Mass Equilibration and Fluctuations in the Angular Momentum Dependent Dynamics of Heavy Element Synthesis Reactions.

Mass and angle distributions for the ^{52}Cr+^{198}Pt and ^{54}Cr+^{196}Pt reactions (both forming ^{250}No) were measured and subtracted, giving new information on fast quasifission mass evolution, and the first direct determination of the dependence of sticking times on angular momentum. TDHF calculations showed good agreement with average experimental values, but experimental mass distributions unexpectedly extended to symmetric splits while the peak yield remained close to the initial masses. This implies a strong role of fluctuations in mass division early in the collision, giving insights into the transition from fast energy dissipative deep-inelastic collisions to quasifission.

Timescales of Quantum Equilibration, Dissipation and Fluctuation in Nuclear Collisions.

Understanding the dynamics of equilibration processes in quantum systems as well as their interplay with dissipation and fluctuation is a major challenge in quantum many-body theory. The timescales of such processes are investigated in collisions of atomic nuclei using fully microscopic approaches. Results from time-dependent Hartree-Fock and time-dependent random-phase approximation calculations are compared for 13 systems over a broad range of energies. The timescale for full mass equilibration (∼2×10^{-20} s) is found to be much larger than timescales for neutron-to-proton equilibration, kinetic energy, and angular momentum dissipations which are on the order of 10^{-21} s. Fluctuations of mass numbers in the fragments and correlations between their neutron and proton numbers build up within only a few 10^{-21} s. This indicates that dissipation is basically not impacted by mass equilibration, but is mostly driven by the exchange of nucleons between the fragments.

Mechanisms Suppressing Superheavy Element Yields in Cold Fusion Reactions.

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of ^{48}Ca, ^{50}Ti, and ^{54}Cr with ^{208}Pb. Moving from ^{48}Ca to ^{54}Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P_{CN} (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for ^{50}Ti and 15 for ^{54}Cr in comparison to ^{48}Ca. The energy dependence of P_{CN} indicates that cold fusion reactions (involving ^{208}Pb) are not driven by a diffusion process.

Exploring Zeptosecond Quantum Equilibration Dynamics: From Deep-Inelastic to Fusion-Fission Outcomes in

Energy dissipative processes play a key role in how quantum many-body systems dynamically evolve toward equilibrium. In closed quantum systems, such processes are attributed to the transfer of energy from collective motion to single-particle degrees of freedom; however, the quantum many-body dynamics of this evolutionary process is poorly understood. To explore energy dissipative phenomena and equilibration dynamics in one such system, an experimental investigation of deep-inelastic and fusion-fission outcomes in the ^{58}Ni+^{60}Ni reaction has been carried out. Experimental outcomes have been compared to theoretical predictions using time dependent Hartree-Fock and time dependent random phase approximation approaches, which, respectively, incorporate one-body energy dissipation and fluctuations. Excellent quantitative agreement has been found between experiment and calculations, indicating that microscopic models incorporating one-body dissipation and fluctuations provide a potential tool for exploring dissipation in low-energy heavy ion collisions.

Evidence for the Role of Proton Shell Closure in Quasifission Reactions from X-Ray Fluorescence of Mass-Identified Fragments.

The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in ^{48}Ti+^{238}U reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at Z=82 is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the N/Z (neutron over proton ratios of the fragments) evolution in quasifission reactions.

Interplay between quantum shells and orientation in quasifission.

The quasifission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in (40)Ca+(238)U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed (238)U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of (238)U produce quasifission fragments in the magic Z=82 region, while collisions with the side are the only ones that may result in fusion.

1n and 2n transfer with the Borromean nucleus 6He near the Coulomb barrier.

Angular distributions for 1n and 2n transfer are reported for the 6He+65Cu system at E_{lab}=22.6 MeV. For the first time, triple coincidences between alpha particles, neutrons, and characteristic gamma rays from the targetlike residues were used to separate the contributions arising from 1n and 2n transfer. The differential cross sections for these channels, elastic scattering, and fusion were analyzed using a coupled reaction channels approach. The large measured ratio of the 2n-to-1n cross section and the strong influence of 2n transfer on other channels indicate that the dineutron configuration of 6He plays a dominant role in the reaction mechanism.

Extraction of weak transition strengths via the (3He, t) reaction at 420 MeV.

Differential cross sections for transitions of known weak strength were measured with the (3He, t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn, and 208Pb. Using these data, it is shown that the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in the distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

Quantum calculations of Coulomb reorientation for sub-barrier fusion.

Classical mechanics and time dependent Hartree-Fock (TDHF) calculations of heavy ions collisions are performed to study the rotation of a deformed nucleus in the Coulomb field of its partner. This reorientation is shown to be independent of the charges and relative energy of the partners. It only depends upon the deformations and inertias. TDHF calculations predict an increase by 30% of the induced rotation due to quantum effects while the nuclear contribution seems negligible. This reorientation modifies strongly the fusion cross section around the barrier for light deformed nuclei on heavy collision partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.

Quantum calculation of the dipole excitation in fusion reactions.

The excitation of the giant dipole resonance induced by fusion reaction is studied with N/Z asymmetry in the entrance channel. The time dependent Hartree-Fock solution exhibits a strong dipole vibration which can be associated with a giant vibration along the main axis of the deformed compound nucleus. This dipole motion appears to be nonlinearly coupled to the shape oscillation, leading to a strong modulation of its frequency. These phenomena can be detected in the gamma-ray emission from hot compound nuclei.

Dual structural requirements for multilineage hematopoietic-suppressive activity of chemokine-derived peptides.

Many chemokines have direct suppressive activity in vitro and in vivo on primitive hematopoietic cells. However, few chemokine-derived peptides have shown a significant activity in inhibiting hematopoiesis. Interestingly, a peptide derived from the 34-58 sequence of the CXC chemokine platelet factor 4 (PF4) produced a 30-40% inhibition of proliferation of murine hematopoietic progenitors (CFU-MK, CFU-GM, and BFU-E) in vitro, at concentrations of 30-60-fold lower than PF4. The aim of the present work was to define the structural parameters and motifs involved in conferring biological activity to the peptide PF4(34-58). Both structural predictions and determinations revealed a new helical motif that was further localized between residues 38 and 46. This helix was necessary for binding of the peptide and for permitting the functional DLQ motif at position 54-56 to activate the putative receptor site. Peptides lacking either the helical or the DLQ motif were devoid of inhibitory activity on the hematopoietic progenitors in vitro. However, among inactive peptides, only those having the helical motif counteracted the inhibition induced by the active peptide PF4(34-58). This suggested that the helix might be required for peptide interactions with a putative receptor site, whereas the DLQ motif would be implicated in the activation of this receptor. These results identify for the first time the dual requirements for the design of chemokine-derived peptides with high suppressive activity on hematopoiesis, as well as for the design of molecules with antagonistic action.

Structural requirements for thymosin beta4 in its contact with actin. An NMR-analysis of thymosin beta4 mutants in solution and correlation with their biological activity.

We examined the conformational preferences of mutants of thymosin beta4, an actin monomer sequestering protein by NMR spectroscopy in 60% (v/v) trifluoroethanol. Under these conditions, the wild-type thymosin beta4 conformation consists of an alpha-helix (helix I) extending from residues 5-16 with a more stable fragment from lysine 11 to lysine 16 and a second alpha-helix (helix II) encompassing residues 31-39. The point mutations studied here are located in helix I or in the LKKTET segment (residues 17-22) that form the two main entities of interaction with the actin molecule. The alpha-1H conformational shifts allow us to investigate the helicity of the polypeptides at the residue level and to correlate these structures with their biological activity. We determine that an extension of helix I at its C-terminal end over the LKK-segment results in loss of activity. The correct termination of this helix is connected to a specific orientation of the polypeptide essential for a cooperative action of the thymosin beta4 binding entities required for full activity.

Molecular organization of the alkali-insoluble fraction of Aspergillus fumigatus cell wall.

Physical and biological properties of the fungal cell wall are determined by the composition and arrangement of the structural polysaccharides. Cell wall polymers of fungi are classically divided into two groups depending on their solubility in hot alkali. We have analyzed the alkali-insoluble fraction of the Aspergillus fumigatus cell wall, which is the fraction believed to be responsible for fungal cell wall rigidity. Using enzymatic digestions with recombinant endo-beta-1,3-glucanase and chitinase, fractionation by gel filtration, affinity chromatography with immobilized lectins, and high performance liquid chromatography, several fractions that contained specific interpolysaccharide covalent linkages were isolated. Unique features of the A. fumigatus cell wall are (i) the absence of beta-1,6-glucan and (ii) the presence of a linear beta-1, 3/1,4-glucan, never previously described in fungi. Galactomannan, chitin, and beta-1,3-glucan were also found in the alkali-insoluble fraction. The beta-1,3-glucan is a branched polymer with 4% of beta-1,6 branch points. Chitin, galactomannan, and the linear beta-1, 3/1,4-glucan were covalently linked to the nonreducing end of beta-1, 3-glucan side chains. As in Saccharomyces cerevisiae, chitin was linked via a beta-1,4 linkage to beta-1,3-glucan. The data obtained suggested that the branching of beta-1,3-glucan is an early event in the construction of the cell wall, resulting in an increase of potential acceptor sites for chitin, galactomannan, and the linear beta-1,3/1,4-glucan.

Differential patterns of activity displayed by two exo-beta-1,3-glucanases associated with the Aspergillus fumigatus cell wall.

Two exo-beta-1,3-glucanases (herein designated exoG-I and exoG-II) were isolated from the cell wall autolysate of the filamentous fungus Aspergillus fumigatus and purified by ion-exchange, hydrophobic-interaction, and gel filtration chromatographies. Molecular masses estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration chromatography were 82 kDa for the monomeric exoG-I and 230 kDa for the dimeric exoG-II. exoG-I and exoG-II were glycosylated, and N glycans accounted, respectively, for 2 and 44 kDa. Their pH optimum is 5.0. Their optimum temperatures are 55 degrees C for exoG-I and 65 degrees C for exoG-II. By a sensitive colorimetric method and high-performance anion-exchange chromatography for product analysis, two patterns of exo-beta-1,3-glucanase activities were found. The 230-kDa exoG-II enzyme acts on p-nitrophenyl-beta-D-glucoside, beta-1,6-glucan, and beta-1,3-glucan. This activity, which retains the anomeric configuration of glucose released, presented a multichain pattern of attack of the glucan chains and a decrease in the maximum initial velocity (Vm) with the increasing size of the substrate. In contrast, the 82-kDa exoG-I, which inverts the anomeric configuration of the glucose released, hydrolyzed exclusively the beta-1,3-glucan chain with a minimal substrate size of 4 glucose residues. This enzyme presented a repetitive-attack pattern, characterized by an increase in Vm with an increase in substrate size and by a degradation of the glucan chain until it reached laminaritetraose, the limit substrate size. The 82-kDa exoG-I and 230-kDa exoG-II enzymes correspond to a beta-1,3-glucan-glucohydrolase (EC 3.2.1.58) and to a beta-D-glucoside-glucohydrolase (EC 3.2.1.21), respectively. The occurrence and functions of these two classes of exo-beta-1,3-glucanases in other fungal species are discussed.

A novel beta-(1-3)-glucanosyltransferase from the cell wall of Aspergillus fumigatus.

Cell wall transferases utilizing beta-(1-3)-glucan chains as substrates may play important roles in cell wall assembly and rearrangement, as beta-(1-3)-glucan is a major structural component of the cell wall of many fungi. A novel beta-(1-3)-glucanosyltransferase was purified to apparent homogenei ty from an autolysate of the cell wall of Aspergillus fumigatus. The enzyme had a molecular mass of 49 kDa and contained approximately 5 kDa of N-linked carbohydrate. The enzyme catalyzed an initial endo-type splitting of a beta-(1-3)-glucan molecule, followed by linkage of the newly generated reducing end to the nonreducing end of another beta-(1-3)-glucan molecule. Laminarioligosaccharides of size G10 and greater were donor substrates for the transferase. Laminarioligosaccharides of size G5 and greater formed acceptors. The enzyme was able to reuse initial transferase products as donors and acceptors in extended incubations, resulting in the formation of increasingly larger transferase products until they became insoluble. The major initial products from an incubation of the transferase with borohydride-reduced G11 (rG11) were rG6 and rG16. 1H NMR analysis of the rG16 transferase product showed it was a laminarioligosaccharide, indicating that the enzyme forms a beta-(1-3)-linkage during transfer. The enzyme may have a key function in vivo by allowing the integration of newly synthesized glucan into the wall and promoting cell wall expansion during cell growth.

Conformational behaviour of a synthetic peptide of the C-terminus of villin that interacts with actin: an NMR, CD and stimulated annealing study.

The solution structure of a synthetic 22-amino acid peptide (P1) corresponding to the extreme C-terminal end and one of the F-actin binding sites of villin has been determined by 1H NMR and CD spectroscopy. The structure of this peptide was compared to that of a peptide in which lysine to glutamic acid substitutions were introduced at positions 17 and 19 (P11), abolishing F-actin binding. Both peptides are largely unstructured in aqueous solution. Changes observed in the NMR and CD spectra of both peptides are consistent with alpha-helix formation in trifluoroethanol/water mixtures. A set of 189 interproton distances derived from nuclear Overhauser enhancement (NOE) measurements, 17 phi-angle constraints obtained from 3JNH alpha coupling constants, as well as about 10 N ... O distance restraints deduced from amide proton exchange kinetics with deuterium, were used for the structure determination. The three-dimensional structure of P1 and P11 is characterized by two helical regions, one extending from residues 2 to 5 and a second covering residues 7 to 17. The central fragment, ranging from Leu-7 to Leu-15, is more stable. The C-terminal residues are less structured, particularly within peptide P11. The significance of these structural results is discussed in relation to the biological activity of villin.

Sequence of a staphylococcal gene, vat, encoding an acetyltransferase inactivating the A-type compounds of virginiamycin-like antibiotics.

The Staphylococcus aureus plasmids, pIP680 and pIP1156, which confer resistance to A-type compounds of virginiamycin-like antibiotics (Vml: streptogramin A, pristinamycin IIA, virginiamycin M) and to synergistic mixtures of the A and B compounds of Vml antibiotics, were shown to direct the modification of A-type compounds by acetylation. The vat gene, encoding the acetyltransferase modifying A-type compounds, was isolated from plasmid pIP680 and sequenced. This gene potentially encodes a 219-amino-acid (aa) protein, VAT, of 24 330 Da showing at least 38% aa identity with two chloramphenicol acetyltransferases encoded by cat genes isolated from Escherichia coli and Agrobacterium tumefaciens. Resistance to A-type compounds of Vml antibiotics conferred to S. aureus by vat was not expressed in E. coli, although a protein having a M(r) similar to that encoded by this gene was detected in E. coli minicells. The vat gene was detected by the polymerase chain reaction in two chromosomally located staphylococcal conjugative elements and in the conjugative plasmid, pIP1156, conferring resistance to A-type compounds.