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Ionization of N(2)O(4) in and on thin water films on surfaces is believed to be a key step in the hydrolysis of NO(2) which generates HONO, a significant precursor to the OH free radical in the lower atmosphere. Molecular dynamics simulations using "on the fly" high-level MP2 potentials are carried out for ONONO(2) x (H(2)O)(n) clusters, n < or = 8, used to mimic the surface reaction, in order to investigate the ionization process and determine its time-scale and mechanism around room temperature. The results are (i) the isolated molecule does not convert to the NO(+)NO(3)(-) ion pair, even for long times; (ii) for ONONO(2) x (H(2)O)(n) with n = 1 and 2, ionization takes place in several picoseconds; (iii) for n > or = 3, ionization is essentially immediate, implying that the neutral species does not have sufficient lifetime to be considered a significant intermediate in the reaction; and (iv) even at ice temperatures, T < or = 250 K, ionization for n > or = 3 is immediate. The implications for hydrolysis of oxides of nitrogen on surfaces in the atmosphere are discussed.

Infrared (IR) spectroscopy is used for identifying a novel noble-gas compound, HXeOXeH, which seems to be the smallest known neutral molecule with two noble-gas atoms. HXeOXeH has represented a first step toward the preparation of [(Xe-O).sub.] chains and it might be relevant to the terrestrial missing xenon problem.

Ab initio vibrational self-consistent field (VSCF) calculations and time-resolved resonance Raman spectroscopy are used to examine the role of anharmonic effects in the vibrational spectroscopy of the dark state and two major chromophore intermediates of the photoactive yellow protein (PYP) photocycle. The results have shown the importance of the inclusion of anharmonicity in spectroscopic calculations and the sensitivity of experiments to anharmonicity.

The overtone-induced processes in [H.sub.2]S[O.sub.4], [H.sub.2]S[O.sub.4]-[H.sub.2]O are characterized by classical trajectory simulations using potential surfaces from semiempirical electronic structure theory (PM3). Fast intramolecular proton transfer is predicted for [H.sub.2]S[O.sub.4] and dissociation into S[O.sub.3] and [H.sub.2]O is also found, with a characteristic time scale of approximately 9 ps.

The theoretical prediction of fluorine-free organic compounds of argon, HArC4H and HArC6H, is reported. The predicted stability of two molecules is in analogy with the prepared noble gas hydrocarbon compounds and these compounds have suggested a new class of argon compounds that widen the scope of argon chemistry.

Experimental and theoretical data is presented, which allows claiming the identification of an organic molecule containing krypton, HKrCCH. The ab initio calculation shows intrinsic stability of HKrCCH owing to sufficient ionic and covalent contributions to the bonding.

Proton transfer and dissociation processes following excitation of the OH or NH stretching modes of the proton-bound complex G1yLysH+ are studied by classical trajectories. "On the fly" simulations with the PM3 semi- empirical electronic structure method for the potential surface are used. Initial conditions are sampled to correspond to the v=1 excited state of the OH or NH stretching modes. Five different conformers of the complex are studied as initial structures. The main findings are (1) Photoinduced proton transfer is on the picosecond time scale. (2) Proton transfer is much faster than the processes of dissociation. (3) Proton transfer involves different sites. Most trajectories show sequences of two proton transfer events. (4) The proton transfer events show high selectivity with regard to the initially excited vibration and the initial structure, (s) Photodissociation of the complex occurs on a typical time scale of 100 Ps. (6) Conformational transitions are found to be often faster than proton transfer. These results have implications for the mass spectrometry of complexes, for dynamics of proton wires) and for proton migration in proteins. [ABSTRACT FROM AUTHOR]

The article offers information about a study conducted to characterize the overtone-induced processes in H2SO4 and H2SO4-H2O by the classical trajectory simulations, using potential surfaces from semiempirical electronic structure theory (PM3). Furthermore, the study supports the validity of the PM3 potential surface for the systems involved. Results predicted a fast intramolecular proton transfer for H2SO4.

Ionization of N2O4 in and on thin water films on surfaces is believed to be a key step in the hydrolysis of NO2 which generates HONO, a significant precursor to the OH free radical in the lower atmosphere. Molecular dynamics simulations using "on the fly" high-level MP2 potentials are carried out for ONONO2•(H2O)n clusters, n ≤ 8, used to mimic the surface reaction, in order to investigate the ionization process and determine its time-scale and mechanism around room temperature. The results are (i) the isolated molecule does not convert to the NO+NO3- ion pair, even for long times; (ii) for ONONO2•(H2O)n with n = 1 and 2, ionization takes place in several picoseconds; (iii) for n ≥ 3, ionization is essentially immediate, implying that the neutral species does not have sufficient lifetime to be considered a significant intermediate in the reaction; and (iv) even at ice temperatures, T ≤ 250 K, ionization for n ≥ 3 is immediate. The implications for hydrolysis of oxides of nitrogen on surfaces in the atmosphere are discussed. [ABSTRACT FROM AUTHOR]

Fumonisins are polyketide-derived mycotoxins produced by several plant pathogenic fungi. The toxins cause several fatal diseases in domestic animals and are associated with esophageal cancer and neural tube defects in humans. Fumonisins contain a highly reduced, acyclic 18-carbon chain, which is synthesized by an iterative polyketide synthase (PKS). This PKS does not contain a thioesterase or cyclase domain that is found in other PKSs for the release of the covalently linked polyketide chain. In this study, we expressed the acyl carrier protein (ACP) of FUM1 and in vitro loaded acyl chains to the ACP from acyl-CoA using a promiscuous 4'-phosphopantetheinyl transferase. We then expressed FUM8, which is homologous to 2-oxoamine synthase genes, in yeast and showed that the enzyme is able to offload the acyl chains from ACP. Products resulted from the decarboxylative condensation between l-alanine and acyl-S-ACP were detected by GC-MS. The enzyme activity was dependent on pyridoxal 5'-phosphate (PLP), and C18-S-ACP was the preferred substrate. The results revealed a novel polyketide chain-releasing mechanism, in which a PLP-dependent enzyme catalyzes the termination and offloading of the polyketide chain as well as the introduction of a new carbon-carbon bond and an amino group to the chain. The mechanism is fundamentally different from the thioesterase/cyclase-catalyzed polyketide chain release found in bacterial and other fungal polyketide biosyntheses.