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We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.
In this contribution, we study using first principles the co-adsorption and catalytic behaviors of CO and O2 on a single gold atom deposited at defective magnesium oxide surfaces. Using cluster models and point charge embedding within a density functional theory framework, we simulate the CO oxidation reaction for Au1 on differently charged oxygen vacancies of MgO(001) to rationalize its experimentally observed lack of catalytic activity. Our results show that: (1) co-adsorption is weakly supported at F0 and F2+ defects but not at F1+ sites, (2) electron redistribution from the F0 vacancy via the Au1 cluster to the adsorbed molecular oxygen weakens the O2 bond, as required for a sustainable catalytic cycle, (3) a metastable carbonate intermediate can form on defects of the F0 type, (4) only a small activation barrier exists for the highly favorable dissociation of CO2 from F0, and (5) the moderate adsorption energy of the gold atom on the F0 defect cannot prevent insertion of molecular oxygen inside the defect. Due to the lack of protection of the color centers, the surface becomes invariably repaired by the surrounding oxygen and the catalytic cycle is irreversibly broken in the first oxidation step.
Information about the strength of donor–acceptor interactions in push–pull alkenes is valuable, as this so-called “push–pull effect” influences their chemical reactivity and dynamic behaviour. In this paper, we discuss the applicability of NMR spectral data and barriers to rotation around the C[double bond, length as m-dash]C double bond to quantify the push–pull effect in biologically important 2-alkylidene-4-oxothiazolidines. While olefinic proton chemical shifts and differences in 13C NMR chemical shifts of the two carbons constituting the C[double bond, length as m-dash]C double bond fail to give the correct trend in the electron withdrawing ability of the substituents attached to the exocyclic carbon of the double bond, barriers to rotation prove to be a reliable quantity in providing information about the extent of donor–acceptor interactions in the push–pull systems studied. In particular all relevant kinetic data, that is the Arrhenius parameters (apparent activation energy Ea and frequency factor A) and activation parameters (ΔS‡, ΔH‡ and ΔG‡), were determined from the data of the experimentally studied configurational isomerization of (E)-9a. These results were compared to previously published related data for other two compounds, (Z)-1b and (2E,5Z)-7, showing that experimentally determined ΔG‡ values are a good indicator of the strength of push–pull character. Theoretical calculations of the rotational barriers of eight selected derivatives excellently correlate with the calculated C[double bond, length as m-dash]C bond lengths and corroborate the applicability of ΔG‡ for estimation of the strength of the push–pull effect in these and related systems.
Coupling of attention and saccades when viewing scenes with central and peripheral degradation
(2016)
Degrading real-world scenes in the central or the peripheral visual field yields a characteristic pattern: Mean saccade amplitudes increase with central and decrease with peripheral degradation. Does this pattern reflect corresponding modulations of selective attention? If so, the observed saccade amplitude pattern should reflect more focused attention in the central region with peripheral degradation and an attentional bias toward the periphery with central degradation. To investigate this hypothesis, we measured the detectability of peripheral (Experiment 1) or central targets (Experiment 2) during scene viewing when low or high spatial frequencies were gaze-contingently filtered in the central or the peripheral visual field. Relative to an unfiltered control condition, peripheral filtering induced a decrease of the detection probability for peripheral but not for central targets (tunnel vision). Central filtering decreased the detectability of central but not of peripheral targets. Additional post hoc analyses are compatible with the interpretation that saccade amplitudes and direction are computed in partial independence. Our experimental results indicate that task-induced modulations of saccade amplitudes reflect attentional modulations.
Molekulare Charakterisierung des Centrosom-assoziierten Proteins CP91 in Dictyostelium discoideum
(2016)
Das Dictyostelium-Centrosom ist ein Modell für acentrioläre Centrosomen. Es besteht aus einer dreischichtigen Kernstruktur und ist von einer Corona umgeben, welche Nukleationskomplexe für Mikrotubuli beinhaltet. Die Verdoppelung der Kernstruktur wird einmal pro Zellzyklus am Übergang der G2 zur M-Phase gestartet. Durch eine Proteomanalyse isolierter Centrosomen konnte CP91 identifiziert werden, ein 91 kDa großes Coiled-Coil Protein, das in der centrosomalen Kernstruktur lokalisiert. GFP-CP91 zeigte fast keine Mobilität in FRAP-Experimenten während der Interphase, was darauf hindeutet, dass es sich bei CP91 um eine Strukturkomponente des Centrosoms handelt. In der Mitose hingegen dissoziieren das GFP-CP91 als auch das endogene CP91 ab und fehlen an den Spindelpolen von der späten Prophase bis zur Anaphase. Dieses Verhalten korreliert mit dem Verschwinden der zentralen Schicht der Kernstruktur zu Beginn der Centrosomenverdopplung. Somit ist CP91 mit großer Wahrscheinlichkeit ein Bestandteil dieser Schicht. CP91-Fragmente der N-terminalen bzw. C-terminalen Domäne (GFP-CP91 N-Terminus, GFP-CP91 C-Terminus) lokalisieren als GFP-Fusionsproteine exprimiert auch am Centrosom, zeigen aber nicht die gleiche mitotische Verteilung des Volllängenproteins. Das CP91-Fragment der zentralen Coiled-Coil Domäne (GFP-CP91cc) lokalisiert als GFP-Fusionsprotein exprimiert, als ein diffuser cytosolische Cluster, in der Nähe des Centrosoms. Es zeigt eine partiell ähnliche mitotische Verteilung wie das Volllängenprotein. Dies lässt eine regulatorische Domäne innerhalb der Coiled-Coil Domäne vermuten. Die Expression der GFP-Fusionsproteine unterdrückt die Expression des endogenen CP91 und bringt überzählige Centrosomen hervor. Dies war auch eine markante Eigenschaft nach der Unterexpression von CP91 durch RNAi. Zusätzlich zeigte sich in CP91-RNAi Zellen eine stark erhöhte Ploidie verursacht durch schwere Defekte in der Chromosomensegregation verbunden mit einer erhöhten Zellgröße und Defekten im Abschnürungsprozess während der Cytokinese. Die Unterexpression von CP91 durch RNAi hatte auch einen direkten Einfluss auf die Menge an den centrosomalen Proteinen CP39, CP55 und CEP192 und dem Centromerprotein Cenp68 in der Interphase. Die Ergebnisse deuten darauf hin, dass CP91 eine zentrale centrosomale Kernkomponente ist und für den Zusammenhalt der beiden äußeren Schichten der Kernstruktur benötigt wird. Zudem spielt CP91 eine wichtige Rolle für eine ordnungsgemäße Centrosomenbiogenese und, unabhängig davon, bei dem Abschnürungsprozess der Tochterzellen während der Cytokinese.
Background: Aggression is a severe behavioral problem that interferes with many developmental challenges individuals face in middle childhood and adolescence. Particularly in the peer and in the academic domain, aggression inhibits the individual from making important learning experiences that are predictive for a healthy transition into adulthood. Furthermore, the resulting developmental deficits have the propensity to feedback and to promote aggression at later developmental stages. The aim of the present PhD thesis was to investigate pathways and processes involved in the etiology of aggression by examining the interrelation between multiple developmental problems in the peer and in the academic domain. More specifically, the relevance of affiliation with deviant peers as a driving mechanism for the development of aggression, factors promoting the affiliation with deviant peers (social rejection; academic failure), and mechanisms by which affiliation with deviant peers leads to aggression (external locus of control) were investigated.
Method: The research questions were addressed by three studies. Three data waves were available for the first study, the second and third study were based on two data waves. The first study specified pathways to antisocial behavior by investigating the temporal interrelation between social rejection, academic failure, and affiliation with deviant peers in a sample of 1,657 male and female children and adolescents aged between 6 and 15 years. The second study examined the role of external control beliefs as a potential mediator in the link between affiliation with deviant peers and aggression in a sample of 1,466 children and adolescents in the age of 9 to 19 years, employing a half-longitudinal design. The third study aimed to expand the findings of Study 1 and Study 2 by examining the differential predictivity of combinations of developmental risks for different functions of aggression, using a sample of 1,479 participants in the age between 9 and 19 years. First, profiles of social rejection, academic failure, and affiliation with deviant peers were identified, using latent profile analysis. Second, prospective pathways between risk-profiles and reactive and proactive aggression were investigated, using latent path analysis.
Results: The first study revealed that antisocial behavior at T1 was associated with social rejection and academic failure at T2. Both mechanisms promoted affiliation with deviant peers at the same data wave, which predicted deviancy at T3. Furthermore, both an indirect pathway via social rejection and affiliation with deviant peers and an indirect pathway via academic failure and affiliation with deviant peers significantly mediated the link between antisocial behavior at the first and the third data wave. Additionally, the proposed pathways generalized across genders and different age groups. The second study yielded that external control beliefs significantly mediated the link between affiliation with deviant peers and aggression, with affiliation with deviant peers at T1 predicting external control beliefs at T2 and external control beliefs at T1 predicting aggressive behavior at T2. Again, the analyses provided no evidence for gender and age specific variations in the proposed pathways. In the third study, three distinct risk groups were identified, made up of a large non-risk group, with low scores on all risk measures, a group characterized by high scores on social rejection (SR group), and a group with the highest scores on measures of affiliation with deviant peers and academic failure (APAF group). Importantly, risk group membership was differentially associated with reactive and proactive aggression. Only membership in the SR group at T1 was associated with the development of reactive aggression at T2 and only membership in the APAF group at T1 predicted proactive aggression at T2. Additionally, proactive aggression at T1 predicted membership in the APAF group at T2, indicating a reciprocal relationship between both constructs.
Conclusion: The results demonstrated that aggression causes severe behavioral deficits in social and academic domains which promote future aggression by increasing individuals’ tendency to affiliate with deviant peers. The stimulation of external control beliefs provides an explanation for deviant peers’ effect on the progression and intensification of aggression. Finally, multiple developmental risks were shown to co-occur within individuals and to be differentially predictive of reactive and proactive aggression. The findings of this doctoral dissertation have possible implications for the conceptualization of prevention and intervention programs aimed to reduce aggression in middle childhood and adolescence.
A majority of studies documented a reduced ankle muscle activity, particularly of the peroneus longus muscle (PL), in patients with functional ankle instability (FI). It is considered valid that foot orthoses as well as sensorimotor training have a positive effect on ankle muscle activity in healthy individuals and those with lower limb overuse injuries or flat arched feet (reduced reaction time by sensorimotor exercises; increased ankle muscle amplitude by orthoses use). However, the acute- and long-term influence of foot orthoses on ankle muscle activity in individuals with FI is unknown.
AIMS: The present thesis addressed (1a) acute- and (1b) long-term effects of foot orthoses compared to sensorimotor training on ankle muscle activity in patients with FI. (2) Further, it was investigated if the orthosis intervention group demonstrate higher ankle muscle activity by additional short-term use of a measurement in-shoe orthosis (compared to short-term use of “shoe only”) after intervention. (3) As prerequisite, it was evaluated if ankle muscle activity can be tested reliably and (4) if this differs between healthy individuals and those with FI.
METHODS: Three intervention groups (orthosis group [OG], sensorimotor training group [SMTG], control group [CG]), consisting of both, healthy individuals and those with FI, underwent one longitudinal investigation (randomised controlled trial). Throughout 6 weeks of intervention, OG wore an in-shoe orthosis with a specific “PL stimulation module”, whereas SMTG conducted home-based exercises. CG served to measure test-retest reliability of ankle muscle activity (PL, M. tibialis anterior [TA] and M. gastrocnemius medialis [GM]). Pre- and post-intervention, ankle muscle activity (EMG amplitude) was recorded during “normal” unperturbed (NW) and perturbed walking (PW) on a split-belt treadmill (stimulus 200 ms post initial heel contact [IC]) as well as during side cutting (SC), each while wearing “shoes only” and additional measurement in-shoe orthoses (randomized order). Normalized RMS values (100% MVC, mean±SD) were calculated pre- (100-50 ms) and post (200-400 ms) - IC.
RESULTS: (3) Test-retest reliability showed a high range of values in healthy individuals and those with FI. (4) Compared to healthy individuals, patients with FI demonstrated lower PL pre-activity during SC, however higher PL pre-activity for NW and PW. (1a) Acute orthoses use did not influence ankle muscle activity. (1b) For most conditions, sensorimotor training was more effective in individuals with FI than long-term orthotic intervention (increased: PL and GM pre-activity and TA reflex-activity for NW, PL pre-activity and TA, PL and GM reflex-activity for SC, PL reflex-activity for PW). However, prolonged orthoses use was more beneficial in terms of an increase in GM pre-activity during SC. For some conditions, long-term orthoses intervention was as effective as sensorimotor training for individuals with FI (increased: PL pre-activity for PW, TA pre-activity for SC, PL and GM reflex-activity for NW). Prolonged orthoses use was also advantageous in healthy individuals (increased: PL and GM pre-activity for NW and PW, PL pre-activity for SC, TA and PL reflex-activity for NW, PL and GM reflex-activity for PW). (2) The orthosis intervention group did not present higher ankle muscle activity by the additional short-term use of a measurement in-shoe orthosis at re-test after intervention.
CONCLUSION: High variations of reproducibility reflect physiological variability in muscle activity during gait and therefore deemed acceptable. The main findings confirm the presence of sensorimotor long-term effects of specific foot orthoses in healthy individuals (primary preventive effect) and those with FI (therapeutic effect). Neuromuscular compensatory feedback- as well as anticipatory feedforward adaptation mechanism to prolonged orthoses use, specifically of the PL muscle, underpins the key role of PL in providing essential dynamic ankle joint stability. Due to its advantages over sensorimotor training (positive subjective feedback in terms of comfort, time-and-cost-effectiveness), long-term foot orthoses use can be recommended as an applicable therapy alternative in the treatment of FI. Long-term effect of foot orthoses in a population with FI must be validated in a larger sample size with longer follow-up periods to substantiate the generalizability of the existing outcomes.
Inhalt
- Reinhard Andress: Ein unveröffentlichter Brief Alexander von Humboldts an den Buchhändler Jean-Georges Treuttel
- Julian Drews: Écriture (auto)biographique dans l‘Examen critique d‘Alexandre de Humboldt
- Alberto Gómez Gutiérrez: Alexander von Humboldt y la cooperación transcontinental en la Geografía de las plantas: una nueva apreciación de la obra fitogeográfica de Francisco José de Caldas
- Karin Reich und Elena Roussanova: Der Briefwechsel zwischen Karl Kreil und Alexander von Humboldt, ein wichtiger Beitrag zur Geschichte des Erdmagnetismus
- Alexander Stöger: Experiment und Wissensvermittlung. Alexander von Humboldts Darstellungsmethoden in seinen Versuchen über die gereizte Muskel- und Nervenfaser
- Friedrich Herneck und Ingo Schwarz: Friedrich Herneck: Hegel und Alexander von Humboldt