[en] Here we review recent studies on the metastable fragmentation of individual DNA and RNA building blocks and their compositions using matrix assisted laser desorption/ionization mass spectrometry (MALDI). To compare the fragmentation channels of small DNA components with larger compositions we have studied the metastable fragmentation of the deprotonated nucleobases, ribose, ribose-monophosphates, the nucleosides, the nucleoside 5'-monophosphates and selected oligonucleotides. Both previously published and unpublished data are reported. To gain a comprehensive picture of the fragmentation of individual components, metastable fragmentation of native components are in many cases compared to chemically modified components and isotopic labelling is used to unambiguously identify fragments. Furthermore, to shed light on the underlying fragmentation mechanisms we complement the experimental studies with classical dynamics simulations of the fragmentation of selected compounds. For the DNA and RNA components where dissociative electron attachment studies have been conducted we compare these to the metastable fragmentation channels observed here. (authors)

[en] Here we present a study on the metastble decay of the deprotonated deoxynucleosides dC, dT, dA and dG. We discuss the underlying mechanisms and compare the fragmentation pattern we observe to the fragment formation observed through dissociative electron attachment. The deprotonated deoxynucleosides show characteristic decay spectra which composition is mainly governed by the nature of the base, i.e., if the deoxynucleoside contains a pyrimidine or a purine base. The deoxynucleosides containing purine decompose predominantly through the simple rupture of the glycosidic bond, leading to the observation of the negatively charged, deprotonated base [B-H]^-. The only other fragment observed for dA and dG is [B+C₂H₂]^-. The deoxynucleosides containing pyrimidine (dC and dT) show a considerably richer fragmentation pattern with one of the dominating fragments being the negative ion of the high electron affinity fragment NCO. In the case of dT all other fragments may be traced to decomposition of the sugar moiety. The fragmentation spectra of dC on the other hand are governed by the decomposition of the base

[en] The dissociation of proteins due to radiation and charged particles is an important process which is not well understood. Measurements of the fragmentation of such large molecules using mass spectrometers can be difficult to describe and understand since for many of the masses observed there are more than one possible combination of atoms. We have simulated the metastable decay of the anions formed by dissociative electron attachment (DEA) to L-Valine to show that it is possible to recognize some of the observed mass to charge ratios by their further fragmentation. By using computer simulations of the classical dynamics of the ions and finding minimum energy paths for intermolecular atom transfer and reorganization, we have identified molecular combinations that can be assigned to the measured m/Z ratios. The atomic forces in these calculations are obtained from density functional theory using a gradient dependent functional.

[en] In the present study we investigate free electron attachment to the amino acid valine. Mass spectra and anion efficiency curves are measured in the electron energy range from about zero eV to about 15 eV and the anionic fragments are analyzed with a double focusing mass spectrometer. The high sensitivity of the present setup allows the detection of 10 fragment anions that have not been reported before and the high mass resolution of our sector field mass spectrometer allows us the separation and identification of isobaric anions. Thus the isobaric ion pairs, CN^-/C₂H₂^-, and O^-/NH₂^-, can be identified and assigned to individual resonances. For some of the heavier fragment anions formed we have studied collision induced dissociation to collect more information on the structures of these anions. (authors)

[en] We review recent results on dissociative electron attachment (DEA) to gas phase D-ribose, tetraacetyl-D-ribose (TAR) and dibutylphosphate (DBP), which serve as model compounds for the DNA or RNA backbone. New results are presented on negative ion formation in D-ribose probed by matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry. The two methods reveal that the transient D-ribose anion R^- decomposes in the same way like the deprotonated D-ribose molecule [R-H]^-, i.e. by abstraction of different numbers of water and formaldehyde units. In DEA the TNI R^- is generated at very low energies close to 0 eV most likely through a vibrational feshbach resonance. The fragmentation pattern and the characteristic resonances of D-ribose are preserved in TAR, where a furanose is bound to four acetyl groups. The presence of an acetyl group leads additionally to fragmentation through a shape resonance. Shape resonances were also observed in DBP, followed by C-O and P-O bond breaking.

[en] Differential cross sections (DCSs) for excitation of the C-H stretch vibrations for CH₃F, CH₃Cl, CH₃Br and CH₃I were measured by electron impact. Resonance enhancement was observed for the C-H stretch at 6.6, 5.9, 5.5 and 4.7 eV respectively. For CH₃F this is attributed to a ²A₁ shape resonance formed through temporary occupation of the LUMO and for the heavier halogens to a ²E shape resonance formed through temporary occupation of the LUMO+1. DCSs for excitation of the C-X stretch were also measured for CH₃F and CH₃Cl. The shift in the resonance energy is discussed in context to the substitution effect of the halogens.

[en] In this paper, we present integral cross sections (ICS) for electron impact excitation of the n=2 levels in helium in the impact energy range of 23.5 to 35 eV. The ICS of each final state, 2³S, 2¹S, 2³P and 2¹P, has been determined by integration of the angular differential cross sections (DCS) over all of 0deg to 180deg, where those DCS were obtained from both our previous experiments and through extrapolation using the convergent close coupling calculation. The present experimental ICS for the optically allowed 2¹P transition state are also compared with those obtained from the BEf-scaling method. Very good agreement between the experimental and BEf-scaled 2¹P ICSs is generally found in the measured impact energy region. (author)

[en] In this paper, we present integral cross sections (ICS) for electron impact excitation of the n = 2 levels in helium in the impact energy range of 23.5 eV to 35 eV. The ICS of each final state, 2³S, 2¹S, 2³P and 2¹P, has been determined by integration of the angular differential cross sections (DCS) over all of 0⁰ to 180⁰, where those DCS were obtained from both our previous experiments and the extrapolation using the convergent close coupling calculation. The present experimental ICS for the optically allowed 2¹P transition state are also compared with those obtained from the BEf-scaling method. Very good agreement between the experimental and BEf-scaled 2¹P ICSs is generally found in the measured impact energy region.

[en] In this paper, we present new normalized experimental differential cross sections for electron impact excitation of the n = 2 states in helium. The incident electrons have energies in the range 23.5-35 eV, while the scattered electrons are detected over the angular range 10-130 deg. Corresponding theoretical results from our convergent close-coupling approach are also presented and in general are in very good accord with our measured data. Where possible, a comparison of the present experimental and theoretical results with those from previous measurements is also made. The case for the n = 2 levels in helium constituting a benchmark cross section data set is examined.

[en] Measurements of the grand total and total positronium formation cross sections for positron scattering from uracil have been performed for energies between 1 and 180 eV, using a trap-based beam apparatus. Angular, quasi-elastic differential cross section measurements at 1, 3, 5, 10, and 20 eV are also presented and discussed. These measurements are compared to existing experimental results and theoretical calculations, including our own calculations using a variant of the independent atom approach