Biological applications of spin trapping

The results of y-radiolysis of aqueous solutions of thymine in the presence of MNP exemplify the unusual, but by no means unprecedented, occurrence of accumulation of spin adduct after reaction has ceased . Thus, when irradiation is discontinued, and the sample is examined spectroscopically, the signal attributed to the MNP adduct of [38] slowly increases in intensity (Joshi et al., 1978). The explanation advanced in this instance was that the spin adduct itself scavenges radicals very efficiently during radiolysis, giving a diamagnetic product which is thermally unstable, and which slowly cleaves, 

Work by Harbour, Chow and Bolton (1974) on the spin adducts of superoxide (or HOO )13 with nitrones paved the way for a number of investigations of superoxide and hydroperoxyl radical chemistry. Harbour and Bolton (1975) used DMPO to trap superoxide formed by spinach chloroplasts in the presence of 02. The signal strength was greatly enhanced when methylviologen was present, consistent with the hypothesis that this bis-pyridinium dication accepts an electron from the primary acceptor of photoprotein I, and then transfers it to molecular oxygen. 

Buettner and Oberley (1978) have drawn attention to difficulties encountered with DMPO which may be attributable to the presence of an impurity in the commercial material that renders it unstable. The presence of this in earlier work might possibly explain inconsistencies reported by Buettner and Oberley regarding the relative stabilities of (DMPO—HO ) and (DMPO—HOO ). 

Very recently, rate constants for scavenging of hydroxyl radicals by DMPO, and by the nitrone [18c], have been determined (Marriott et al., 1980) (see Table 5). As might be expected, the figures are close to the diffusion-controlled limit. The report of this work includes a concise and informative discussion of some of the difficulties with, and limitations of, the spin trapping method, especially where these relate to reactions involving hydroxyl radicals. 

Spin-trapping experiments connected with biological superoxide production have also been used to examine the biochemistry of anti-tumour antibiotics bleomycin (Sugiura and Kikuchi, 1978) and mitomycin C (Lown et al., 1978), as well as the effect of iron concentration on xanthine oxidase reactions (Buettner et al., 1978), and the photochemistry of melanins (Felix et al., 1978). 

---