Unravelling the nature of the α-keratin EPR signal: an ab initio study

Fingernail as a biodosimetry material, analyzed by the EPR technique, has attracted great attention in several experimental studies. One of the most challenging issues that should be addressed is additional signals, masking the radiation-induced signals (RIS) in EPR dosimetry analyses. In this work, we conducted a theoretical study of the RIS radicals and mechanisms to propose robust methods to distinguish the original signal from the irradiated nails' unwanted noise. Also, the proposed approach includes procedures to improve the accuracy of the dosimetry measurements. In our research, three categories of cysteine, DOPA and cystine radicals were considered due to their dominant abundance during the α-keratin reduction-oxidation processes. The SOMO-HOMO inversion is observed while investigating the electronic structure in these quasi-closed-shell systems. Furthermore, we demonstrated that the SOMO-HOMO gap is proportional to the spin localization. Indeed, new peaks in the EPR signals are not observed when the amino acid sequences are different. Moreover, the studied structures' neighborhood effect merely leads to a small change in the peak broadening of the EPR signals. On-the-fly magnetic parameter calculations were used to evaluate the system dynamics' effect on the broadening of the EPR signals in a molecular dynamics simulation. Comparing the calculated parameters with computational and experimental results in other studies helps assign low dose peaks to the corresponding tyrosyl phenoxyl radical.