The field of oscillating chemical reactions has grown dramatically over the last 15 years. The reasons for this activity are the intrinsic chemical interest in the behavior and mechanisms of oscillating chemical reactions and the connection between oscillating reactions and the properties of nonlinear differential equations.

We demonstrate the phenomenon of stochastic resonance in a nonlinear chemical reaction. The term “stochastic resonance” (SR) denotes the detection of a weak periodic signal in a noisy system displaying a threshold. If the sum of the periodic signal and the noise amplitude crosses the threshold, an output pulse is triggered. At an optimal noise amplitude the distribution of pulse intervals as well as the signal-to-noise ratio will pass through a maximum. In the continuously stirred tank reactor (CSTR) experiments we superimpose a periodic flow rate signal on an excitable focal steady state located near a Hopf bifurcation in the Belousov−Zhabotinsky (BZ) reaction. In the Showalter−Noyes−Bar-Eli model of this reaction we vary the perturbation frequency and amplitude as well as the pulse length of the applied noise to elaborate the optimal conditions for stochastic resonance to occur in the model.

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Mechanistic investigations of the photoinduced DNA damage by acridizinium salts (4a-azonia-anthracene derivatives) are presented. Irradiation of 9-bromoacridizinium in the presence of defined double- and single-stranded DNA oligomers under aerobic conditions leads to both frank strand breaks and alkali-labile sites as determined by polyacrylamide gel electrophoresis (PAGE). The extent of the DNA damage increases significantly in D(2)O and occurs selectively at guanosine residues. These observations reveal the formation of singlet oxygen ((1)O(2)) as reactive species, which oxidizes the DNA bases, above all the guanine bases. Further evidence for (1)O(2) formation was obtained from laser-flash spectroscopic investigations, which show intersystem crossing (S(1) to T(1)) of the excited states of the parent acridizinium and of the 9-bromo- and 9-amino-substituted derivatives. The resulting triplet state is efficiently quenched by oxygen (k(q) > 10(9) s(-)(1)M(-)(1)) to yield (1)O(2). Under anaerobic conditions, no significant alkali-labile lesions are observed, but frank strand breaks are induced; however, to lesser extent than under aerobic conditions. The DNA damage is suppressed in the presence of a radical scavenger, namely t-BuOH, and hydroxyl radicals are shown to be the reactive intermediates by trapping experiments with terephthalic acid. Moreover, the intercalated acridizinium molecules are not involved in the DNA damage reactions. The intercalated acridizinium salt leads to a primary PET reaction with the DNA bases; however, a fast BET transfer is proposed that regains the dye and the DNA, so that the excited intercalated dye does not contribute significantly to the overall DNA damage.