arXiv subjects

E. Vigezzi

Publications and source records attributed to E. Vigezzi.

62 records · Page 4Linked to original sources

arXiv1998

Folding and Aggregation of Designed Proteins

Studies of how protein fold have shown that the way protein clumps form in the test tube is similar to how proteins form the so-called ``amyloid'' deposits that are the pathological signal of a variety of diseases, among them the memory disorder Alzheimer's. Protein aggregation have traditionally been connected to either unfolded or native states. Inclusion body formation (disordered aggregation) has been assumed to arise from hydrophobic aggregation of the unfolded or denaturated states, while the amyloid fibrils (ordered aggregation) have been assumed to arise from native-like conformations in a process analogous to the polymerization of hemoglobin S. Making use of lattice-model simulations we find that both ordered and disordered aggregation arise from elementary structures which eventually build the folding nucleus of the heteropolymers, and takes place when some of the most strongly interacting amino acids establish their contacts leading to the formation of a specific subset of the native structure. These elementary structures can be viewed as the partially folded intermediates suggested to be involved in the aggregation of a number of proteins. These results have evolutionary implications, as the elementary structures forming the folding core of designed proteins contain the residues which are conserved among the members of homologous sequences.

cond-mat↗

arXiv1998

Electron-Phonon Coupling in Charged Buckminsterfullerene

A simple, yet accurate solution of the electron-phonon coupling problem in C_{60} is presented. The basic idea behind it is to be found in the parametrization of the ground state electronic density of the system calculated making use of ab-initio methods, in term of sp$^{2+x}$ hybridized orbitals. This parametrization allows for an economic determination of the deformation potential associated with the fullerene's normal modes. The resulting electron-phonon coupling constants are used to calculate Jahn-Teller effects in C_{60}^-, and multiple satellite peaks in the corresponding photoemission reaction. Theory provides an accurate account of the experimental findings.

cond-mat↗

arXiv1997

Folding and Misfolding of Designed Heteropolymer Chains with Mutations

We study the impact of mutations (changes in amino acid sequence) on the thermodynamics of simple protein-like heteropolymers consisting of N monomers, representing the amino acid sequence. The sequence is designed to fold into its native conformation on a cubic lattice. It is found that quite a large fraction, between one half and one third of the substitutions, which we call 'cold errors', make important contributions to the dynamics of the folding process, increasing folding times typically by a factor of two, the altered chain still folding into the native structure. Few mutations ('hot errors'), have quite dramatic effects, leading to protein misfolding. Our analysis reveals that mutations affect primarily the energetics of the native conformation and to a much lesser extent the ensemble of unfolded conformations, corroborating the utility of the ``energy gap'' concept for the analysis of folding properties of protein-like heteropolymers.

cond-mat↗

arXiv1997

Level Statistics of Near-Yrast States in Rapidly Rotating Nuclei

The nearest neighbour level spacing distribution and the $Δ_3$ statistics of level fluctuations associated with very high spin states ($I \gesim 30$) in rare-earth deformed nuclei are analysed by means of a cranked shell model. The many particle-many hole configurations created in the rotating Nilsson potential are mixed by the surface-delta two-body residual interaction. The levels in the near-yrast region show a Poisson-like level spacing distribution. As the intrinsic excitation energy $U$ increases, the level statistics shows a gradual transition from order to chaos, reaching at $U \gesim 2$ MeV the Wigner distribution typical of the Gaussian orthogonal ensemble of random matrices. This transition is caused by the residual two-body interaction. On the other hand, the level spacings between the yrast and the first excited state show a peculiar behaviour, displaying a Wigner-like distribution instead of the Poisson-like distribution seen for the other near-yrast rotational states. The lowest spacings reflect the properties of the single-particle orbits in the mean-field, and are only weakly affected by the residual two-body interaction.

nucl-th↗

arXiv1997

Shell Model for Warm Rotating Nuclei

In order to provide a microscopic description of levels and E2 transitions in rapidly rotating nuclei with internal excitation energy up to a few MeV, use is made of a shell model which combines the cranked Nilsson mean-field and the residual surface delta two-body force. The damping of collective rotational motion is investigated in the case of a typical rare-earth nucleus, namely \Yb. It is found that rotational damping sets in at around 0.8 MeV above the yrast line, and the levels which form rotational band structures are thus limited. We predict at a given rotational frequency existence of about 30 rotational bands of various lengths, in overall agreement with the experimental findings. The onset of the rotational damping proceeds quite gradually as a function of the internal excitation energy. The transition region extends up to around 2 MeV above yrast and it is characterized by the presence of scars of discrete rotational bands which extend over few spin values and stand out among the damped transitions, and by a two-component profile in the $E_γ-E_γ$ correlation. The important role played by the high-multipole components of the two-body residual interaction is emphasized.

nucl-th↗

arXiv1996

The Spectral Line Shape of Exotic Nuclei

The quadrupole strength function of $^{28}O$ is calculated making use of the SIII interaction, within the framework of continuum-RPA and taking into account collisions among the nucleons (doorway coupling). The centroid of the giant resonance is predicted at $\approx 14$ MeV, that is much below the energy expected for both isoscalar and isovector quadrupole resonances in nuclei along the stability valley. About half of this width arises from the coupling of the resonance to the continuum and about half is due to doorway coupling. This result is similar to that obtained in the study of giant resonances in light, $β$-stable nuclei, and shows the lack of basis for the expectation, entertained until now in the literature, that continuum decay was the main damping mechanism of giant resonances in halo nuclei.

nucl-th↗

arXiv1992

Chaotic Behavior in Warm Deformed Nuclei Induced by Residual Two-Body Interactions

Band mixing calculations in rapidly rotating well-deformed nuclei are presented, investigating the properties of energy levels and rotational transitions as a function of excitation energy. Substantial fragmentation of E2 transitions is found for $E_x \gsim$ 800 keV above yrast, which represents the onset of rotational damping. Above $E_x \approx $ 2 MeV, energy levels and E2 strengths display fluctuations typical of quantum chaotic systems, which are determined by the high multipole components of the two-body residual interaction.

nucl-th↗

arXiv1992

Microscopic Origin of Quantum Chaos in Rotational Damping

The rotational spectrum of $^{168}$Yb is calculated diagonalizing different effective interactions within the basis of unperturbed rotational bands provided by the cranked shell model. A transition between order and chaos taking place in the energy region between 1 and 2 MeV above the yrast line is observed, associated with the onset of rotational damping. It can be related to the higher multipole components of the force acting among the unperturbed rotational bands.

nucl-th↗