Protomer-Dependent Electronic Spectroscopy and Photochemistry of the Model Flavin Chromophore Alloxazine

Flavin chromophores play key roles in a wide range of photoactive proteins, but key questions exist in relation to their fundamental spectroscopic and photochemical properties. In this work, we report the first gas-phase spectroscopy study of protonated alloxazine (AL∙H+), a model flavin chromophore. Laser photodissociation is employed across a wide range (2.34–5.64 eV) to obtain the electronic spectrum and characterize the photofragmentation pathways. By comparison to TDDFT quantum chemical calculations, the spectrum is assigned to two AL∙H+ protomers; an N5 (dominant) and O4 (minor) form. The protomers have distinctly different spectral profiles in the region above 4.8 eV due to the presence of a strong electronic transition for the O4 protomer corresponding to an electron-density shift from the benzene to uracil moiety. AL∙H+ photoexcitation leads to fragmentation via loss of HCN and HNCO (along with small molecules such as CO2 and H2O), but the photofragmentation patterns differ dramatically from those observed upon collision excitation of the ground electronic state. This reveals that fragmentation is occurring during the excited state lifetime. Finally, our results show that the N5 protomer is associated primarily with HNCO loss while the O4 protomer is associated with HCN loss, indicating that the ring-opening dynamics are dependent on the location of protonation in the ground-state molecule.

ENDOR Spectra of Pyrene Excited Triplet State in Naphthalene-TCNB Crystals

Triplet state traps, observed on optically excited single crystals of Naphthalene-TCNB doped with Pyrene, have been studied by EPR and ENDOR spectroscopy. The magnetic resonance lines recorded at low temperature (20 K) are shown to be due to triplet excitations localized almost completely on a Pyrene molecule. The complete hyperfine tensors of all the protons have been determined. The spin distribution is shown to be distorted in such a way that the triplet Pyrene molecule lacks the in-plane symmetry axes of the ground state molecule. The orientation of the triplet molecule in the crystal lattice and the relative orientation of Pyrene and TCNB are also discussed.

Uv Multiphoton Induced Chemistry of Nitrobenzene in Solution

The technique of Multiphoton Induced Chemistry (MPIC) has been employed to initiate ion-molecule chemistry of organic molecules in solution. We report one of the first examples of the use of liquid phase multiphoton ionization (MPI) to prepare organic cations, which then react with the solvent in ionmolecule processes. The products obtained in this chemical sequence are significantly different from those observed in conventional or multiphoton-induced neutral chemistry in the same solvent. The particular example explored in this work is the reactivity of the nitrobenzene cation in methanol solvent. Products of the ion-molecule chemistry, detected by gas chromatography/mass spectrometry, are phenol and benzyl alcohol. These products depend upon the square of the laser intensity. It is shown by ionization current measurements in a conductance cell, that ionic species are produced as precursors to the observed products. The implications of this application of MPI are briefly discussed. A preliminary report on the unimolecular chemistry of the highly excited neutral molecule is also included. The product of this channel is nitrosobenzene. It is shown, in this case, that the reactive state is most likely a highly vibrationally excited ground state molecule, not the lowest triplet level invoked in conventional photochemistry.

Photochemical studies of unimolecular processes I. The photoisomerization of cycloheptatriene and cycloheptatriene-d 8

The photochemical isomerization of cyclo [1. 3. 5] heptatriene to toluene was studied at seven wavelengths between 228.8 and 313.0 nm. It was shown to occur via a vibrationally excited ground state molecule, the isomerization being quenched by added gases (the parent molecule, toluene, SF 6 , CO 2 , He) according to the Stern-Volmer law. The perdeuteriated compound was also studied.