Stability of dry Phage Lambda DNA irradiated with swift heavy ions

Highlights

• Damages created in irradiated dry DNA by swift heavy ions were studied by electrophoresis.

• The inverse dependence between the portion of large DNA fragments and ion fluence is presented.

• Restriction analysis of irradiated DNA is informative for damage reveal.

Abstract

Effects of irradiations of Phage Lambda dry DNA samples at room and cryogenic temperatures with 158 MeV Xe and 48 MeV Ar ions were investigated. These ions primary transfer energy into the electronic subsystem of a target: dE/dx = S_e, S_e^Xe = 10.8 keV/nm and S_e^Ar = 3.5 keV/nm. Site-specific enzymes restriction endonucleases were applied to indicate DNA damages induced by these ions. Electrophoresis was applied to analyze the dependencies of the distributions of DNA fragments sizes on the ion fluence ranging from 10⁸ cm⁻² to 10¹⁰ cm⁻².

Electron paramagnetic resonance (EPR) technique was used to investigate damages of a dry DNA sample irradiated at the cryogenic temperature (140 K) with 158 MeV ¹³²₅₄Xe²⁶⁺ ions up to the fluence 8.6·10⁹cm⁻². A poorly resolved signal centered at the g-factor value for the free electron g ≈ g_e was detected, which probably results from a mixture of different-type radicals trapped in the DNA film. The total concentration of paramagnetic species in this sample was estimated to be 1.3 × 10¹⁹ spin/g.

Introduction

Radiation conditions of long distance space missions e.g. manned flights to the Moon, Mars and to the deep space as well as man's staying on the surfaces of celestial bodies considerably differ from the conditions of Earth-orbit flights. Effects of complex spectrum of solar and galactic cosmic rays and secondary radiations on the human body and its genetic apparatus are not well understood stimulating a fundamental interest in their investigations (see a review (Cortese et al., 2018) for details).

In particular, long-term stay in the deep space is associated with exposure to heavy charged particles. Passages of these particles through a cell cause DNA damage which, according to some authors, may cause cancer development (Cucinotta and Durante, 2006; Cucinotta et al., 2017). Numerous studies on cell cultures and experimental animals (Kiefer, 2002; Masumura et al., 2002) demonstrated that effects of radiation with such projectiles depend on parameters of charged ions, their energy and fluences (Turker et al., 2017).

Irradiations of biological materials with swift heavy ions (SHI) having masses M > 10m_p, m_p is the proton mass, and energies E > 1–10 MeV/nucl) can be applied for modeling of cells damage caused by heavy components of cosmic rays. Such ions transfer the most part of their energy (>95%) to the electron system of a target in the nanometric vicinities of their trajectories, realizing the Bragg peak of the electronic energy losses at extremely high values: S_e = dE/dx = 5–50 keV/nm (Komarov, 2017; Solov'yov, 2017). During subsequent 10 fs, electron cascades convert the initial excitation to a cloud of hot electrons and holes spreading up to 10–50 nm from the ion path. Relaxation of this cloud occurs within about 50–100 fs causing subsequent nanometric structural and chemical transformations of a target along the SHI trajectory. Mechanisms of the above mentioned kinetics are still under discussions in a swift heavy ions community.

A lack of knowledge about physical, chemical and biological processes starting at sub-picosecond temporal and ranging in the nanometric spatial scales forms the basic problem in the present understanding of effects of SHI irradiations of biological systems.

Dry DNA is a good model material for investigations of the effects of SHI interactions with biological substances (Terato et al., 2008). Dehydration considerably improves DNA radiation resistance. Absence of water radicals results in suppression of indirect radiation effects mediated by water which usually forms 80–90% of the all damages and largely determines inactivation of a cell by ionizing radiation (Korystov, 1992). This effect was demonstrated in a series of experiments compared sensitivity to irradiations of wet vs. dehydrated cells (Ionin et al., 2013), as well as dried vs. frozen and wet cells. Finally, application of natural DNA instead of, for example, pBR322 plasmid (a DNA plasmid with 4361 base pairs, (b.p.), and a molecular weight of 2.85 × 10⁶ Da) (Souici et al., 2017) in radiation experiments is more suitable for understanding of both mechanisms of radiation damage of cells and in vivo radiation effects. These reasons justify applications of dry DNA in model studies simulating the radiation effects under the conditions of long-term spaceflights and radiotherapy, e.g. space PHOENIX experiment at the International Space Station (ISS) (Karganov et al., 2017).

The presented paper is focused on development of experimental methods for investigation of the kinetics of damage produced by SHIs in Phage Lambda dry DNA samples.