Structure of transient radicals from cytostatic-active p-alkoxyphenols by continuous-flow EPR

Para-alkoxyphenols are of medical significance as futural cytostatic drugs in antimelanoma chemotherapy. They take part in a radical redox-reaction in which the catalytically essential protein-linked tyrosyl radical in the functional subunit R2 of the growth-regulating enzyme ribonucleotide reductase (RR) is quenched. EPR spectroscopy has been employed in conjunction with a continuous-flow system to study the structure of transient radicals from p-alkoxyphenols with different alkyl chain lengths. Radicals of p-alkoxyphenols were generated by oxidation in a Fenton system (Ti3+/H2O2, pH1) after rapid mixing in a novel continuous-flow EPR cavity designed especially for low consumption of substance. Hyperfine structures identified by spectral simulation show that the structure of transient radicals from oxidized para-alkoxyphenols (methyl-, ethyl-, alkyl-, propyl-,iso-propyl-, butyl-, iso-butyl-) belong to the type of phenoxyl radicals formed after abstraction of the OH proton. Hyperfine coupling constants are similar and vary only slightly with alkyl substituents.