Spectral properties and absolute rate constants for beta-scission of ring-substituted cumyloxyl radicals. A laser flash photolysis study

A laser flash photolysis study of the spectral properties and beta-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after beta-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and sigma(+) substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H(2)O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for beta-scission of the cumyloxyl radicals, which varies between 7.1 x 10(5) and 1.1 x 10(6) s(-1), a result that suggests that cumyloxyl radical beta-scission is not governed by the stability of the resulting acetophenone. Finally, k(beta) increases on going from MeCN to the more polar MeCN/H(2)O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for beta-scission through increased solvation of the incipient acetophenone product.

Photo- and radiation-chemical formation and electrophilic and electron transfer reactivities of enolether radical cations in aqueous solution

In aqueous solution, enolether radical cations (EE.+) were generated by photoionization (lambda < or = 222 nm) or by electron transfer to radiation-chemically produced oxidizing radicals. Like other radical cations, the EE.+ exhibit electrophilic reactivity with respect to nucleophiles such as water or phosphate as well as electron transfer reactivity, for example, towards one-electron reductants such as phenols, amines, vitamins C and E, and guanine nucleosides. The reactivity of these electron donors with the radical cation of cis-1,2-dimethoxyethene.+ (DME.+) can be described by the Marcus equation with the reorganization energy lambda = 16.5 kcalmol(-1). By equilibrating DME.+ with the redox standard 1,2,4-trimethoxybenzene, the reduction potential of DME.+ is determined to be 1.08 +/- 0.02 V/NHE. The oxidizing power of the radical cation of 2,3-dihydrofuran, which can be considered a model for the enolether formed on strand breakage of DNA, is estimated to be in the range 1.27-1.44 V/NHE.

Is "frank" DNA-strand breakage via the guanine radical thermodynamically and sterically possible?

Using the reduction potential of one-electron oxidized guanosine in water and the pKa values of the radical and of the parent, the N1-H bond energy of the 2'-deoxyguanosine moiety is determined to be (94.3+/-0.5) kcal mol(-1). Using the DFT method, the energy of the N1-centered guanosine radical is calculated and compared with those of the C1'- and C4'-radicals formed by H-abstraction from the 2'-deoxyribose moiety of the molecule. The result is that these deoxyribose-centered radicals appear to be more stable than the N1-centered one by up to 3 kcalmol(-1). Therefore, H-abstraction from a 2'-deoxyribose C-H bond by an isolated guanosine radical should be thermodynamically feasible. However, if the stabilization of a guanine radical by intrastrand pi-pi interaction with adjacent guanines and the likely lowering of the oxidation potential of guanine by interstrand proton transfer to the complementary cytosine base are taken into account, there is no more thermodynamic driving force for H-abstraction from a deoxyribose unit. As a further criterion for judging the probability of occurrence of such a reaction in DNA, the stereochemical situation that a DNA-guanosine radical faces was investigated utilizing X-ray data for relevant model oligonucleotides. The result is that the closest H-atoms from the neighboring 2'-deoxyribose units are at distances too large for efficient reaction. As a consequence, H-abstraction from 2'-deoxyribose by the DNA guanine radical leading subsequently to a "frank" DNA strand break is very unlikely. The competing reaction of the guanine radical cation with a water molecule which eventually yields 8-oxo-2'-deoxyguanosine (leading to "alkali-inducible" strand breaks) has thus a chance to proceed.