Quenching and generation of singlet oxygen by hydroethidine and related chromophores

We have found that the oxidative probe hydroethidine (HE) is an efficient 1O2 quencher with kq = 1.1 × 109 M−1 s−1 in CH3CN. The structurally related MitoSOX Red had kq = 2.1 × 109 M−1 s−1 in D2O–CH3CN containing DNA. Ethidium cation, the main oxidation product, and three model compounds, 5-ethyl-5,6-dihydro-3,8-dinitro-6-phenyl-6-phenanthridinol, 3-amino-2-naphthenol, and 2-aminophenol, were lesser 1O2 quenchers. Another product, 2-OH-ethidium cation, quenched 1O2 with kq = 5.5 × 108 M−1 s−1 in a D2O–CH3CN mixture. Interestingly, both ethidium and 2-OH-ethidium cations are weak 1O2 photosensitizers with quantum yields of 1O2 production in CH3CN of 0.06 and 0.03, respectively. Thus, 1O2 can be involved in photochemical experiments with MitoSOX Red and HE.