Cross-polarization electron-nuclear double resonance spectroscopy.

Magnetic nuclei in the proximity of a paramagnetic center can be polarized through electron-nuclear cross-polarization and detected in electron-nuclear double resonance (ENDOR) spectroscopy. This principle is demonstrated in a single-crystal model sample as well as on a protein, the β2 subunit of E.coli ribonucleotide reductase (RNR), which contains an essential tyrosyl radical. ENDOR is a fundamental technique to detect magnetic nuclei coupled to paramagnetic centers. It is widely employed in biological and materials sciences. Despite its utility, its sensitivity in real samples is about one to two orders of magnitude lower than conventional electron paramagnetic resonance, thus restricting its application potential. Herein, we report the performance of a recently introduced concept to polarize nuclear spins and detect their ENDOR spectrum, which is based on electron-nuclear cross polarization (eNCP). A single-crystal study permits us to disentangle eNCP conditions and CP-ENDOR intensities, providing the experimental foundation in agreement with the theoretical prediction. The CP-ENDOR performance on a real protein sample is best demonstrated with the spectra of the essential tyrosyl radical in the β2 subunit of E.coli RNR

Standard map in magnetized relativistic systems: fixed points and regular acceleration

We investigate the concept of a standard map for the interaction of relativistic particles and electrostatic waves of arbitrary amplitudes, under the action of external magnetic fields. The map is adequate for physical settings where waves and particles interact impulsively, and allows for a series of analytical result to be exactly obtained. Unlike the traditional form of the standard map, the present map is nonlinear in the wave amplitude and displays a series of peculiar properties. Among these properties we discuss the relation involving fixed points of the maps and accelerator regimes.Comment: Work to appear in Phys. Rev. E. 2 figure

Nonlinear dynamics of inhomogeneous mismatched charged particle beams

This work analyzes the transversal dynamics of an inhomogeneous and mismatched charged particle beam. The beam is azimuthally symmetric, initially cold, and evolves in a linear channel permeated by an external constant magnetic field. Based on a Lagrangian approach, a low-dimensional model for the description of the beam dynamics has been obtained. The small set of nonlinear dynamical equations provided results that are in reasonable agreement with that ones observed in full self-consistent N-particle beam numerical simulations

Alternate islands of multiple isochronous chains in wave-particle interactions

We analyze the dynamics of a relativistic particle moving in a uniform magnetic field and perturbed by a standing electrostatic wave. We show that a pulsed wave produces an infinite number of perturbative terms with the same winding number, which may generate islands in the same region of phase space. As a consequence, the number of isochronous island chains varies as a function of the wave parameters. We observe that in all the resonances, the number of chains is related to the amplitude of the various resonant terms. We determine analytically the position of the periodic points and the number of island chains as a function of the wave number and wave period. Such information is very important when one is concerned with regular particle acceleration, since it is necessary to adjust the initial conditions of the particle to obtain the maximum acceleration.Comment: Submitte

Optimal regularizations for data generation with probabilistic graphical models

Understanding the role of regularization is a central question in Statistical Inference. Empirically, well-chosen regularization schemes often dramatically improve the quality of the inferred models by avoiding overfitting of the training data. We consider here the particular case of L 2 and L 1 regularizations in the Maximum A Posteriori (MAP) inference of generative pairwise graphical models. Based on analytical calculations on Gaussian multivariate distributions and numerical experiments on Gaussian and Potts models we study the likelihoods of the training, test, and 'generated data' (with the inferred models) sets as functions of the regularization strengths. We show in particular that, at its maximum, the test likelihood and the 'generated' likelihood, which quantifies the quality of the generated samples, have remarkably close values. The optimal value for the regularization strength is found to be approximately equal to the inverse sum of the squared couplings incoming on sites on the underlying network of interactions. Our results seem largely independent of the structure of the true underlying interactions that generated the data, of the regularization scheme considered, and are valid when small fluctuations of the posterior distribution around the MAP estimator are taken into account. Connections with empirical works on protein models learned from homologous sequences are discussed

Nonlinear dynamics of relativistic charged particle beams

The idea behind this work is to analyze the transversal dynamics of a relativistic charged particle beam. The beam is azimuthally symmetric, focused by a constant magnetic field and supposed to be initially cold. While mismatched, nonrelativistic, and homogeneous beams oscillate with an invariant cold density profile, it is shown that relativistic homogeneous beams progressively heat and lose an important amount of constituents during its magnetic confinement. This heating process starts with phase-space wave-breaking, a mechanism observed before in initially inhomogeneous beams. The results have been obtained with full self-consistent N-particle beam numerical simulations

Chaotic Interaction of Langmuir Solitons and Long Wavelength Radiation

In this work we analyze the interaction of isolated solitary structures and ion-acoustic radiation. If the radiation amplitude is small solitary structures persists, but when the amplitude grows energy transfer towards small spatial scales occurs. We show that transfer is particularly fast when a fixed point of a low dimensional model is destroyed.Comment: LaTex + 4 eps file

Statistical Mechanics of Unbound Two Dimensional Self-Gravitating Systems

We study, using both theory and molecular dynamics simulations, the relaxation dynamics of a microcanonical two dimensional self-gravitating system. After a sufficiently large time, a gravitational cluster of N particles relaxes to the Maxwell-Boltzmann distribution. The time to reach the thermodynamic equilibrium, however, scales with the number of particles. In the thermodynamic limit, $N\to\infty$ at fixed total mass, equilibrium state is never reached and the system becomes trapped in a non-ergodic stationary state. An analytical theory is presented which allows us to quantitatively described this final stationary state, without any adjustable parameters

Nickel (0) complexes as promising chemosensors for detecting the “cork taint” in wine

2,4,6-Trichloroanisole (TCA) is well recognized as one of the most responsible molecules of cork taint, an organoleptic defect of wine which represents a serious problem for wine industries. Up to now, very few examples of TCA-biosensors have been developed and we report herein a promising nickel (0) complex that can be employed as chemosensor for the TCA detection in cork stoppers. Among the three Ni (0) complexes studied in this work, complex Ni(0)(BINAP)(η2-PhCN) (2) showed the best reactivity towards pure TCA affording the oxidative addition product 4 in four hours at room temperature. Compound 4 represents an appealing probe for the indirect quantification of TCA due to the presence of the characteristic UV-adsorption band at 444 nm. Statistical studies on real samples confirmed that the presence of TCA can be detected by employing UV-Visible spectroscopy, as demonstrated by PCA analyses which allowed distinguishing TCA-contaminated samples from non-contaminated ones. Even if the present study has to be considered a preliminary approach for the realization of a chemosensor usable in real systems, the here reported Ni (0)-based sensing procedure represents the first examples of TCA chemical detection