SUBSTITUENT EFFECTS ON THE SPECTRAL, ACID-BASE, AND REDOX PROPERTIES OF INDOLYL RADICALS - A PULSE-RADIOLYSIS STUDY

Spectral and acid-base properties and reduction potentials of various substituted indolyl radicals were studied by pulse radiolysis in aqueous solutions at 20-degrees-C. Except for the 5-methoxyindolyl and 5-carboxyindolyl radicals, the spectra of the substituted indolyl radicals resemble the previously published 320- and 520-nm spectra of the neutral and 330- and 580-nm spectra of the cation indolyl and tryptophan radicals. The substitution of indolyl radical cation by electron-attracting groups (positive e) results in a blue shift of the 580-nm band by - 30 nm, whereas the spectra of methylindolyl (sigma+ = -0.31) are similar to those of unsubstituted indolyl radicals. The 430- and 455-nm bands appearing in the spectra of the 5-carboxyindolyl and 5-methoxyindolyl radical cations, respectively, indicate even stronger interaction of the unpaired electron with the 5-substituent. The radical cations of various indole-3-acetic acids decarboxylate at pH values below their pK(a) to produce allyl radicals. The 5-bromoindolyl radical undergoes solvolysis to 5-hydroxyindolyl radical in acidic and alkaline media. The dissociation constants and reduction potentials of the substituted indolyl radicals correlate with the Brown substituent constants: pK(r) = 4.14 - 2.13-SIGMA-sigma+, correlation coefficient 0.987, and E0/0.059 = 22.29 + 3.5-SIGMA-sigma+, correlation coefficient 0.980. The rho-values from these correlations (-2.13 and 3.5) are similar to that of the Hammett correlation of the dissociation constants of the protonated indole nitrogen in various substituted indoles, rho = -2.49, but smaller than the rho-value of the dependence on the substituent of the reduction potentials of phenoxyl radicals, rho = 5.4.