| This dissertation presents a detailed investigation of the spectroscopic and kinetic properties of a number of short-lived radicals which are models for systems of biochemical relevance. Focus is on radicals produced in the oxidation and reduction of 4-hydroxy- and 4-amino-benzoic acids, isonicotinic acid and its amide and riboflavin. Experimental data on reaction rate constants and absorption and vibrational characteristics of the radicals are presented, and their formation and decay mechanisms are discussed.; The radicals were prepared under controlled conditions by electron pulse irradiation of appropriate chemical precursors in aqueous solutions. Time-resolved optical absorption and resonance Raman spectroscopic methods were used for their detection. Studies were performed in very acidic to highly basic solutions to examine the structural and kinetic behavior of intermediates in their different states of protonation.; The work on 4-hydroxybenzoic acid (a substrate of the enzyme 4-hydroxybenzoate hydroxylase) clarifies the mechanism of the base catalyzed water elimination from the OH-adducts of phenols (dihydroxycyclohexadienyl radicals). First, H{dollar}sp+{dollar} is lost by reaction with base, followed by intramolecular electron transfer and OH{dollar}sp-{dollar} elimination, and phenoxyl radical is formed. The OH{dollar}cdot{dollar}O{dollar}sp-cdot{dollar} radicals react with phenoxide ions by addition, and not by electron transfer. In a related study on 4-aminobenzoic acid, adduct radicals have been observed to decay into aniline cation (pKa {dollar}sim{dollar} 6.8), which deprotonates in basic solution to form anilino radical.; In the study of reduction of isonicotinic acid and its amide, the anionic form of the electron adduct in the latter system has been observed for the first time. Proton ionization involves amide N-H (pKa {dollar}sim{dollar} 13.5) and not ring N-H. It has been also found that the hydrogen atom bonds exclusively to the isonicotinamide ring N site if it is not protonated. In acidic solutions, where the ring N is protonated, {dollar}sim{dollar}29% of hydrogen atoms add to amide CO and rest to ring carbons. Thus, the protonation state of the pyridine ring affects the chemistry.; The resonance Raman studies of riboflavin (vitamin {dollar}rm{lcub}Bsb2{rcub}{dollar}) semiquinone radicals have provided, for the first time, reference vibrational frequencies for probing protein interactions. |
