Photosensitization of DNA by β-carbolines: Kinetic analysis and photoproduct characterization

beta-Carbolines (beta Cs) are a group of alkaloids present in many plants and animals. It has been suggested that these alkaloids participate in a variety of significant photosensitized processes. Despite their well-established natural occurrence, the main biological role of these alkaloids and the mechanisms involved are, to date, poorly understood. In the present work, we examined the capability of three important beta Cs (norharmane, harmane and harmine) and two of its derivatives (N-methyl-norharmane and N-methyl-harmane) to induce DNA damage upon UV-A excitation, correlating the type and extent of the damage with the photophysical characteristics and DNA binding properties of the compounds. The results indicate that DNA damage is mostly mediated by a direct type-I photoreaction of the protonated beta Cs after non-intercalative electrostatic binding. Reactive oxygen species such as singlet oxygen and superoxide are not involved to a major extent, as indicated by the only small influence of D2O and of superoxide dismutase on damage generation. An analysis with repair enzymes revealed that oxidative purine modifications such as 8-oxo-7,8-dihydroguanine, sites of base loss and single-strand breaks (SSB) are generated by all beta Cs, while only photoexcited harmine gives rise to the formation of cyclobutane pyrimidine dimers as well.