Heme ferrous–hydroperoxo complexes: some theoretical considerations Radu Silaghi-Dumitrescu * Department of Chemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, GA 30602, USA Department of Chemistry, Babes-Bolyai University, Cluj-Napoca RO-3400, Romania Received 17 January 2004, and in revised form 17 February 2004 Abstract We report density functional calculations on complexes of ferrous hemes with hydroperoxide, where the axial ligand trans to OOH  is imidazole, thiolate, or phenoxide. The geometrical parameters and charge distributions within the Fe–O–O–H moiety are identical between the ferrous complexes reported here and their ferric counterparts previously described, even though the latter contain one unpaired electron on iron as opposed to the former, which are diamagnetic. The extra negative charge upon going from a formally ferric state to formally ferrous appears to be distributed essentially on the porphyrin. These findings support recent experimental data showing that the ferrous state of certain hemoproteins can interact with peroxides in a catalytically competent fashion, cleaving the O–O bond heterolytically in a manner reminiscent of the ‘‘canonical’’ ferric–peroxo complexes, and contrary to any expectations based on the Fenton concept commonly invoked in non-heme chemistry. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Heme; DFT; Peroxide; Peroxidase; Catalase; Alkylhydroperoxide reductase; Hemoprotein; Fenton; Free radical Ferric heme active sites of various proteins are known to react readily with peroxides, in a process that results in immediate heterolytic cleavage of the O–O bond within a ferric–hydroperoxo (or ferric–alkylperoxo) complex [1–3]. This reaction is accepted to define the physiological function of peroxidases [2,4] and catalases [3,5], while for other hemoproteins (e.g., cytochromes P450) [1], this reaction has been observed in vitro but is generally assumed to have little physiological relevance. The product of O–O bond cleavage is a species denoted Compound I, currently described as iron(IV)–oxo þ porphyrin cation radical [6]. One electron reduction of Compound I further yields Compound II, formally an iron(IV)–oxo species. One electron reduction of Com- pound II then yields the resting ferric state of the heme [1,4]. By contrast, the ferrous form of the above-mentioned active sites was proposed to be less reactive with per- oxides, and this reactivity was thought to involve liber- ation of hydroxyl radicals via homolytic cleavage of the O–O bond in Fe(II)–O–OH complexes—the so-called Fenton chemistry [7,8]. Protein damage has been re- ported to occur upon reaction of the ferrous form of hemoglobin with hydrogen peroxide and this damage has been associated with hydroxyl radicals [7]. It is still unclear as to what extent such protein damage can be attributed to hydroxyl radical-independent pathways— such as heterolytic cleavage of the O–O bond within a ferrous–hydroperoxo complex (yielding the high-valent intermediate Compound II). On the other hand, ferrous horseradish peroxidase was shown to induce heterolytic cleavage of the O–O bond in H 2 O 2 [9]. A similar (het- erolytic) behavior was reported for myeloperoxidase [10]. Likewise, under anaerobic conditions, ferrous fla- vohemoglobin (the active site of which features a histi- dine-ligated heme similar to that of canonical peroxidases) efficiently catalyzes heterolytic cleavage of the O–O bond in alkylperoxides, forming alcohols and Compound II [11]. With these previous results in mind, we have investigated the origin of the unexpected non-Fenton reactivity of these ferrous hemes towards * Fax: 1-706-542-9454. E-mail address: silaghi@chem.uga.edu. 0003-9861/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.abb.2004.02.017 Archives of Biochemistry and Biophysics 424 (2004) 137–140 ABB www.elsevier.com/locate/yabbi