title: Absorption and emission spectroscopic characterization of lumichrome in aqueous solutions.
abstract: The spectroscopic behavior of lumichrome (7,8-dimethyl-alloxazine, LC) in aqueous solutions in a pH range from -1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields, and lifetimes are determined. The ionization stage of ground-state LC changes with rising pH from the cationic form (LCH(2)(+)) to the neutral form (LCH) with a mid-point pH of pK(c) ≈ -0.53, and to the anionic form (LC(-)) with a mid-point pH of pK(a) ≈ 12.5. Above pH 7 a partial ground-state tautomerization of LCH to 7,8-dimethyl-isoalloxazine (IAH) occurs by N1-N10 intra-molecular proton transfer. For pH > pK(a) ≈ 12.5 LCH and IAH change to the anionic forms LC(-) and IA(-), and above pH 14 LC(-) tautomerizes completely to IA(-). In the excited state some neutral lumichrome (LCH*) converts to cationic lumichrome (LCH(2)(+)) at low pH by proton transfer from H(3)O(+) to LCH*. No photoinduced excited-state tautomerization of lumichrome was observed. LCH for pH > 3 and IAH are reasonably fluorescent. The fluorescence efficiencies of LC(-) and IA(-) are lower than those of LCH and IAH. The fluorescence of LCH(2)(+) is strongly quenched likely by intra-molecular diabatic charge transfer and excited-state relaxation by potential surface touching with the ground state.
authors.affiliation: Fakultät für Physik, Universität Regensburg, Regensburg, Germany.
authors.forename: Amit
authors.initials: A
authors.lastname: Tyagi
authors.forename: Alfons
authors.initials: A
authors.lastname: Penzkofer
mesh_terms: D000838:Anions / Q000378:metabolism; D001419:Bacteria; D002412:Cations / Q000378:metabolism; D005415:Flavins / Q000737:chemistry / Q000378:metabolism*; D005453:Fluorescence; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D015394:Molecular Structure; D055668:Photochemical Processes; D010782:Photolysis; D011522:Protons; D012996:Solutions / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013237:Stereoisomerism; D014867:Water
chemical_list: D000838:Anions; D002412:Cations; D005415:Flavins; D011522:Protons; D012996:Solutions; D014867:Water; C001559:7,8-dimethylalloxazine