Iron—Oxygen Adducts

Addition of O2 to the Fe(II)—BLM complex produces an ESR-silent adduct which rapidly breaks down but is stabilized in the presence of DNA the Fe(II)BLM—O2 reaction thus results in considerable drug decomposition, unless DNA is present [64]. Spectral and kinetic studies distinguished three events in the Fe(II)—O2 reaction (a) formation of the short-lived, ESR silent species with ti/2 == 6 s at 2 C (b) the decay of this species to an activated complex, capable of cleaving DNA and (c) a slower decay of this second species to Fe(III)BLM, t 2 = 60 s at 2°C [70]. Upon addition of Fe(II) to a bleomycin—DNA mixture in air, a long-lived hn 45 min), ESR silent species is formed [71]. The formation of this species consumes one mole of oxygen and the complex eventually decomposes to Fe(III)BLM [71]. The initial Fe(II)02BLM species was shown initially to yield a 1 1 mixture of activated BLM and Fe(III)BLM, the reaction perhaps occurring by reduction of Fe(II)02—BLM by Fe(II)— BLM [72]  

The reduction potential of Fe(III)BLM is +129 12 mV (vs. NHE) and thus the Fe(II) form can be easily regenerated under biological conditions by reducing agents such as GSH or Oj [73]. The chemistry of activated bleomycin has been recently reviewed [111]. 

The Fe(II)02 BLM adduct formation is first order with respect to both [O2] and [Fe(II)BLM] [70]. Mossbauer studies confirm that the 

The final species in the above scheme contains the active oxygen (independent of exact electronic description). This analogy is reinforced by the similar chemical reactivity of the activated form of bleomycin and chloroperoxidase [77]. 

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One of the major diflSculties encountered in attempts to prepare monomeric iron-oxygen adducts, particularly of iron porphyrins, is in preventing the conversion of the adduct into the jM-oxo-dimer ... 

Hi. Zeolites exchanged with transition metal ions. In the first row, scandium-, titanium-, cobalt-, and nickel-exchanged zeolites have been the most studied. Cobalt-exchanged zeolites are discussed in Section IV,E since they lead to oxygen adducts on adsorption of oxygen. There are several cases where copper and particularly iron ions are found as impurity cations which affect the oxygen adsorption properties of the zeolite. 

In the mechanism of cytochrome P450, it is assumed that the high-valent iron-oxo species is formed from Fe "-i] -OOH, that may derive from the one-electron reduction of oxy adducts, [Fe-Oj] probably via an [Fe-tJ -02l It is well assumed that these types of iron-oxygen species are involved in the mechanisms of nonheme iron oxygenases. As an activated bleomycin A2 (BLM), formation of a low spin Fe "(BLM)-tj -OOH by the reaction of Fe BLM with O2 or by Fe BLM with H2O2 was shown kinetically and by As a synthetic... 

Reversible changes in the ultraviolet (UV)-visible and electron-spin resonance (ESR) spectra are observed as in Fig. 1 in toluene solution when oxygen or nitrogen gas is bubbled into the solution at room temperature. The orange solution is reversibly colored brownish-black in response to the partial pressure of oxygen. The uptake of oxygen was measured by volumetry and was equimolar to the cobalt iron ([02]/[Co] = 0.9 1). The oxygen adduct remained stable for a few days without irreversible oxidation. 

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