Femtoradical events in aqueous molecular environments: the tenuous borderline between direct and indirect radiation damages

The complex links existing between radiation physics and radiobiology concern the complete understanding of spatio-temporal events triggered by an initial energy deposition in confined spaces called spurs. Microscopic radiation effects (photons or relativistic particles) on integrated biological targets such as water 'the solvent of life' and biomolecular architectures (DNA, histones, enzymes) cannot be satisfactorily described from an absorbed dose delivery profile or a linear energy transfer (LET) approach. Primary radiation damages on biological targets being dependent on the survival probability of secondary electrons and short-lived radicals inside nascent nanometric clusters of ionisation, a thorough knowledge of these processes require the real-time probing of early events on sub-micrometric scale, in the temporal range 10^-15 - 10^-10 s. Major strides concern early water damages: primary water cation formation (H₂O^•+ or positive hole), concerted electron-proton couplings, attachment dynamics of p-like excited prehydrated electron on biomolecule, short-lived radical pairs involving water-bridged radical OH^• and hydronium ion H₃O⁺. The deactivation frequency of electron-radical pairs is comparable to an H-OH deactivation of excited water molecules (v_H2O^* ~ 0.33 × 10¹³ s^-1). These short-lived events take place in the prethermal regime of delocalized secondary electrons and represent a tenuous borderline between direct and indirect molecular damages.