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 Delta V(++) 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) complexes.

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

Nitrite binds reversibly to the ferriheme proteins metmyoglobin and methemoglobin in aqueous buffer solution at a physiological pH of 7.4. The spectral changes recorded for the formation of metMb(NO2-) differ significantly from those observed for the nitrosylation of metMb, which can be accounted for in terms of the different reaction products. Nitric oxide binding to metMb produces a nitrosyl product with Fe(II)-NO+ character, whereas the reaction with nitrite produces an Fe(III)-NO2- complex. The kinetics of the binding and release of nitrite by metMb and metHb were investigated by stopped-flow techniques at ambient and high pressure. The kinetic traces recorded for the reaction of nitrite with metMb exhibit excellent single-exponential fits, whereas nitrite binding to metHb is characterized by double-exponential kinetics which were assigned to the reactions of the alpha- and beta-chains of metHb with NO2-. 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 impurities will not affect the latter reaction. The activation parameters (deltaH++,deltaS(ne),deltaV++) obtained from the temperature and pressure dependence of the reactions support 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.

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(H(2)O)(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 degrees C were found to be k(on) = 1.42 x 10(6) M(-1) s(-1), DeltaH(++)(on) = 37.1 +/- 0.5 kJ mol(-1), DeltaS(++)(on) = -3 +/- 2 J K(-1) mol(-1), DeltaV(++)(on) = +6.1 +/- 0.4 cm(3) mol(-1), and k(off) = 3240 +/- 750 s(-1), DeltaH(++)(off) = 48.4 +/- 1.4 kJ mol(-1), DeltaS(++)(off) = -15 +/- 5 J K(-1) mol(-1), DeltaV(++)(off) = +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(H(2)O)(6)](2+). In addition, Mössbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H(2)O)(5)(NO)](2+). The Mössbauer 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)(H(2)O)(5)(NO(-))](2+), and not [Fe(I)(H(2)O)(5)(NO(+))](2+) as usually quoted in undergraduate text books.