Positron Binding and Annihilation Properties of Amino Acid Systems

Despite the fact that the positron annihilation has been used in biomedical applications, the detailed mechanism of the positron annihilation on biological molecules remains poorly understood so far. In this work, we investigated the positron binding and positron annihilation properties for both global minimum and hydrogen-bonded structures of 20 amino acid molecules using the multicomponent molecular orbital method. By regression analysis, we confirmed that positron affinity can increase with an increase of the permanent dipole moment of the parent amino acids as reported in previous studies, while the annihilation rate linearly increases with respect to the square root of positron affinity. By the one-particle property analyses for probabilities of electron-positron contacts, we found that delocalization characteristics of both electrons and positrons play key roles to enhance the positron annihilation rate arising from both the valence electrons in σ- and π-type molecular orbitals from 2p atomic orbitals but not from the highest occupied molecular orbital electrons, particularly for comparatively weakly bound positronic amino acid systems.