An EPR/ENDOR study of aminoxyls (nitroxides) capable of intramolecular bonding: hydroxyalkyl radical spin adducts of nitrones

A comparison of the effect of solvent and 13C labeling (of the radical addend) on the EPR (electron paramagnetic resonance) spectra of hydroxyalkyl vs. alkyl radical adducts of α-phenyl-N-tert-butyl nitrone (PBN) and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) has been investigated. The solution ENDOR spectra in toluene and in ethanol are the first examples studied of aminoxyls with hydroxyl substituents in close proximity to the free radical centre. Diastereomeric mixtures of hydroxyalkyl radical adducts are clearly resolved by ENDOR spectroscopy. Conformations of these radicals have been assigned based on the 1H and 13C hyperfine splittings (HFS's) and by differential optimum ENDOR temperatures. Definitive assignments of carbon-centered radical adducts of DMPO and PBN are shown to be feasible by monitoring the β-13C HFS's of the radical addend. Long-range γ-H HFS's from the added radical group can be observed when the deuterated spin trap PBN-d14 is used. The production of 13C labeled hydroxyalkyl adducts of nitrones (e.g., DMPO–13CH2OH, from the reaction of hydroxyl radicals with added 13CH3OH) is shown to be useful as an improved EPR spectroscopic method for the verification of the presence of hydroxyl radicals.

Photochemistry of cyclic α-hydroxy ketones. I. The nature and genesis of the photoproducts from steroidal 5-hydroxy 6-keto steroids and related compounds

Photolyses of 5-hydroxy-5α- and 5β-cholestan-6-one and their 3β-acetoxy- and 3β-benzyloxy derivatives in benzene or ethanol proceed stereospecifically with retention of configuration at C-5 to give the corresponding lactones, 6-oxa-B-homocholestan-7-ones. Photolyses of 3β-acetoxy-7α-deutero-5-hydroxy-5α- and 5β-cholestan-6-ones also proceed stereospecifically to give the corresponding 5-deutero lactones. 3β-Acetoxy-5-deuteroxy-5α- and 5β-cholestan-6-one give on irradiation 1:3 and 7:1 mixtures, respectively, of the corresponding 7aα- and 7aβ-deutero lactones. Irradiation of 3β-acetoxy-5-methoxy-5α-cholestan-6-one in ethanol leads to the stereoselective formation of ethyl 3β-acetoxy-5-methoxy-5,6-seco-5α-cholestan-6-oate, while that of 3β-acetoxy-5α-cholestan-6-one gives mainly photoreduction products. These observations are interpreted in terms of α-cleavage of the C-5—C-6 bond of the ketols to give alkyl acyl diradicals that undergo hydrogen transfer to give hydroxy ketenes, which then form the lactones. It is proposed that retention of configuration at C-5 results from two major factors—the nonplanar geometry of the hydroxyalkyl radical center, and fast hydrogen transfer in the diradical, the latter resulting from restricted rotation about the C-9—C-10 bond. The specific transfer of the 7α-deuterium atom in the 7α-deutero ketols is attributed to these factors and to the preferred direction of opening to the diradical on α-cleavage. The O-deuterium labelling results are interpreted in terms of product development control in the conversion of the hydroxy ketenes to the lactones and are in accord with restricted rotation about the C-9—C-10 bond. The photolysis of 3β-acetoxy-5-amino-5α-cholestan-6-one proceeds stereoselectively to give the 5β-lactam analogue of the 5β-lactone formed from the analogous 5α-ketol.

Formation and Structure of Radicals from ᴅ-Ribose and 2-Deoxy- ᴅ-ribose by Reactions with SO4·̅ Radicals in Aqueous Solution. An in-situ Electron Spin Resonance Study

ESR spectroscopy has been used to analyse the conformation of the radicals produced by the reaction of SO4·̅ with ᴅ-ribose (1), and 2-deoxy-ᴅ-ribose (6), at pH 1.3-5. From ribose three different types of radicals formed by H abstraction at C-1, C-2 and C-3 followed by a regio-selective α,β-water elimination have been identified: the 2-deoxy-ribonolacton-2-yl (3), the 1-deoxy-pentopyranos-2-ulos-1-yl (4). and the 4-deoxy-pentopyranos-3-ulos-4-yl (2). Using deoxyribose two radicals of similar type, formed by H abstraction at C-3 and C-4 followed by water elimination, have been observed: the 2,4-dideoxy-3-ulos-4-yl (7) and the 2,3-dideoxy-4-ulos-3-yl (8). In addition, from both sugars an a-hydroxyalkyl radical has been identified based in part on the timing of their conformational motions: the ribos-3-yl (5) (the precursor of 2) and the 2-deoxy-ribos-1-yl (9), respectively. For radical 5 the rate constant k(e) for the water elimination and hence transformation into radical 2 was estimated. From the analysis of selective line broadening the frequencies of conformational changes of radicals 2 and 7 have been estimated. For 7 the frequencies of exchange of the two methylene groups were found to differ by more than 3 orders of magnitude.

Elimination of Ammonium Ion from the a-Hydroxyalkyl Radicals of Serine and Threonine in Aqueous Solution and the Difference in the Reaction Mechanism

The zwitterionic radicals HO-ĊH-CH(COO-)NH3+ (4a) and HO-Ċ(CH3)-CH(COO-)NH3+ (4b) are the main species produced upon OH· radical attack in aqueous solutions at pH 3-7 at the amino acids serine, HO-CH2-CH(COO-)NH3+, or threonine, HO-CH(CH3)-CH(COO-)NH3+, respectively. Both radicals undergo elimination of NH4+ ion to form the radicals O=CH-ĊH-COO- (7) or CH3-CO-ĊH-COO- (9) respectively.The pKa of the serine-derived cationic radical HO-ĊH-CH(COOH)NH3+ (3a) (3a ⇄ 4a + H+), was determined by ESR spectroscopy to 2.2 ± 0.1 at 276 K. From kinetic data the pKa(OH) of radical 4a (4a ⇄ O-ĊH-CH(COO-)NH3+ (5a) + H+) was calculated to 7.0. The elimination of NH3 takes place from the ketyl radical 5a (type-B mechanism), the rate constant was calculated from kinetic data to 2.4 × 106 s-1 at 290 K.The half-lives of radicals 4a and 4b were measured by time-resolved conductivity changes upon pulse radiolysis, 170 ± 10 μs for 4a and 26 ± 2 μs for 4b, at 290 K and pH 5.8 .With the threonine derived radicals elimination of NH3 takes place at the stage of the α-hydroxyalkyl radical 4b (type-A mechanism). In this series the pKa of the product radical CH3-CO-ĊH-COOH (8) (8 ⇄ 9 + H+), was determined by ERS spectroscopy to 2.7 ± 0.1. The reasons for the observed mechanistic differences (type-A versus type-B decay) are discussed. As further examples for a type-B decay some preliminary data on the elimination of HF from the radicals CF3-Ċ(OH)-CF3 and CF3-ĊH-OH have been added.

The oxidation of oxyhaemoglobin by glyceraldehyde and other simple monosaccharides

Glyceraldehyde and other simple monosaccharides oxidize oxyhaemoglobin to methaemoglobin in phosphate buffer at pH 7.4 and 37 degrees C, with the concomitant production of H2O2 and an alpha-oxo aldehyde derivative of the monosaccharide. Simple monosaccharides also reduce methaemoglobin to ferrohaemichromes (non-intact haemoglobin) at pH 7.4 and 37 degrees C. Carbonmonoxyhaemoglobin is unreactive towards oxidation by autoxidizing glyceraldehyde. Free-radical production from autoxidizing monosaccharides with haemoglobins was observed by the e.s.r. technique of spin trapping with the spin trap 5,5-dimethyl-l-pyrroline N-oxide. Hydroxyl and l-hydroxyalkyl radical production observed from monosaccharide autoxidation was quenched in the presence of oxyhaemoglobin and methaemoglobin. The haemoglobins appear to quench the free radicals by reaction with the free radicals and/or the ene-diol precursor of the free radical.