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Marianne Engeser

Publications and source records attributed to Marianne Engeser.

6 recordsLinked to original sources
PubMed2006

Degradation of ionized OV(OCH3)3 in the gas phase. From the neutral compound all the way down to the quasi-terminal fragments VO+ and VOH+.

The consecutive fragmentation of ionized trimethyl vanadate(V), OV(OCH3)3 (1), is examined by experiment and theory. After an elimination of formaldehyde from the molecular ion 1+, subsequent dissociations proceed via losses of first H2 and then two molecules of formaldehyde to finally yield the VOH+ cation; these redox reactions involve the V(II)/V(IV) manifold. At elevated energies, expulsion of CH3O* from 1+ can efficiently compete to afford OV(OCH3)2+, a formal V(V) compound, from which subsequent losses of H2 and two units of CH2O lead to bare VO+, thereby exploring the V(III)/V(V) redox manifold. Experiments using complementary mass spectrometric techniques, i.e., neutralization-reionization experiments and ion/molecule reactions, in conjunction with extensive computational studies provide deep insight into the ion structures and the relative energetics of these dissociation reactions. In particular, a quantitative energetic scheme is obtained that ranges from neutral OV(OCH3)3 all the way down to the quasi-terminal fragment ions VOH+ and VO+, respectively.

Journal Article↗
PubMed2005

A versatile toolbox for variable DNA functionalization at high density.

To broaden the applicability of chemically modified DNAs in nano- and biotechnology, material science, sensor development, and molecular recognition, strategies are required for introducing a large variety of different modifications into the same nucleic acid sequence at once. Here, we investigate the scope and limits for obtaining functionalized dsDNA by primer extension and PCR, using a broad variety of chemically modified deoxynucleotide triphosphates (dNTPs), DNA polymerases, and templates. All natural nucleobases in each strand were substituted with up to four different base-modified analogues. We studied the sequence dependence of enzymatic amplification to yield high-density functionalized DNA (fDNA) from modified dNTPs, and of fDNA templates, and found that GC-rich sequences are amplified with decreased efficiency as compared to AT-rich ones. There is also a strong dependence on the polymerase used. While family A polymerases generally performed poorly on "demanding" templates containing consecutive stretches of a particular base, family B polymerases were better suited for this purpose, in particular Pwo and Vent (exo-) DNA polymerase. A systematic analysis of fDNAs modified at increasing densities by CD spectroscopy revealed that single modified bases do not alter the overall B-type DNA structure, regardless of their chemical nature. A density of three modified bases induces conformational changes in the double helix, reflected by an inversion of the CD spectra. Our study provides a basis for establishing a generally applicable toolbox of enzymes, templates, and monomers for generating high-density functionalized DNAs for a broad range of applications.

Base Sequence↗
PubMed2005

Gas-phase dehydrogenation of methanol with mononuclear vanadium-oxide cations.

The reactions of methanol with mass-selected V+, VOH+, VO+, and VO2(+) cations are studied by Fourier-transform ion-cyclotron resonance (FT-ICR) mass spectrometry in order to investigate the influence of the formal oxidation state of the metal on the reactivity of vanadium-oxide compounds. Interestingly, the most reactive species is the low-valent hydroxide cation VOH+, for which a formal condensation reaction prevails to afford VOCH3(+). In contrast, atomic V+ is oxidized and the high-valent dioxide cation VO2(+) is reduced by methanol. The dehydrogenation of methanol mediated by VO+ does not involve any change of the metal's oxidation state. For the latter reaction, the experimental results are complemented by a theoretical investigation by using density functional theory.

Journal Article↗
PubMed2004

Ion chemistry of OV(OCH3)3 in the gas phase: molecular cations and anions and their primary fragmentations.

Trimethyl vanadate(V), OV(OCH(3))(3) (1), is examined by various mass spectrometric means. Photoionization experiments yield an ionization energy of IE(OV(OCH(3))(3)) = 9.54 +/- 0.05 eV for the neutral molecule. The primary fragmentation of the molecular cation 1(+), i.e., loss of neutral formaldehyde, can occur via two independent routes of hydrogen migrations to afford the formal V(IV) compounds HOV(OCH(3))(2)(+) and OV(OCH(3))(CH(3)OH)(+), respectively. These two pathways are associated with low-lying activation barriers of almost identical height. At elevated energies, direct V-O bond cleavage of 1(+) allows for expulsion of a methoxy radical concomitant with the generation of the cationic fragment OV(OCH(3))(2)(+), a formal V(V) compound. Trimethyl vanadate can also form a molecular anion, 1(-), whose most abundant dissociation channel involves loss of a methyl radical, thereby leading to the formal V(V) compound OV(OCH(3))(2)O(-). Various mass spectrometric experiments and extensive theoretical studies provide detailed insight into the ion structures and the relative energetics of the primary dissociation reactions of the molecular cations and anions of 1.

Journal Article↗
PubMed2002

Energetics of the ligated vanadium dications VO2+, VOH2+, and [V,O,H2]2+.

Charge-stripping mass spectrometry and electronic structure calculations are combined to examine the energetics of the vanadium-containing dications VO2+, VOH2+, [HVOH]2+, and [V(OH2)]2+. Energy-resolved measurements, in conjunction with quantum-mechanical calculations, led to ionization energies (IE) of (VO+) = 16.6 +/- 0.3 eV and IE(VOH+) = 16.0 +/- 0.4 eV. Born-Haber cycles reveal that VO2+ and VOH2+ are both thermochemically stable dications with respect to their charge-separation asymptotes. Depending on the mode of ion generation, the [V,O,H2]+ monocations are formed as variable mixtures of the structural isomers [HVOH]+ and [V(OH2)]+. The measured IE([V,O,H2]+) = 13.7 +/- 0.3 eV is between the theoretical predictions for these isomers, indicating that mixtures of both isomers are sampled in the experiments. According to the theoretical results, [V(OH2)]2+ is also a thermochemically stable dication, whereas [HVOH]2+ is suggested to be metastable.

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