Reaction of hydroxyl radicals with alkyl phosphates and the oxidation of phosphatoalkyl radicals by nitro compounds

The rate constants for reactions of hydroxyl radicals with a number of alkyl phosphates have been determined by competition with KSCN. Hydroxyl radicals react with alkyl phosphates preferentially by H-abstraction at the alpha-position of the phosphate functions. The resulting alpha-phosphatoalkyl radicals are not very efficient one-electron reducing agents towards nitro compounds. They react with tetranitromethane (TNM) by addition to form adduct intermediates with absorption maxima at about 300 nm. The rate constants for decay of these TNM adducts to produce the nitroform anion (NF-) and the corresponding alpha-phosphato-alcohols have been determined by optical and/or conductance detection. The stability of these TNM adducts varies considerably with the chain length (methyl > ethyl > isopropyl) and number (trialkyl > dialkyl > monoalkyl) of the alkyl substituents. Additional formation of proton during or after the decay of the TNM adducts has been tentatively attributed to the hydrolysis of the alpha-phosphato-alcohols. Alpha-Phosphatoalkyl radicals derived from trimethyl, triethyl, triisopropyl, and diethyl phosphates react with p-nitroacetophenone (PNAP) very slowly (k < 5 x 10(7) dm3mol-1S-1) possibly forming adducts. One-electron reduction of PNAP by these radicals to PNAP.- was not observed under pulse radiolysis conditions. The rate constants for the reactions of .OH with glycerol 1-phosphate and glycerol 2-phosphate have been redetermined by competition with KSCN. Using the radical scavengers N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and TNM, the percentage of .OH attack at each carbon atom was obtained. Contrary to the simple alkyl phosphates described above, the alpha-position to the phosphate function is the least favoured (10-15% in glycerol 1-phosphate and 6% in glycerol 2-phosphate). These so-formed alpha-phosphatoalkyl radicals react with TNM also by forming adducts. The beta-phosphatoalkyl radicals in both cases eliminate inorganic phosphate on formation (k > 10(6)S-1). The gamma-phosphatoalkyl radical from glycerol 1-phosphate undergoes base-catalysed water elimination (kobs = 1.8 x 10(5)S-1 at pH 10.6) to give an oxidizing radical. Products in the gamma-radiolysis of N2O-saturated solutions of glycerol 1-phosphate and glycerol 2-phosphate have been identified and their yields determined. The mechanisms for their formation are discussed.

Radiosensitivity and hydroxyl radical reactivity of phosphate esters as measured by radiation-induced dephosphorylation

The yields of inorganic phosphate from irradiated phosphate esters of biological interest have been measured in the presence of various free-radical scavengers. These studies indicate that hydroxyl radical attack on the phosphate esters accounts for most of this damage. Competition kinetics using different hydroxyl radical scavengers whose reactivities are known have been used to estimate the rate-constants for the overall hydroxyl radical reactivities of the phosphate esters. The extent of hydroxyl-radical-induced dephosphorylation is very low for nucleotide derivatives, but high for some sugar and glycerol phosphates, reflecting the probability of hydroxyl radical abstraction at the alpha or beta carbon atoms adjacent to the phosphate ester linkage. The hydroxyl radical reactivities of nucleotides, coenzymes, sugar phosphates and phospholipid components are all high (1-10 X 10(9) M-1 sec-1), indicating the importance of hydroxyl radical attack in the inactivation of these components in living cells, although not necessarily by dephosphorylation.