RESUMO
The hypervalent muscle pigment ferrylmyoglobin, formed by activation of metmyoglobin by hydrogen peroxide, was found to be reduced in a second-order reaction by N-tert-butyl-alpha-phenylnitrone (PBN, often used as a spin trap). In acidic aqueous solution at ambient temperature, the reduction is relatively slow (deltaH++ = 65+/-2kJ x mol(-1) and deltaS++ = -54+/-7 J x mol(-1). K(-1) for pH = 5.6), but phase transitions during freezing of the buffered solutions accelerates the reaction between ferrylmyoglobin and PBN. In these heterogenous systems at low temperature (but not when ice-formation was inhibited by glycerol), a PBN-derived radical intermediate was detected by ESR-spectroscopy, identified as a nitroxyl radical by a parallel nitrogen hyperfine coupling constant of 31.8 G, and from microwave power saturation behavior concluded not to be located in the heme-cleft of the protein. The acceleration of the reaction is most likely caused by a lowering of the pH during the freezing of the buffered solutions whereby ferrylmyoglobin becomes more oxidizing.
Assuntos
Congelamento , Metamioglobina/química , Óxidos de Nitrogênio/química , Óxidos N-Cíclicos , Espectroscopia de Ressonância de Spin Eletrônica , Radicais Livres , Glicerol/farmacologia , Peróxido de Hidrogênio/química , Concentração de Íons de Hidrogênio , Oxirredução , Soluções , Espectrofotometria , Marcadores de Spin , Termodinâmica , ÁguaRESUMO
Decay of the hypervalent muscle pigment ferrylmyoglobin, formed by activation of metmyoglobin by hydrogen peroxide, was found, when studied by a combination of ESR and UV/VIS spectroscopy in aqueous solution at physiological pH, to proceed by parallel second- and first-order kinetics. At pH below 6.5 a sharp ESR signal (g = 2.003) with an increasing intensity for decreasing pH were observed in solutions frozen in liquid nitrogen, and a broad signal (g = 2.005) was seen throughout the studied pH range also in frozen solutions. The g = 2.005 signal is suggested to arise from an intermediate formed in an intramolecular rate-determining electron-transfer in ferrylmyoglobin, whereas the g 2.003 signal is caused by a radical formed in a proton-assisted electron-transfer initiating the specific acid-catalysed autoreduction.
Assuntos
Espectroscopia de Ressonância de Spin Eletrônica , Congelamento , Metamioglobina/química , Radicais Livres , Peróxido de Hidrogênio/química , Concentração de Íons de Hidrogênio , Cinética , Oxirredução , Soluções , Espectrofotometria , Espectrofotometria Ultravioleta , ÁguaRESUMO
Kinetics of reduction of iron(IV) in ferrylmyoglobin by chlorogenate in neutral or moderately acidic aqueous solutions (0.16 M NaCl) to yield metmyoglobin was studied using stopped flow absorption spectroscopy. The reaction occurs by direct bimolecular electron transfer with (2.7 +/- 0.3) x 10(3) M(-)(1).s(-)(1) at 25.0 degrees C (DeltaH( )(#) = 59 +/- 6 kJ.mol(-)(1), DeltaS(#) = 15 +/- 22 J. mol(-)(1).K(-)(1)) for protonated ferrylmyoglobin (pK(a) = 4.95) and with 216 +/- 50 M(-)(1).s(-)(1) (DeltaH( )(#) = 73 +/- 8 kJ. mol(-)(1), DeltaS( )(#) = 41 +/- 30 J.mol(-)(1).K(-)(1)) for nonprotonated ferrylmyoglobin in parallel with reduction of a chlorogenate/ferrylmyoglobin complex by a second chlorogenate molecule with (8.6 +/- 1.1) x 10(2) M(-)(1).s(-)(1) (DeltaH( )(#) = 74 +/- 8 kJ.mol(-)(1), DeltaS( )(#) = 59 +/- 28 J.mol(-)(1).K(-)(1)) for protonated ferrylmyoglobin and with 61 +/- 9 M(-)(1).s(-)(1) (DeltaH( )(#) = 82 +/- 12 kJ.mol(-)(1), DeltaS( )(#) = 63 +/- 41 J. mol(-)(1).K(-)(1)) for nonprotonated ferrylmyoglobin. Previously published data on ascorbate reduction of ferrylmyoglobin are reevaluated according to a similar mechanism. For both protonated and nonprotonated ferrylmyoglobin the binding constant of chlorogenate is approximately 300 M(-)(1), and the modulation of ferrylmyoglobin as an oxidant by chlorogenate (or ascorbate) leads to a novel antioxidant interaction for reduction of ferrylmyoglobin by ascorbate in mixtures with chlorogenate.