The exposure of DNA to ionising radiation may directly induce oxidation via deprotonation or electron removal, again producing photolesions such as 8-oxoguanine [
96]. Hydroxyl radicals produced from water radiolysis can also disrupt the bonds in the sugar backbone of DNA, resulting in SSBs [
49,
97]. As ionising radiation is highly energetic, electrons ejected from radical formation could potentially cause further radiolysis of nearby water molecules, resulting in a high density of hydroxyl radicals [
95,
98], increasing the probability of SSB occurring close enough to each other (within 10 base pairs) to promote the formation of double-stranded breaks (DSBs) [
28,
99]. DSBs are potentially highly cytotoxic due to the risk of failed repair, such as in non-homologous end joining (NHEJ) or homologous recombination, resulting in gene mutations [
100,
101], clastogenic effects [
102], teratogenesis [
103] and carcinogenesis [
99].
Ionising radiation-induced water radiolysis can cause significant ROS-mediated damage to proteins through the disruption of peptide bonds, thereby altering their structure and function [
67,
104,
105]. This leads to similar outcomes to those already described in
Section 2 including both protein oxidation [
106] and lipid peroxidation [
107]. The direct impact of ionising radiation on proteins can be observed during X-ray diffraction studies of protein crystals, where cryogenic temperatures reduce the effects of radicals produced by the solvent [
108]. These studies demonstrate that di-sulphide bonds and carboxyl groups are most susceptible to localised radiation damage [
109,
110]. However, this damage may not be evenly distributed throughout the protein [
111]. For example, Weik et al. (2000) have shown that the specific disulphide bond between Cys-254 and Cys-265 residues for
Torpedo californica acetylcholinesterase, as well as the disulphide bond between Cys-6 and Cys-127 for hen egg white lysozyme, are most susceptible to radiation damage. Radiation damage may also localise at active sites in proteins [
110,
112,
113] such as for bacteriorhodopsin [
114], DNA photolyase [
115], malate dehydrogenases [
116], and carbonic anhydrase [
117]. This damage localisation has been hypothesised to be mediated either by the presence of metal ions, which have high proton numbers and hence more electrons for photo-absorption to propagate subsequent ionisation events [
118], or by the relative accessibility of exposed active sites to ROS [
110]. Key extracellular protein targets of ionising radiation are discussed in
Section 4 and
Section 5.