Induction of strand breaks in DNA films by low energy electrons and soft X-ray under nitrous oxide atmosphere

Abstract

Five-monolayer (5 ML) plasmid DNA films deposited on glass and tantalum substrates were exposed to Al K α X-rays of 1.5 keV under gaseous nitrous oxide (N 2O) at atmospheric pressure and temperature. Whereas the damage yields for DNA deposited on glass are due to soft X-rays, those arising from DNA on tantalum are due to both the interaction of low energy photoelectrons from the metal and X-rays. Then, the differences in the yields of damage on glass and tantalum substrates, essentially arises from interaction of essentially low-energy electrons (LEEs) with DNA molecules and the surrounding atmosphere. The G-values (i.e., the number of moles of product per Joule of energy absorbed) for DNA strand breaks induced by LEEs ( G LEE) and the lower limit of G-values for soft X-ray photons ( G XL) were calculated and the results compared to those from previous studies under atmospheric conditions and other ambient gases, such as N 2 and O 2. Under N 2O, the G-values for loss of supercoiled DNA are 103±15 nmol/J for X-rays, and 737±110 nmol/J for LEEs. Compared to corresponding values in an O 2 atmosphere, the effectiveness of X-rays to damage DNA in N 2O is less, but the G value for LEEs in N 2O is more than twice the corresponding value for an oxygenated environment. This result indicates a higher effectiveness for LEEs relative to N 2 and O 2 environments in causing SSB and DSB in an N 2O environment. Thus, the previously observed radiosensitization of cells by N 2O may not be only due to OH radicals but also to the reaction of LEE with N 2O molecules near DNA. The previous experiments with N 2 and O 2 and the present one demonstrate the possibility to investigate damage induced by LEEs to biomolecules under various types of surrounding atmospheres.