Effective Excited State Deactivation of Flavins in Archeal Dodecin Proteins

Dodecin from Halobacterium salinarum is a flavin binding protein, able to incorporate two flavin molecules in each of its six identical binding pockets, building an aromatic tetrade with two tryptophan residues [1]. Its function in the cell is flavin homeostasis by preventing riboflavin from photodegradation via ultrafast depopulation of the reactive excited state [2].

Transient spectroscopy of dodecin with bound riboflavin shows, that two species with different lifetimes contribute to this ultrafast quenching mechanism. By systematic variation of the binding pocket, the molecular mechanism of the excited state deactivation can be studied. One possible mechanism is an ultrafast electron transfer from tryptophan to the photoexcited flavin, which was also observed for other flavin binding proteins [3]. Since flavins are known to undergo fluorescence self quenching at high concentrations [4], the influence of the second flavin was studied for dodecin with binding pockets where one flavin was exchanged by another aromatic moiety. By insertion of lumiflavin, the role of the ribityl chain can be examined. Proton transfer processes were studied by site-specific mutations and studies of the kinetic isotope effect on samples in deuterated solvent. Furthermore, the electron transfer mechanism in dodecin can be modulated by incorporating tryptophan derivates with varying ionization potentials; these investigations are currently in progress. These comparative studies allow to derive a detailed molecular picture of the functional photodynamics in archeal dodecin.