Protonation and complexation of nucleic acid bases with metal ions have a profound influence on their structure and reactivity.1–3Although the interaction between carbony bases and the Li+ion has been studied by vibrational spectroscopy and ab initio calculations8–15, no experimental results are available for the complexation between this ion and carbonyl bases derived from nucleic acids. In this work, the interaction between 1,3-dimethyluracil (DMU)