Mechanistic information on the reversible binding of NO to selected iron(II) chelates from activation parameters

Mechanistic insight on the reversible binding of NO to Fe II chelate complexes as potential catalysts for the removal of NO from effluent gas streams has been obtained from the temperature and pressure parameters for the “on” and “off” reactions determined using a combination of flash photolysis and stopped-flow techniques. These parameters are correlated with those for water exchange reactions on the corresponding Fe II and Fe III chelate complexes, from which mechanistic conclusions are drawn. Small and positive ¢ V q values are found for NO binding to and release from all the selected complexes, consistent with a dissociative interchange (I d ) mechanism. The only exception in the series of studied complexes is the binding of NO to [Fe II (nta)(H 2 O) 2 ] - . The negative volume of activation observed for this reaction supports the operation of an I a ligand substitution mechanism. The apparent mechanistic differences can be accounted for in terms of the electronic and structural features of the studied complexes. The results indicate that the aminocarboxylate chelates affect the rate and overall equilibrium constants, as well as the nature of the substitution mechanism by which NO coordinates to the selected complexes. There is, however, no simple correlation between the rate and activation parameters and the selected donor groups or overall charge on the iron(II) complexe

Thermodynamics and kinetics of RuIII(edta)Ru^{III}(edta) as an efficient scavenger for nitric oxide in aqueous solution

The edta complex of Ru III reacts very rapidly with NO in aqueous solution at pH = 5 to form a stable nitrosyl complex. The results from FT-IR (ATR) and 15 N-NMR studies clearly support the NO character of coordinated NO, such that the nitrosyl product can be formulated as [Ru II (edta)NO] . A combination of UV-Vis spectroscopy and electrochemical detection of NO was used to determine the overall equilibrium constant K NO as (9.1 ± 1.2) × 10 7 M 1 at 25 C and pH = 5.0. Stopped- fl ow kinetic studies on the reaction of acetate-bu ff ered solutions of [Ru III (edta)H 2 O] with NO gave k on values two orders of magnitude lower than that reported in the literature as a result of bu ff er e ff ects. The values of k on determined at low and high pH, viz . 3.8 × 10 4 and 1.2 × 10 5 M 1 s 1 , respectively, are signi fi cantly smaller than that found at pH = 5.0, and in agreement with that observed for the substitution reactions of Ru III (edta) with other entering nucleophiles. Attempts to determine k on for the binding of NO to [Ru III (edta)H 2 O] using laser fl ash photolysis failed due to the occurrence of side reactions. Under speci fi c conditions (NO in excess and NO 2 as impurity), the formation of the disubstituted [Ru II (edta)(NO )(NO 2 )] 2 species was detected using 15 N-NMR spectroscopy. Laser fl ash photolysis of this complex leads to multiple chemical reaction steps as a result of the formation of two primary photoproducts, which decay with di ff erent rate constants to the starting complex. Possible mechanisms for these photoinduced reactions are proposed and compared to related systems reported in the literature

Ligand effects on the kinetics of the reversible binding of NO to selected aminocarboxylato complexes of iron(II) in aqueous solution

Rate constants for the formation and dissociation of FeII(L)NO (L = aminocarboxylato) have been determined using stopped-flow, temperature jump, flash photolysis, and pulse radiolysis techniques. For a series of ligands the formation rate constants vary between 1.6 × 106 and 2.4 × 108M−1 s−1, whereas the dissociation rate constants vary between 0.11 and 3.2 × 103 s−1 at 25 °C. These rate constants result in stability constants (KNO = kf/kd) ranging from 5.0 × 102 to 1.1 × 107M−1, which are in good agreement with values of KNO determined by a combined spectrophotometric and potentiometric technique. The results are discussed with reference to available literature data, and interpreted in terms of a generalized reaction mechanism

Kinetics, mechanism, and spectroscopy of the reversible binding of nitric oxide to aquated iron(II). An undergraduate text book reaction revisited

A detailed kinetic and mechanistic analysis of the classical "brown-ring" reaction of [Fe(H2O)(6)](2+) with NO was performed using stopped-flow and laser flash photolysis techniques at ambient and high pressure. The kinetic parameters for the "on" and "off" reactions at 25 degreesC were found to be k(on) = 1.42 x 10(6) M-1 s(-1), DeltaH(on)(double dagger) = 37.1 +/- 0.5 kJ mol (-1), DeltaS(on)(double dagger) = -3 +/- 2 J K-1 mol(-1), DeltaV(on)(double dagger) = + 6.1 +/- 0.4 cm(3) mol(- 1), and k(off) = 3240 +/- 750 s(-1), Delta(off)(double dagger) = 48.4 +/- 1.4 kJ mol(-1), DeltaS(off)(double dagger) = -15 +/- 5 J K-1 mol-1, DeltaV(off)(double dagger) = +1.3 +/- 0.2 cm(3) mol(-1). These parameters suggest that both reactions follow an interchange dissociative (I-d) ligand substitution mechanism, which correlates well with the suggested mechanism for the water exchange reaction on [Fe(H2O)(6)](2+). In addition, Mossbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H2O)(5)(NO)](2+). The Mossbauer and EPR parameters closely resemble those of the {FeNO}(7) units in any of the other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of high-spin Fe-III antiferromagnetically coupled to NO- (S = 1) yielding the observed spin quartet ground state (S= 3/2), i.e., [Fe-III(H2O)(5)(NO-)](2+), and not [Fe- I(H(2)0)(5)(N0(-))](2+) as usually quoted in undergraduate text books

Laser flash photolysis as tool in the elucidation of the nitric oxide binding mechanism to metallobiomolecules

The article presents a sampling of mechanistic studies on nitric oxide binding to metallobiomolecules. The main emphasis falls on the application of ambient and high pressure laser flash photolysis techniques in the elucidation of the mechanism of the reaction of NO with metals in acti v e centres of biomolecules and complexes of potential medicinal application. # 2002 Else v ier Science B.V. All rights reserved

Reactions of the [Fe(CN)5NO]2−[Fe(CN)_5NO]^{2−} complex with biologically relevant thiols

Reactions of the [Fe(CN)5NO]2− complex with biologically relevant thiols (HnRS[thin space (1/6-em)]=[thin space (1/6-em)]cysteine, N-acetylcysteine, ethyl cysteinate and glutathione) are initiated by the nucleophilic attack of a thiolate (RSn−) on the N atom of the NO+ ligand in the complex to form [Fe(CN)5N(O)SR](n+2)−. The N–S bond in the latter complex is, however, weak and can undergo both heterolytic and homolytic splitting. The former process makes the synthesis reaction reversible, whereas the latter is responsible for the spontaneous redox decomposition: [Fe(CN)5N(O)SR](n+2)−[thin space (1/6-em)]→[thin space (1/6-em)][FeI(CN)5NO]3−[thin space (1/6-em)]+[thin space (1/6-em)]RS˙(n−1)−. The rate of the monomolecular reaction is controlled by an inductive effect in the thiol with an additional stabilisation coming from formation of a six-membered ring in the case of the N-acylated compounds. In the presence of thiolate excess, the RS˙(n−1)− radicals are transformed into the more stable RSSR˙(2n−1)− radicals, which are scavenged by both [Fe(CN)5N(O)SR](n+2)− and [Fe(CN)5NO]2−. The former reaction initiates, whereas the latter terminates, chain reactions of the catalysed redox decomposition. The catalytic decomposition (in the thiol excess) is much faster than the spontaneous decay (in the nitroprusside excess) but leads to the same final products. The Fe(I) reduction product is identified by UV/Vis, IR, electrochemical and EPR methods. The effect of molecular oxygen is investigated and explained. The mechanism is interpreted in terms of intermediate [Fe(CN)5N(O)(SR)2](2n+2)− complex formation via nucleophilic attack and its decay mainly via homolytic splitting of the N–S bond. To verify the mechanism, a simple reaction model is constructed, based on the assumption that the RSNO(n−1)− ligands are mostly responsible for the [Fe(CN)5N(O)(SR)](n+2)− reactivity and their electronic properties are discussed within the DFT framework

Nitrite binding to metmyoglobin and methemoglobin in comparison to nitric oxide binding

Nitrite binds reversibly to the ferriheme pro- teins metmyoglobin and methemoglobin in aqueous bu er solution at a physiological pH of 7.4. The spectral changes recorded for the formation of metMb2NO 2 ) di er signi®cantly from those observed for the nitrosy- lation of metMb, which can be accounted for in terms of the di erent reaction products. Nitric oxide binding to metMb produces a nitrosyl product with Fe2II)-NO + character, whereas the reaction with nitrite produces an Fe2III)-NO 2 ± complex. The kinetics of the binding and release of nitrite by metMb and metHb were investigated by stopped- ̄ow techniques at ambient and high pres- sure. The kinetic traces recorded for the reaction of ni- trite with metMb exhibit excellent single-exponential ®ts, whereas nitrite binding to metHb is characterized by double-exponential kinetics which were assigned to the reactions of the a -and b -chains of metHb with NO 2 . The rate constants for the binding of nitrite to metMb and metHb were found to be much smaller than those reported for the binding of NO, such that nitrite impu- rities will not a ect the latter reaction. The activation parameters 2 D H 6 ; D S ne ; D V 6 ) obtained from the tem- perature and pressure dependence of the reactions sup- port the operation of a dissociative mechanism for the binding and release of nitrite, similar to that found for the binding and release of NO in metMb

Mechanistic studies on the reversible binding of nitric oxide to metmyoglobin

The ferriheme protein metmyoglobin (metMb) in buffer solution at physiological pH 7.4 reversibly binds the biomessenger molecule nitric oxide to yield the nitrosyl adduct (metMb(NO)). The kinetics of the association and dissociation processes were investigated by both laser flash photolysis and stopped-flow kinetics techniques at ambient and high pressure, in three laboratories using several different sources of metMb. The activation parameters ¢ H q , ¢ S q , and ¢ V q were calculated from the kinetic effects of varying temperature and hydrostatic pressure. For the “on” reaction of metMb plus NO, reasonable agreement was found between the various techniques with ¢ H on q , ¢ S on q , and ¢ V on q determined to have the respective values 65 kJ mol - 1 , 60 J mol - 1 K - 1 , and 20 cm 3 mol - 1 . The large and positive ¢ S q and ¢ V q values are consistent with the operation of a limiting dissociative ligand substitution mechanism whereby dissociation of the H 2 O occupying the sixth distal coordination site of metMb must precede formation of the Fe - NO bond. While the activation enthalpies of the “off” reaction displayed reasonable agreement between the various techniques (ranging from 68 to 83 kJ mol - 1 ), poorer agreement was found for the ¢ S off q values. For this reason, the kinetics for the “off” reaction were determined more directly via NO trapping experiments, which gave the respective activation parameters ¢ H off q ) 76 kJ mol - 1 , ¢ S off q ) 41 J mol - 1 K - 1 , and ¢ V off q ) 20 cm 3 mol - 1 ), again consistent with a limiting dissociative mechanism. These results are discussed in reference to other investigations of the reactions of NO with both model systems and metalloproteins