UV action spectroscopy of protonated PAH derivatives

Aims. We investigate the production of molecular photofragments upon UV excitation of PAH derivatives, relevant for the interstellar medium. Methods. The action absorption spectra of protonated gas-phase methyl-substituted quinolines (CH3 − C9H7NH+) have been recorded in the 215–338 nm spectral range using the electrostatic storage ring ELISA, an electrospray ion source and 3 ns UV laser pulses. Results. It is shown that the absorption profile is both redshifted and broadened when moving the methyl group from the heterocycle containing nitrogen to the homoatomic ring. The absorption profiles are explained by TD-DFT calculations. The dissociation time of the studied molecules is found to be of several milliseconds at 230 nm and it is shown that after redistribution of the absorbed energy the molecules dissociate in several channels. The dissociation time found is an order of magnitude faster than the estimated IR relaxation time. Photophysical properties of both nitrogen containing and methyl-substituted PAHs are interesting in an astrophysical context in connection with identifying the aromatic component of the interstellar medium

Formation and stability of hydrogenated PAHs in the gas phase

Aims. We report on experimental and computational studies of hydrogenation of polycyclic aromatic hydrocarbon (PAH) cations, HnPAHs, which are relevant to the interstellar medium. Methods. The yield of the hydrogenated PAH cations produced in a plasma-ion source and by electrospray ionization was measured. DFT calculations at the B3LYP/6-311+G(d, p) and B3LYP/6-31+G(d, p) level of theory were performed to investigate the hydrogenation pattern. Results. A clear pattern in the yield and binding energies of hydrogen is revealed. Hydrogenated closed shell molecules with an even number of attached hydrogen atoms are significantly more stable than molecules with an odd number of hydrogen atoms and show as a consequence to be more abundant in mass spectra of HnPAHs. The binding energy of a hydrogen atom to Hn−1PAH with an even n is  ~2 eV higher than for odd n. The exact distribution in n observed in the experimental mass spectra remains to be solved due to the unknown internal ion source conditions. Conclusions. The HnPAH cations have been produced under very different conditions, and the measured yield indicates high stability and likely high abundance in the interstellar medium

Aquaporin homologues in plants and mammals transport ammonia

AbstractUsing functional complementation and a yeast mutant deficient in ammonium (NH4+) transport (Δmep1–3), three wheat (Triticum aestivum) TIP2 aquaporin homologues were isolated that restored the ability of the mutant to grow when 2 mM NH4+ was supplied as the sole nitrogen source. When expressed in Xenopus oocytes, TaTIP2;1 increased the uptake of NH4+ analogues methylammonium and formamide. Furthermore, expression of TaTIP2;1 increased acidification of the oocyte-bathing medium containing NH4+ in accordance with NH3 diffusion through the aquaporin. Homology modeling of TaTIP2;1 in combination with site directed mutagenesis suggested a new subgroup of NH3-transporting aquaporins here called aquaammoniaporins. Mammalian AQP8 sharing the aquaammoniaporin signature also complemented NH4+ transport deficiency in yeast

Sub-microsecond photodissociation pathways of gas phase adenosine 5′-monophosphate nucleotide ions

International audienceThe sub-microsecond dissociation pathways for the protonated and deprotonated forms of adenosine 5′-monophosphate were probed in the gas phase using a linear time of flight spectrometer. The studies show two dissociation pathways for the AMP ions indicating dominant ergodic pathways in the photodissociation of these species. The photofragmentation was determined to be a single photon process for the AMP ions. Photodetachment of the AMP anion excited at 266 nm was not observed, leaving dissociation as the prominent pathway for relaxation of the excess energy in the biomolecule. The photofragments were analysed at the electrostatic ion storage ring (ELISA) and found to be similar to collision induced fragments in the case of anions but different in the case of cations