Teaching and scientific research.
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Research↗
Biomedical subjects
M C Symons
Publications and source records attributed to M C Symons.
Sulphur radicals formed by cutting alpha-keratin.
Finger nails are composed largely of alpha-keratin, a protein in which three right-handed alpha-helical peptide chains are twisted into a left-handed coil strengthened by disulphide cross links. We have found that the act of cutting nails generates trapped radicals, giving intense electron spin resonance (ESR) signals characteristic of sulphur centred radicals. The nature of the primary radical and the possible modes of generation are discussed. These radical reactions should be considered when using human nail parings to estimate accidental exposure to ionizing radiation.
Disulfides↗
Structure and mobility of electron gain and loss centres in proteins.
Radiation damage to proteins is a topic of intense interest to those involved in radiation effects in biology, and also to those involved in radiotherapy. Although it has been widely studied, fundamental processes in protein damage are very hard to specify because of the complexity of the final damage products. But the non-invasive technique of electron-spin resonance is ideally suited to the task of detecting and identifying the primary and secondary products as these are expected to contains unpaired electrons (that is, free-radicals) and such species are uniquely detected by this sensitive form of spectroscopy. Our present study shows that a major radical species formed by electron loss in a range of proteins is the backbone amido radical, -N.(CO)-, characterized by hyperfine coupling to one 14N nucleus. These centres are efficiently trapped in proteins at low temperatures. In contrast, the expected backbone electron-capture centres, -NH(CO.-)-, are not readily trapped and electron transfer occurs until the ejected electron is trapped by some electrophilic centre. Such electron mobility was in fact established in our previous work on oxyhaemoglobin (FeO2----FeO2-), superoxide dismutase (Cu(II)----Cu(I] haemocyanin (Cu(II)O2Cu(I)----Cu(I)O2Cu(II] and various proteins containing S-S bonds (-S-S-)----(-S.-S-) (refs 1-4 respectively). This is strongly supported by our observation that such electrons are captured by DNA molecules, giving T.- centres, when nucleohistones are irradiated, and that Fe(CN)3-(6) ions readily scavenge such electrons from proteins which are devoid of highly electrophilic centres.
Electron Spin Resonance Spectroscopy↗
Electron transfer from protein to DNA in irradiated chromatin.
Irradiation of dry or fully hydrated frozen DNA systems (conditions of direct damage) has been shown by electron-spin resonance spectroscopy to give rise to electron-gain centres localized on thymine (T.-) and electron loss centres ('holes') localized on guanine (G.+) with approximately equal yields. Our parallel studies on the development of both single- and double-strand breaks under comparable conditions provide good evidence that these radical centres are the precursors to such damage, and we and others have argued that this may be of relevance to the damage pathways in vivo. We now report evidence that when DNA is complexed to proteins as it is in the nuclei of eukaryotes, electron transfer from the histone to DNA is facile, leading to a significant increase in the yield of electron-gain centres in DNA as judged from their electron-spin resonance spectra. In contrast 'holes' generated in the protein are trapped and do not lead to any detectable increase in the yields of G.+.
Animals↗