Computational Study on Structural, Electron States, and Muon Trapping Sites in Guanine

Knowledge of muon trapping sites in matters is needed in the interpretation of Avoided Level Crossing-Muon Spin Resonance (ALC-µSR) experiment. Additionally, accurate computation on the trapped muon hyperfine interaction is instrumental in the planning of applied magnetic field strength for the experimental measurement. Density Functional Theory quantum mechanical method was employed to predict the most probable sites of muon addition in isolated guanine nucleobase. Utilizing B3LYP functional coupled with 6-311G++(d,p) basis set, the HOMO-LUMO gap was calculated to be 5.147 eV. The frontier molecular orbitals show significant delocalization characteristics. Molecular electronic potential map indicates that N2, O5, and N7 areas are favoured for alkylation. Out of eight possible muon addition sites, C8 was found to have the lowest total energy and therefore is the most probable location for the trapping of implanted muon. Natural Population Analysis reveals that the trapped muon is covalently bonded to the C8 atom with a bond order of 0.903. The calculated muon hyperfine coupling constant at the C8 site is 337.4 MHz. The applied magnetic field strength for ALC-µSR resonance to occur for C8 site is predicted to be around 14,000 Gauss. These findings provide valuable insights into muon behavior in nucleobases, contributing to a more precise interpretation of ALC-µSR experimental data and enhancing the design of future studies on muon interactions in biomolecular systems.