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Recent results from the Crystal Barrel experiment
No full textThe Crystal Barrel experiment has been constructed and installed at the Low Energy Antiproton Ring (LEAR) at CERN. It has been fully operational since late 1989. In this talk, recent results of meson spectroscopy in p{bar p}-annihilations are presented. The main emphasis is on all-neutral annihilations, the study of the strange quark content of the proton, and the investigation of the decay mode of il particles. A 2{sup ++} resonance decaying into {pi}{degrees}{pi}{degrees}at a mass of 1515 {plus_minus} 10 MeV with a width of 120 {plus_minus} 10 MeV has been seen in a 3{pi}{degrees} final state
Conservation of the folding mechanism between a thermostable (βα)8-barrel enzyme, its evolutionary relatives, and ancestral precursors generated by protein design
No full textThe (βα)8-barrel is amongst the most ancient, frequent and versatile enzyme folds. Sequence and structure comparisons suggest that modern (βα)8-barrels have divergently evolved from a common ancestor, which raises the question whether they share the same folding mechanism. Here, this problem was appraoched by using the thermostable cyclase subunit HisF of imidazole glycerol phosphate (ImGP) synthase from Thermotoga maritima as a test case.
In the first part of the work, the folding mechanism of HisF was elucidated and compared to the folding mechanisms of other (βα)8-barrel proteins with low sequence identity. The equilibrium unfolding transition of HisF is reversible and adequately decribed by the two-state model without intermediates. Upon refolding at low denaturant concentrations a burst-phase intermediate is formed rapidly. It contains a significant amount of secondary structure but is probably an off-pathway species. In the further course of folding two phases are observed, which are associated with changes in both intrinsic protein fluorescence and far-UV circular dichroism (CD). Double mixing experiments revealed a sequential mechanism with an on-pathway intermediate which is formed in the first phase and converted into the native protein in the second phase. Refolding experiments in presence of the glutaminase subunit HisH of ImGP synthase showed that the formation of native HisF is a prerequisite for complex formation. The refolding of HisF grossly resembles other (βα)8-barrel proteins, suggesting that the general folding properties within this structural class of proteins have been conserved upon the vast sequence and functional diversification during evolution. The increased thermodynamic stability of HisF compared to (βα)8-barrels from mesophiles is entirely due to a drastically decreased unfolding rate.
The pronounced two-fold symmetry of the sequence and the (βα)8-barrel structure of HisF suggest that its fold has arisen by the duplication and fusion of a (βα)4-half-barrel. Primordial (βα)8-barrels mimicking the precursor of HisF were previously constructed by fusing two copies of its C-terminal half-barrel HisF-C. The resulting HisF-CC construct was optimized by rational protein design and directed laboratory evolution, yielding the artificial (βα)8-barrels Sym1 and Sym2.
In the second part of this thesis, the structures, stabilities and folding mechanisms of Sym1 and Sym2 were analyzed and compared with HisF. Sym1 was shown to be less stable than HisF and its crystal structure shows disorder in the contact regions between the two half-barrels. The next-generation construct Sym2 turned out to be more stable than HisF, and the contact regions are well resolved. However, important characteristics of the kinetic folding pathway are identical for the three proteins: At moderate denaturant conditions, unfolding and refolding are mono-exponential reactions, but at native-like conditions, refolding is complex. For all three proteins, HisF, Sym1, and Sym2, an off-pathway burst-phase folding intermediate IBP is observed by far-UV CD, followed by a fast reaction that occurs with comparable rates (τ ~ 3 s) and leads to the on-pathway intermediate I. The subsequent slow and rate-determining folding reaction has a similar rate for HisF and Sym2 (τ ~ 20 s) but a lower rate for Sym1 (τ = 175 s) and leads to the native state N. For Sym2, the additional on-pathway intermediate I’ is formed in an extremely fast reaction (τ = 0.27 s). These findings point to a similar sequential folding pathway for the three proteins (IBP→U→I’→I→N) with I’ being energetically favored in the case of Sym2 but too unstable to be populated in the case of HisF and Sym1. In contrast to the similarities in refolding, the unfolding kinetics of HisF, Sym1, and Sym2 are characterized by extremely different rates which are responsible for their different thermodynamic stabilities. The conservation of the folding mechanism further backs the hypothesis that (βα)8-barrels have evolved from an ancestral (βα)4-half-barrel and provide compelling evidence that Sym1 and Sym2 are realistic models for the early fusion protein product. Moreover, the different stabilities of Sym1 and Sym2 emphasize the significance of optimized contact regions of the fused half-barrels for the generation of a stable (βα)8-barrel with a well-defined folding landscape
Photo-excitation of nucleon resonances from complex nuclei
No full textPhotoproduction of [n] mesons off 12C, 40Ca, 93Nb, and natPb nuclei has been
measured with the tagged-photon-beam technique for incident photon energies
between 0.6 and 2.2 GeV. The experiment was performed at the Bonn ELSA ac-
celerator with the combined setup of the Crystal Barrel and TAPS calorimeters.
It aimed at the in-medium properties of the S11(1535) nucleon resonance and
the study of the absorption properties of nuclear matter for [n] mesons. Careful
consideration was given to contributions from [np] final states and secondary pro-
duction mechanisms of [n]-mesons e.g. from inelastic [p]N reactions of intermediate
pions. The analysis of the mass number scaling shows that the nuclear absortpion
cross section [o]N[n] for [n] mesons is constant over a wide range of the [n] momen-
tum. The comparison of the excitation functions to data off the deuteron and to
calculations in the framework of a BUU-model show no unexplained in-medium
modifications of the S11(1535)
Annihilation at rest of antiprotons and protons into neutral particles
No full textAnnihilation of antiprotons and protons at rest into neutral particles has been studied with the Crystal Barrel detector at LEAR. Annihilation frequencies are determined for final states containing !0, ", "" and # mesons using a liquid and a room temperature, 12 bar, gaseous hydrogen target. Including annihilation frequencies for production of neutral kaons from other experiments, the identified reactions for annihilation in liquid hydrogen add up to a branching fraction of (3.56 ± 0.28)% per annihilation compared to the frequency of (3.50±0.30)%withwhichwe observe the allneutral decay modes inclusively. Since the exclusive final states are normalized to the Crystal Barrel measurement of the !0!0 branching ratio, the latter result is strongly supported by this present study
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