A. A. Sosnovskaya

Quinones as free-radical fragmentation inhibitors in biologically important molecules

Effects of a number of quinones and diphenols of various structures on free-radical fragmentation processes taking place in f -diols, glycerol, 2-aminoethanol, glycero-1-phosphate, ethylene glycol monobutyrate, maltose, and some lipids were investigated. Quinone additions have been found to change the direction of free-radical transformations of the compounds cited above by inhibiting formation of the respective fragmentation products owing to oxidation of radicals of the starting compounds. The results obtained and literature data available allow a suggestion to be made that the system quinone/diphenol is able to not only deactivate or generate such active species as O 2 ” m but also control the realization probability of free-radical processes of peroxidation and fragmentation in biologically important molecules.

C-N bond cleavage reactions on the radiolysis of amino-containing organic compounds and their derivatives in aqueous solutions.

Purpose : To investigate regularities observed in the course of reactions involving cleavage of C-N bonds on the radiolysis of amino-containing organic compounds and their derivatives in aqueous solutions. Materials and methods : Aqueous solutions of amino-containing compounds, saturated with Ar or O 2, were exposed to γ-radiation from a 137 Cs source. The absorbed dose-rate was 0.33 Gy s −1, the absorbed dose range was 0.1-7.5 kGy. Analyses for ammonia and amino-containing organic compounds were performed using an amino acid analyser, and analyses for other radiolysis products were made by gas-liquid chromatography. Results : The presence of a hydroxyl group in the β-position to the amino group promotes deamination of amino alcohols, amino acids, di- and tripeptides, these processes being associated with the possible occurrence of monomolecular fragmentation of the respective radicals. Oxygen inhibits the deamination of α, β-amino alcohols and hydroxyl-containing amino acids. The presence of alkyl substituents in the amino group favours C-N bond cleavage on the radiolysis of amino alcohols, whereas the presence of an acyl group sharply suppresses this process. Replacement of the hydroxyl group in molecules of the initial amino alcohols by a hydrogen atom (i.e. a changeover to alkylamines) or by a methoxy group decreases the efficiency of deamination sharply. The presence of hydroxyl groups in side residues of di- and tripeptides promotes destruction of the peptide chain with formation of amides of amino acids, while suppression of deamination and main chain destruction processes in these compounds is observed in the presence of oxygen. Conclusions : The most effective processes leading to the C-N bond cleavage on the radiolysis of aqueous solutions of amino-containing organic compounds are those in which the key role is played by reactions of monomolecular decomposition of radicals of the initial compounds.

Effects of various vitamins and coenzymes Q on reactions involving α-hydroxyl-containing radicals

Effects of vitamins B, C, E, K and P, as well as coenzymes Q, on formation of final products of radiation-induced free-radical transformations of ethanol, ethylene glycol, α-methylglycoside and glucose in aqueous solutions were studied. Based on the obtained results, it can be concluded that there are substances among vitamins and coenzymes that effectively interact with α-hydroxyl-containing radicals. In the presence of these substances, recombination reactions of α-hydroxyalkyl radicals and fragmentation of α-hydroxy-β-substituted organic radicals are suppressed. It has been established that the observed effects are due to the ability of the vitamins and coenzymes under study to either oxidize α-hydroxyl-containing radicals yielding the respective carbonyl compounds or reduce them into the initial molecules.

Radiation-induced transformations of threonine and its derivatives in aqueous solutions

It was found that the presence of the α,β-aminoalcohol group in the molecules of the hydroxyl-containing amino acid threonine and its derivatives is responsible for their ability to undergo free-radical degradation with C-C bond cleavage. This degradation occurs by the fragmentation of the nitrogen- and carbon-centered radicals of the initial substances.

Effect of phenolic compounds and quinones on radiation-induced oxidation processes of hexane and ethanol

The radiation-chemical yields were determined for the products of γ-radiation induced oxidation of hexane and ethanol in the presence of natural and synthetic phenolic compounds and quinones. It was found that the commercial antioxidants ionol and bisphenol 2246 more effectively inhibit the hexane oxidation process while α-tocopherol and quinones are superior to other test compounds in the suppression of ethanol oxidation. The results indicate that the antioxidant properties of phenolic compounds and quinones depend not only on their structure but also on the nature of the oxidation substrate. The cause of this phenomenon is discussed.

Deamination and degradation of hydroxyl-containing dipeptides and tripeptides in the radiolysis of their aqueous solutions

The effect of the presence of hydroxyl groups at the side chains of di- and tripeptides on the radiation-initiated deamination and main-chain cleavage was examined. It was found that the presence of hydroxyl groups is favorable to peptide main-chain degradation with the formation of amino acid amides. The deamination was intensified or inhibited depending on the positions of OH groups in the molecules of test compounds. As a rule, both deamination and main-chain degradation processes in oxygenated solutions of di- and tripeptides were inhibited because of oxidation of hydroxyl-containing groups in the starting substances.

Reactions of aminyl radicals during radiolysis and photolysis of aqueous solutions of amino alcohols and their derivatives

It has been found that the radiolysis of the aqueous solutions of α,β-amino alcohols leads to the formation of degradation products of the parent substances. The experimental data suggest that the degradation process includes the stage of the formation of aminyl radicals, which undergo decomposition with the simultaneous cleavage of -C-C- and -O-H bonds through a five-membered transition state. The radiation-induced degradation of amino alcohols is enhanced in an alkaline medium, in which the amino group is deprotonated, and is blocked via the etherification of the hydroxyl group in the parent substances or the introduction of reducing agents.

γ- and UV-radiation-induced degradation of sphingomyelin, lysosphingomyelin, and related compounds

The experimental data on the formation of final products in the radiolysis of 2-aminoglycerol and lysosphingomyelin (sphingosine phosphocholine) and in the photolysis of N-(2-hydroxypropyl)hexanamide and sphingomyelin suggest the occurrence of the radiation-induced degradation of the above substances with C-C bond cleavage. It was hypothesized that this process occurs by the formation and subsequent degradation of the N-centered radicals of the substrates.

Radiation-induced degradation of hydroxyl-containing amino acids in aqueous solutions

The set of data presented in this work shows that the radiolysis of hydroxyl-containing amino acids in aqueous solutions results in their degradation with -C-C- bond cleavage, which leads to both the decarboxylation of parent molecules and the removal of side substituents. The probability of these reactions mainly depends on the pH of the solution. Aminyl radicals or aminium radical cations, which are formed in the interaction of serine and threonine with ·OH radicals via the mechanism of electron or hydrogen-atom transfer from the amino groups of the parent substances, play an important role in these degradation processes.