Oxidative damage to DNA by ionizing radiation, carcinogenic agents, and photosensitizers occurs predominately at guanine (G) bases, [1, 2] a result which can be rationalized by the hierarchy of in vitro oxidation potentials of the isolated nucleobases (G < A ( C,T). [3] Strand cleavage reactions, induced, for instance, by piperidine treatment, [4] have shown consistently that multiple guanine tracts in DNA are more susceptible to oxidative damage than isolated guanine bases.
further stirred for 5 min and directly analyzed by GC. The competitive hydroxylation of cyclohexane and [D 12 ]cyclohexane was performed with a mixture of cyclohexane (0.3 mmol) and [D 12 ]cyclohexane (1 mmol).
Labeled water (H 2
18 O) experiment: H 2 O 2 (0.04 mmol, diluted in CH 3 CN (0.5 mL)) was slowly added over a period of 1 h to a stirred solution containing [Fe(tpfpp)(NO 3 )] (1 Â 10 À3 mmol), substrate (1 mmol), and H 2 18 O (50 mL, 95% 18 O enriched) in a solvent mixture (1 mL) of CH 3 CN and CH 2 Cl 2 (3:1). The reaction mixture was further stirred for 5 min and directly analyzed by GC/MS. The 16 O and 18 O compositions in cyclohexanol and cyclohexene oxide were determined by the relative abundances of mass peaks at m/z 57 and 59 for cyclohexanol and at m/z 83 and 85 for cyclohexene oxide. Control reactions, performed by stirring cyclohexanol-16 O or cyclohexene oxide-16 O in a solution containing [Fe(tpfpp)(NO 3 )] and H 2 18 O, showed that the oxygen of the products did not exchange with labeled water under the reaction conditions. Electrochemical measurements: All electrochemical experiments were performed under an N 2 atmosphere in a glove box using a BAS 50W voltammetric analyzer. The cyclic voltammetric measurements were carried out in a solvent mixture of CH 3 CN/CH 2 Cl 2 (1:1) containing iron porphyrin (0.2 mm) and tBu 4 NPF 6 (40 mm) as a supporting electrolyte in one compartment. The working electrode was a glassy carbon disk and the counter electrode was a platinum wire. The potential was measured by using a Ag/Ag (0.01m) reference electrode and reported versus a Fc/Fc couple. The cyclic voltammograms were run at a scan rate of 50 mV s À1 .