A Rigorous Theoretical Study of Electron Attachment in DNA: A Molecular Orbital Approach to Charge Transfer, Stability, and Radiation-Induced Effects

This research paper by Mirza Adnan Mohtashim and Aaryan K. Rathod presents a comprehensive theoretical study of how low-energy electrons attach to DNA and cause radiation-induced damage, using molecular orbital theory and advanced quantum chemical methods. The work addresses a paradigm shift in radiation biology that began with the groundbreaking experiments of Boudaffa, Sanche, and colleagues in 2000, who demonstrated that secondary electrons with energies below 20 eV can induce single and double strand breaks in DNA with surprising efficiency, fundamentally changing our understanding of how ionizing radiation damages genetic material. The paper develops rigorous mathematical expressions for vertical attachment energies and adiabatic electron affinities of DNA nucleobases, extending Koopmans' theorem to anionic systems and deriving corrections through electron propagator theory and post-Hartree-Fock methods including Møller-Plesset perturbation theory and coupled-cluster theory. The computational validations are performed at the CCSD(T)/CBS level, representing the gold standard in quantum chemical accuracy, achieving mean absolute deviations of only 0.04 eV from experimental vertical attachment energies and 0.03 eV from adiabatic electron affinities measured by electron transmission spectroscopy and laser photodetachment experiments. The stability hierarchy of DNA transient negative ions is established as thymine being most stable, followed by cytosine, guanine, and adenine, with calculated vertical attachment energies consistent with experimental observations. For density functional theory approaches, the paper emphasizes the critical importance of long-range corrected functionals such as LC-ωPBE, which properly reproduce the correct asymptotic behavior of the exchange-correlation potential decaying as minus one over r, unlike standard functionals like B3LYP that systematically underestimate electron affinities due to incorrect asymptotic behavior and self-interaction error. The comparison shows that LC-ωPBE achieves errors typically less than 0.05 eV compared to CCSD(T) reference values, while B3LYP shows errors of 0.2 to 0.4 eV. The charge transfer section formulates a generalized Marcus theory for DNA that accounts for the multi-site nature of the system and coupling to low-frequency vibrational modes of the double helix. Two primary mechanisms are identified: superexchange, where electrons tunnel through virtual states of bridging nucleobases with exponential distance dependence characterized by a decay constant beta approximately 0.6 to 1.0 per angstrom, and hopping, where electrons localize on intermediate bases with incoherent sequential stepping and weaker distance dependence. The crossover between these regimes occurs at approximately 3 to 4 nanometers, marking the transition from coherent tunneling to incoherent hopping. This has profound implications for how electrons migrate along DNA and where damage ultimately occurs. The stability of DNA transient anions is analyzed using the Hylleraas-Undheim-MacDonald theorem to establish rigorous bounds on electron affinity, providing mathematical certainty about the existence and stability of anionic states without solving the full many-electron problem. The resonance parameters reveal that thymine has the most stable transient anion with the longest lifetime of 7.7 femtoseconds, allowing sufficient time for dissociative processes to compete with autodetachment, which helps explain the observed preference for thymine in radiation-induced damage. The dissociative electron attachment section establishes non-adiabatic quantum dynamics models for strand break formation, coupling electronic and nuclear motions through the diabatic representation. The potential energy surfaces for the neutral and anion states of the C3 prime to O3 prime phosphodiester bond show a crossing point at approximately 1.85 angstroms corresponding to a dissociative electron attachment threshold of about 0.8 eV, in excellent agreement with experimental strand break cross-sections. The threshold behavior follows the Wigner threshold law, with cross-sections rising rapidly above threshold and peaking at 2 to 3 eV before decreasing at higher energies due to the declining secondary electron spectrum. The radiation-induced effects section connects microscopic cross-sections to macroscopic biological damage, deriving expressions for strand break yield per unit dose per base pair and analyzing how low linear energy transfer radiation such as X-rays produces sparse secondary electrons and isolated single-strand breaks, while high linear energy transfer radiation such as alpha particles produces dense clusters of ionization leading to complex double-strand breaks with multiple base lesions and cross-links that are significantly more difficult to repair and more likely to lead to mutations or cell death. The discussion highlights that the sequence dependence of electron attachment and charge transfer implies radiation damage is not randomly distributed but follows patterns reflecting the electronic structure of the DNA helix, with GC-rich regions preferentially capturing secondary electrons due to the higher electron affinities of guanine and cytosine, potentially explaining the radiosensitivity of gene promoter regions. The favorable energetics of electron transfer from guanine to thymine suggests a mechanism for long-range repair of oxidative damage where electrons migrate along the DNA helix to neutralize damage at distant sites. The conclusions emphasize that this theoretical framework bridges fundamental quantum chemistry with biological radiation effects, offering quantitative predictions for mutation hotspots, radiosensitivity patterns, and electron transfer repair mechanisms that could guide the development of more effective radioprotective agents and radiosensitizers for cancer therapy, as well as inform DNA nanotechnology design and astrobiology research regarding prebiotic chemistry and the search for life beyond Earth. Future directions include addressing explicit solvent effects, multireference correlation methods for stacked base pairs, incorporation of chromatin structure, development of machine learning potentials, and exploration of quantum computing applications for DNA electronic structure calculations.