Effects of protonation and deprotonation on the excitation energies of lumiflavin

Two protonated (N(1) and N(5) position) and a deprotonated (N(3) position) lumiflavin species have been investigated by means of the combined density functional/multi-reference configuration interaction method (DFT/MRCI). Vertical excitation energies are presented for the vacuum and for aqueous solution simulated with a conductor-like screening model. Additionally, protonation energies for the ground state and the first excited singlet and triplet states are presented. While in the ground state the N(1) position is favorably protonated, protonation in the excited states is preferred in N(5) position.