Spectroscopy and photophysics of 9-methylalloxazine. Experimental and theoretical study

Initial Excited State Dynamics of Lumichrome upon Ultraviolet Excitation

Lumichrome (LC) is the major photodegradation product of biologically important flavin cofactors. Since LC serves as a structural comparison to the flavins; understanding excited states of LC is fundamentally important to establish a connection with photophysics of different flavins, such as lumiflavin (LF), riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Herein, we deduce the initial excited state structural dynamics of LC using UV resonance Raman (UVRR) intensity analysis. The UVRR spectra at wavelengths across the 260 nm absorption band of LC were measured and resulting Raman excitation profiles and absorption spectrum were self consistently simulated using a time-dependent wave packet formalism to extract the initial excited state structural and solvent broadening parameters. These results are compared with those obtained for other flavins following UV excitations. We find that LC undergoes a very distinct instantaneous charge redistribution than flavins, which is attributed to the extended π-conjugation present in flavins but missing in LC. The homogeneous broadening linewidth of LC appears to be lower than that of LF, while the inhomogeneous broadening values are comparable, indicating greater solvent interaction with excited flavin on ultrafast timescale compared to LC, whereas on longer timescale these interactions are almost similar.

Spectroscopy and Photophysics of Monomethyl-Substituted Derivatives of 5-Deazaalloxazine and 10-Ethyl-5-Deaza-Isoalloxazine

Steady-state and time-resolved spectra were used to describe the singlet and triplet states of 8-methyl-5-deazaalloxazine (8-Me-5-DAll), 9-methyl-5-deazaalloxazine (9-Me-5-DAll) and 10-ethyl-5-deaza-isoalloxazine (10-Et-5-DIAll). Solvatochromic properties were described using different polarity scales, including Δf and the four-parameter scale proposed by Catalán. The results indicate that the Catalán scale shows a strong influence of solvent acidity (hydrogen-bond donating ability) on the emission properties of 8-Me-5-DAll and 9-Me-5-DAll. These results indicate the importance of intermolecular solute-solvent hydrogen-bonding interactions in the excited state of these compounds. Contrary to deazaalloxazines, solvent acidity affects the absorption spectra of 10-Et-5-DIAll. Fluorescence lifetimes and quantum yields and also transient absorption spectra were determined for all of the compounds studied. Electronic structure and S(0)-S(i), S(0)-T(i ), T(1)-T(i) transitions energies and oscillator strengths were calculated using the TD-DFT methods. Theoretical calculations were compared to experimental data.

Ground- and Excited-State Double Proton Transfer in Lumichrome/Acetic Acid System: Theoretical and Experimental Approach

Experimental time-resolved spectral and photon counting kinetic results confirm formation of an isoalloxazinic excited state via excited-state double proton transfer (ESDPT) catalyzed by a carboxylic acid molecule that forms a hydrogen-bond complex with the parent alloxazine molecule. This isoalloxazinic tautomer manifests itself as a distinct long-lived emissive species formed only in such alloxazine derivatives that were not substituted at the N1 nitrogen atom, being a product of the excited-state reaction occurring from the alloxazinic excited state. Theoretical calculations support the idea that the ESDPT occurs by the concerted mechanism. The calculated activation barrier in the excited state is much lower than the same barrier in the ground state and even disappears for the HOMO-1 to LUMO excitation, which explains the fact that the reaction takes place in the excited-state only. The reaction rate estimated from the emission kinetics is ca. 1.4 x 10(8) dm3 mol(-1) s(-1) in ethanolic solutions of lumichrome with added acetic acid.