe(CN)(6)](4-) binding to the protein in the His119 region, which agrees with a high local positive electrostatic potential and the presence of a cavity large enough to accommodate [Fe(CN)(6)](4-) in that region. The protonation of the nearby His113 and especially His116 plays a very important role in the catalysis, accelerating the oxidation rate of bound [Fe(CN)(6)](4-) by dissolved oxygen. The simultaneous occurrence of both these factors (i.e. specific binding of [Fe(CN)(6)](4-) to the protein and its fast reoxidation by oxygen) is necessary for the efficient ferrocyanide-catalyzed oxidation of oxymyoglobin.     

Electron transfer kinetics and mechanistic study of the thionicotinamide coordinated to the pentacyanoferrate(III)/(II) complexes: a model system for the in vitro activation of thioamides anti-tuberculosis drugs

The mechanism of activation thioamide-pyridine anti-tuberculosis prodrugs is poorly described in the literature. It has recently been shown that ethionamide, an important component of second-line therapy for the treatment of multi-drug-resistant tuberculosis, is activated through an enzymatic electron transfer (ET) reaction. In an attempt to shed light on the activation of thioamide drugs, we have mimicked a redox process involving the thionicotinamide (thio) ligand, investigating its reactivity through coordination to the redox reversible [Fe(III/II)(CN)(5)(H(2)O)](2-/3-) metal center. The reaction of the Fe(III) complex with thionicotinamide leads to the ligand conversion to the 3-cyanopyridine species coordinated to a Fe(II) metal center. The rate constant, k(et)=10 s(-1), was determined for this intra-molecular ET reaction. A kinetic study for the cross-reaction of thionicotinamide and [Fe(CN)(6)](3-) was also carried out. The oxidation of thionicotinamide by [Fe(CN)(6)](3-) leads to formation of mainly 3-cyanopyridine and [Fe(CN)(6)](4-) with a k(et)=(5.38+/-0.03) M(-1)s(-1) at 25 degrees C, pH 12.0. The rate of this reaction is strongly dependent on pH due to an acid-base equilibrium related to the deprotonation of the R-SH functional group of the imidothiol form of thionicotinamide. The kinetic results reinforced the assignment of an intra-molecular mechanism for the ET reaction of [Fe(III)(CN)(5)(H(2)O)](2-) and the thioamide ligand. These results can be valuable for the design of new thiocarbonyl-containing drugs against resistant strains of Mycobacterium tuberculosis by a self-activating mechanism.     

Michaelis constants of mushroom tyrosinase with respect to oxygen in the presence of monophenols and diphenols

The complex reaction mechanism of tyrosinase involves three enzymatic forms, two overlapping catalytic cycles and a dead-end complex. Analytical expressions for the catalytic and Michaelis constants of tyrosinase towards phenols and oxygen were derived for both, monophenolase and diphenolase activities of the enzyme. Thus, the Michaelis constants of tyrosinase towards the oxygen (K(mO(2))) are related with the respective catalytic constants for monphenols (k(M)(cat)) and o-diphenols (k(D)(cat)), as well as with the rate constant, k(+8). We recently determined the experimental value of the rate constant for the binding of oxygen to deoxytyrosinase (k(+8)) by stopped-flow assays. In this paper, we calculate theoretical values of K(mO(2)) from the experimental values of catalytic constants and k(+8) towards several monophenols and o-diphenols. The reliability and the significance of the values of K(mO(2)) are discussed.     

Ionization dependence of camphor binding and spin conversion of the complex between cytochrome P-450 and camphor. Kinetic and static studies at sub-zero temperatures.

The kinetic rate constants of formation and dissociation of the cytochrome-P-450 - camphor complex (Fe3+-RH) have been obtained by low-temperature (+ 5 degrees C to -20 degrees C) stopped-flow experiments. Simiarly the high-spin/low-spin equilibrium of this complex has been studied as a function of temperature and protonic activity. Both the camphor-binding mechanism and the high-spin/low-spin thermodynamic parameters of Fe3+-RH depend on the protonic activity of the medium in the physiological pH range. The binding rate constants are shown to depend on the ionization of a residue of the protein, probably a histidine. Linear enthalpy-entropy compensation is observed for the camphor binding as well as for the spin-state transition. A camphor-binding-induced change of the electrostatic potential is discussed.     

Studies on the biosynthesis of cytochrome c

A soluble cytochrome was isolated and purified from the slime mould Physarum polycephalum and identified as cytochrome c by room-temperature and low-temperature (77 degrees K) difference spectroscopy. A close similarity between P. polycephalum and mammalian cytochromes c was suggested by a comparison of the initial rates of oxidation of both proteins by mammalian mitochondria. This similarity was further emphasized by redox titrations and gel-electrophoretic studies which indicated that P. polycephalum cytochrome c has an oxidation-reduction midpoint potential of +257mV at pH7.0 and a molecular weight of 12500+/-1500 (mean+/-maximum deviation for a set of six measurements). P. polycephalum exhibits an absolute requirement for protohaemin for growth. The (59)Fe-labelled haemin was prepared by chemical synthesis from protoporphyrin. The purified product had a specific radioactivity of 0.8+/-0.02muCi/mol. Growth of P. polycephalum in the presence of [(59)Fe]haemin resulted in the incorporation of (59)Fe into the plasmodial cytochrome c. The specific radioactivity of the cytochrome c haem was 0.36+/-0.02muCi/mol. The high specific radioactivity of the cytochrome haem indicates that synthesis of the holoenzyme must proceed by direct attachment of haem to the apoprotein rather than by the intermediate formation of a protoporphyrinogen-apoprotein complex. The observed decrease in the specific radioactivity of the haem group is attributed to exchange of the (59)Fe with unlabelled iron in the plasmodia either before or during attachment of the haem group to the apoprotein.     

Partitioning of electrostatic and conformational contributions in the redox reactions of modified cytochromes c.

The reduction of acetylated, fully succinylated and dicarboxymethyl horse cytochromes c by the radicals CH3CH(OH), CO2.-, O2.-, and e-aq' and the oxidation of the reduced cytochrome c derivatives by Fe(CN)3-6 were studied using the pulse radiolysis technique. Many of the reactions were also examined as a function of ionic strength. By obtaining rate constants for the reactions of differently charged small molecules redox agents with the differently charged cytochrome c derivatives at both zero ionic strength and infinite ionic strength, electrostatic and conformational contributions to the electron transfer mechanism were effectively partioned from each other in some cases. In regard to cytochrome c electron transfer mechanism, the results, especially those for which conformational influences predominate, are supportive of the electron being transferred in the heme edge region.     

Thermodynamics of nucleotide binding to actomyosin V and VI: a positive heat capacity change accompanies strong ADP binding

We have measured the energetics of ATP and ADP binding to single-headed actomyosin V and VI from the temperature dependence of the rate and equilibrium binding constants. Nucleotide binding to actomyosin V and VI can be modeled as two-step binding mechanisms involving the formation of collision complexes followed by isomerization to states with high nucleotide affinity. Formation of the actomyosin VI-ATP collision complex is much weaker and slower than for actomyosin V. A three-step binding mechanism where actomyosin VI isomerizes between two conformations, one competent to bind ATP and one not, followed by rapid ATP binding best accounts for the data. ADP binds to actomyosin V more tightly than actomyosin VI. At 25 degrees C, the strong ADP-binding equilibria are comparable for actomyosin V and VI, and the different overall ADP affinities arise from differences in the ADP collision complex affinity. The actomyosin-ADP isomerization leading to strong ADP binding is entropy driven at >15 degrees C and occurs with a large, positive change in heat capacity (DeltaC(P) degrees ) for both actomyosin V and VI. Sucrose slows ADP binding and dissociation from actomyosin V and VI but not the overall equilibrium constants for strong ADP binding, indicating that solvent viscosity dampens ADP-dependent kinetic transitions, presumably a tail swing that occurs with ADP binding and release. We favor a mechanism where strong ADP binding increases the dynamics and flexibility of the actomyosin complex. The heat capacity (DeltaC(P) degrees ) and entropy (DeltaS degrees ) changes are greater for actomyosin VI than actomyosin V, suggesting different extents of ADP-induced structural rearrangement.     

Ternary nickel(II) complexes as hydrolytic DNA-cleavage agents

A series of small model complexes made from Ni(II) and the ligands ethylenediamine (en), histamine (hist), and histidylleucine (HisLeu) were prepared and studied as potential hydrolytic DNA-cleavage agents. The stability constants and species-distribution curves for these complexes were determined as a function of pH. The 1 : 1 : 1 ternary complexes [Ni(II)(en)(HisLeu)] (1) and [Ni(II)(hist)(HisLeu)] (2) were the only major species present at the physiologically relevant pH of 6-7, as further corroborated by ESI-MS analysis. The complex geometries of 1 and 2 were analyzed by UV/VIS experiments and molecular dynamics (MD) simulations. Both ternary complexes were found to intercalate with DNA, as shown by UV/VIS, thermal-denaturation, and fluorescence-titration studies with ethidium bromide (EB). The intrinsic binding constants (K(b)) for the bound complexes 1DNA and 2DNA were determined as 150 and 290, resp. Gel-electrophoresis experiments revealed that 1 and 2 cleave supercoiled (type-I) to nicked-circular (type-II) DNA at physiological pH, with rate constants of 0.64 and 0.75 h(-1), resp. A tentative mechanism for this hydrolytic cleavage is proposed.     

FeFe]-Hydrogenase active site models with relatively low reduction potentials: Diiron dithiolate complexes containing rigid bridges

Three diiron dithiolate complexes containing rigid and conjugated bridges, [mu-SC(6)H(4)-2-(CO)S-mu]Fe(2)(CO)(6) (1), [2-mu-SC(5)H(3)N-3-(CO)S-mu]Fe(2)(CO)(6) (2), and the PPh(3)-monosubstituted complex [mu-SC(6)H(4)-2-(CO)S-mu]Fe(2)(CO)(5)(PPh(3)) (1-P), were prepared as biomimetic models for the [FeFe]-hydrogenase active site. The structures of complexes 1 and 2 were determined by single crystal X-ray analysis, which shows that each complex features a rigid coplanar dithiolate bridge with a 2-3 degrees deviation from the bisect plane of the molecule. The influence of the rigid bridge on the reduction potentials of complexes 1, 2 and 1-P was investigated by electrochemistry. The cyclic voltammograms of complexes 1 and 2 display large positive shifts for the primary reduction potentials, that is, 380-480mV in comparison to that of the pdt-bridged (pdt=propane-1,3-dithiolato) complex (mu-pdt)Fe(2)(CO)(6) and 160-260mV to that of the bdt-bridged (bdt=benzene-1,2-dithiolato) analogue (mu-bdt)Fe(2)(CO)(6).     

When being straight bends rules: a rationale for the linear FeNO unit in the low-spin square-pyramidal {FeNO}7 tetracyanonitrosylferrate(2-) anion

All low-spin S=1/2 heme-NO complexes feature FeNO angles of about 140 degrees . In contrast, the square-pyramidal [Fe(CN)(4)(NO)](2-) complex features an exactly linear {FeNO}(7) unit. We have sought here to determine a possible, simple molecular orbital (MO) rationale for these structural variations. A DFT-based (DFT=density functional theory) MO analysis shows that the linearity of the latter stems from the greater pyramidalization of the Fe center, relative to nitrosylheme, which results in significant differences in d orbital hybridization. Thus, the singly occupied molecular orbital (SOMO) of [Fe(CN)(4)(NO)](2-) , while primarily Fe dz2-based, also has a significant amount of 4p(z) character, which makes it less stereochemically active, accounting for the linearity of the FeNO unit.     

2-Amino-3-carboxy-1,4-naphthoquinone affects the end-product profile of bifidobacteria through the mediated oxidation of NAD(P)H

Glucose metabolism of bifidobacteria in the presence of 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ), a specific growth stimulator for bifidobacteria, and ferricyanide (Fe(CN)(6)(3-)) as an extracellular electron acceptor was examined using resting cells of Bifidobacterium longum and Bifidobacterium breve. NAD(P)H in the cells is oxidized by ACNQ with the aid of diaphorase activity, and reduced ACNQ donates the electron to Fe(CN)(6)(3-). Exogenous oxidation of NADH by the ACNQ/Fe(CN)(6)(3-) system suppresses the endogenous lactate dehydrogenase reaction competitively, which results in the remarkable generation of pyruvate and a decrease in lactate production. In addition, a decrease in acetate generation is also observed in the presence of ACNQ and Fe(CN)(6)(3-). This phenomenon could not be explained in terms of the fructose-6-phosphate phosphoketolase pathway, but suggests rather that glucose is partially metabolized via the hexose monophosphate pathway. This was verified by NADP(+)-induced reduction of Fe(CN)(6)(3-) in cell-free extracts in the presence of ACNQ. Effects of the ACNQ/Fe(CN)(6)(3-) system on anaerobically harvested cells were also examined. Stoichiometric analysis of the metabolites from the pyruvate-formate lyase pathway suggests that exogenous oxidation of NADH is an efficient method to produce ATP in this pathway.     

Kinetic data for redox reactions of cytochrome c with Fe(CN)5X complexes and the question of association prior to electron transfer

Use of rigorous equilibration kinetics to evaluate rate constants for the Fe(CN)6 4- reduction of horse-heart cytochrome c in the oxidized form, cyt c (III), has shown that limiting kinetics do not apply with concentrations of Fe(CN)6 4- (the reactant in excess) in the range 2-10 x 10(-4) M, I = 0.10 M (NaCl). The reaction conforms to a first-order rate law in each reactant, and at 25 degrees C, pH 7.2 (Tris), it is concluded that K for association prior to electron transfer is less than 200 M-1. From previous studies at 25 degrees C, ph 7.0 (10(-1) M phosphate), I = 0.242 M (NaCl), a value K = 2.4 x 10(3) M-1 has been reported. Had such a value applied, some or all of the redox inactive complexes Mo(CN)8 4-, Co(CN)6 3-, Cr(CN)6 3-, Zr(C2O4)4 4- present in amounts 5-20 x 10(-4) M would have been expected to associate at the same site and partially block the redox process. No effect on rats was observed. With the reductants Fe(CN)5(4-NH2-py)3- and Fe(CN)5(imid)3-, reactions proceeded to greater than 90% completion and rate laws were again first order in each reactant. Rate constants (M-1 sec-1) at 25 degrees C, pH 7.2 (Tris), I = 0.10 M (NaCl), are Fe(CN)6 4- (3.5 x 10(4)), Fe(CN)5(4-NH2py)3- (6.7 x 10(5), and Fe(CN)5(imid)3- (4.2 x 10(5). Related reactions in which cyt c(II) is oxidized are also first order in each reactant, Fe(CN)6 3- (9.1 x 10(6)), Fe(CN)5(NCS)3- (1.3 x 10(6)), Fe(CN)5(4-NH2py)2- (3.8 x 10(6) at pH 9.4), and Fe(CN)5(NH3)2- (2.75 x 10(6) at ph 8). Redox inactive Co(CN)6 3- (1.0 x 10(-3) M) has no effect on the reaction of Fe(CN)6 3- which suggests that a recent interpretation for the Fe(CN)6 3- oxidation of cyt c(II), I = 0.07 M, may also require reappraisal.     

Iron(III)- and copper(II) complexes of an asymmetric, pentadentate salen-like ligand bearing a pendant carboxylate group

The equilibrium and solution structural properties of the iron(III) and copper(II) complexes of an asymmetric salen-like ligand (N,N'-bis(2-hydroxybenzyl)-2,3-diamino-propionic acid, H(3)bhbdpa) bearing a pendant carboxylate group were characterized in aqueous solution by potentiometric, pH-dependent electron paramagnetic resonance (EPR) and UV-Vis (UV-Visible) measurements. In the equimolar systems the pentadentate ligand forms very stable, differently protonated mononuclear complexes with both metal ions. In the presence of iron(III) {NH, PhO(-), COO(-)}, {2NH, 2PhO(-), COO(-)} and {2NH, 2PhO(-), COO(-), OH(-)} coordinated complexes are dominant. The EPR titrations reflected the presence of microscopic complex formation pathways, leading to the formation of binding isomers in case of Cu(H(2)bhbdpa)(+), Cu(Hbhbdpa) and Cu(bhbdpa)(-). The {2NH, 2PhO(-)+COO(-)/H(2)O} coordinated Cu(bhbdpa) is the only species between pH 6-11. At twofold excess of metal ion dinuclear complexes were detected with both iron(III) and copper(II). In presence of iron(III) a mu-carboxylato-mu-hydroxo-bridged dinuclear complex (Fe(2)(bhbdpa)(OH)(3)) is formed from Fe(H(2)bhbdpa)(2+) through overlapping proton release processes, providing one of the rare examples for the stabilization of an endogenous carboxylate bridged diiron core in aqueous solution. The complex Cu(2)(bhbdpa)(+) detected in the presence of copper(II) is a paramagnetic (S=1) species with relatively weakly coupled metal ions.     

Ion binding to cytochrome c

This paper is a further study of ion binding to protein surfaces and builds on the studies of the binding of [Cr(CN)6]3- and [Fe(edta)(H2O)]- previously reported [Williams et al. (1982) FEBS Lett. 15, 293-299; Eley et al. (1982) Eur. J. Biochem. 124, 295-303]. In the present paper the binding of polyaminocarboxylate complexes of gadolinium have been studied. Eight ion-binding sites have been identified on the surface of cytochrome c. These exhibit different binding specificities which, in some cases, are not full understood. However it is clear that simple outer-sphere interactions are not the sole determining factor for the association of metal ion complexes with proteins. The NMR paramagnetic difference spectrum method has been shown to be good at locating binding sites and revealing qualitative differences in their relative affinities for a range of complex types. However the use of relaxation probes is not a good method for the quantitative determination of binding constants; for this, isostructural shift probes must be sought.     

Metal chelates with biological activity. Part 4. Solution properties of iron(III)-histidinehydroxamic acid

Reactions of carbon monoxide with iron(II) diethyldithiocarbamate and iron(II) ethylxanthate were followed using solution IR spectroscopy. In DMF and CH₃CN solutions, the only Fe—dithiocarbamate—carbon monoxide complex observed was cis-[Fe(CO)₂(dedtc)₂]. This complex formed rapidly and appeared to be very stable, resisting displacement of the coordinated CO molecules by other ligands. Fe(exa)₂ showed very little coordination of CO in DMF solution, but in CH₃CN solution formed the complex cis-[Fe(CO)₂(exa)₂] rapidly via the monocarbonyl intermediate [Fe(CO)(exa)₂CH₃CN]. In CHCl₃ solution, in the presence of CO and added bases, a series of complexes, [Fe(CO)(exa)₂L], where L = pyridine, pyrrolidine, diethylamine and triphenylphosphine, was formed. However, with the exception of [Fe(CO)(exa)₂(ø₃P)], these monocarbonyl complexes were unstable with respect to disproportionation to cis-[Fe(CO)₂(exa)₂] and [Fe(exa)₂L₂]. No mixed-ligand monocarbonyl complexes were observed with Fe(dedtc)₂.     

Synthesis and electrochemistry of Co(III) and Co(I) complexes having C5Me5 auxiliary

The synthesis and electrochemistry of half-sandwich type of Co(III) complexes [(C(5)Me(5))Co(bidentate)(CH(3)CN)](BF(4))(2) {bidentate=dppe (1,2-bis(diphenylphosphino)ethane), dppp (1,3-bis(diphenylphosphino)propane), bpy (2,2'-bipyridine), en (ethylenediamine)) are reported. Cyclic voltammograms of [(C(5)Me(5))Co(bidentate)(CH(3)CN)](BF(4))(2) in CH(3)CN s showed two redox couples assignable to Co(II)/Co(III) and Co(I)/Co(II). The Co(I) complex having C(5)Me(5) and dppe was also prepared. Two redox couples of this Co(I) complex, (C(5)Me(5))Co(dppe), in CH(3)CN coincided with those of [(C(5)Me(5))Co(dppe)(CH(3)CN)](BF(4))(2) in spite of the structural change around the metal center.     

Study of redox potential in cytochrome <Emphasis Type="Italic">c</Emphasis> covalently bound to terminal oxidase of alkaliphilic <Emphasis Type="Italic">Bacillus pseudofirmus</Emphasis> FTU

Spectroelectrochemistry was used to determine the midpoint redox potentials of heme cofactors of the caa3-type cytochrome oxidase from the alkaliphilic bacterium Bacillus pseudofirmus FTU. The apparent midpoint potentials (E(m)(app)) for the most prominent transitions of hemes a and a3 (+193 and +334 mV, respectively) were found to be similar to the values reported for other enzymes with high homology to the caa3-type oxidase. In contrast, the midpoint potential of the covalently bound cytochrome c (+89 mV) was 150-170 mV lower than in cytochromes c, either low molecular weight or covalently bound to the caa3 complex in all known aerobic neutralophilic and thermo-neutralophilic bacteria. Such an unusually low redox potential of the covalently bound cytochrome c of the caa3-type oxidase of alkaliphilic bacteria, together with high redox potentials of hemes a and a3, ensures more than twice higher difference in redox potentials inside the respiratory complex compared to the homologous mitochondrial enzyme. The energy released during this redox transition might be stored in the transmembrane H+ gradient even under low Deltap in the alkaline environment of the bacteria at the expense of a significant increase in DeltaG of the coupled redox reaction.     

Infrared and electron paramagnetic resonance study of nitrosyl(protoporphyrin IX dimethyl ester)iron(II) and its complexes with nitrogenous bases as model systems for nitrosylhemoproteins: effect of solvent polarity

Infrared and electron paramagnetic resonance spectra of nitrosyl(protoporphyrin IX dimethyl ester)iron(II)(Fe(PPDME)(NO)) and its complexes with nitrogenous bases (N bases) such as imidazoles, pyridines, aliphatic amines, and anilines have been measured in various solvents. At room temperature, giso, Aiso, and nu NO values of five-coordinate Fe(PPDME)(NO) decreased with an increase in solvent polarity parameter ET, indicating the interaction between the solvent and the vacant axial coordination position. It has been found that the nu NO value of six-coordinate species is very sensitive to the solvent polarity, while the giso value is less sensitive. The solvent effect on the equilibrium constants, which are evaluated from the intensity change of the NO stretching band for five- and six-coordinate species, is discussed.