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We investigate the properties of plasma turbulence by means of two-dimensional Hall-magnetohydrodynamic (HMHD) and hybrid particle-in-cell (HPIC) numerical simulations. We find that the HMHD simulations exhibit spectral properties that are in most cases in agreement with the results of the HPIC simulations and with solar wind observations. The energy spectra of magnetic fluctuations exhibit a double power law with spectral index −5/3 at MHD scales and −3 at kinetic scales, while for velocity fluctuations the spectral index is −3/2 at MHD scales. The break between the MHD and the kinetic scales occurs at the same scale in both simulations. In the MHD range the slopes of the total energy and residual energy spectra satisfy a fast Alfvén-dynamo balance. The development of a turbulent cascade is concurrently characterized by magnetic reconnection events taking place in thin current sheets that form between large eddies. A statistical analysis reveals that reconnection is qualitatively the same and fast in both the HMHD and HPIC models, characterized by inverse reconnection rates much smaller than the characteristic large-eddy nonlinear time. The agreement extends to other statistical properties, such us the kurtosis of the magnetic field. Moreover, the observation of a direct energy transfer from the large vortices to the small sub-ion scales, triggered by magnetic reconnection, further supports the existence of a reconnection-mediated turbulent regime at kinetic scales. We conclude that the HMHD fluid description captures to a large extent the transition of the turbulent cascade between the large MHD scales and the sub-ion scales. [ABSTRACT FROM AUTHOR]

Several aspects of the mechanism of action of Pro-rich antimicrobial peptides, together with their low toxicity in mammalian cells, make them good candidates for the development of new antibiotic agents. We investigated the effect induced in the insect antimicrobial peptide apidaecin Ib by the replacement of a single arginine/leucine residue with a N-substituted glycine. The resulting peptoid−peptide hybrids are more resistant to proteolysis and devoid of any significant cytotoxic activity, but moving the [NArg]residue from the N- to the C-terminal end of the molecule progressively reduces the antibacterial activity. Cell uptake experiments in E. colicells suggest that the loss of antibacterial activity of [NArg17]apidaecin is a consequence of its inability to translocate into bacterial cells. Conversely, apidaecin and its peptoid−peptide hybrids are able to cross the plasma membrane in eukaryotic cells and to diffuse in the cytosol, although their translocating ability is far less effective than that of other known cell permeant peptides. [ABSTRACT FROM AUTHOR]

The relationship between a decaying plasma turbulence and proton fire hose instabilities in a slowly expanding plasma is investigated using three-dimensional hybrid expanding box simulations. We impose an initial ambient magnetic field along the radial direction, and we start with an isotropic spectrum of large-scale, linearly polarized, random-phase Alfvénic fluctuations with zero cross-helicity. A turbulent cascade rapidly develops and leads to a weak proton heating that is not sufficient to overcome the expansion-driven perpendicular cooling. The plasma system eventually drives the parallel and oblique fire hose instabilities that generate quasi-monochromatic wave packets that reduce the proton temperature anisotropy. The fire hose wave activity has a low amplitude with wave vectors quasi-parallel/oblique with respect to the ambient magnetic field outside of the region dominated by the turbulent cascade and is discernible in one-dimensional power spectra taken only in the direction quasi-parallel/oblique with respect to the ambient magnetic field; at quasi-perpendicular angles the wave activity is hidden by the turbulent background. These waves are partly reabsorbed by protons and partly couple to and participate in the turbulent cascade. Their presence reduces kurtosis, a measure of intermittency, and the Shannon entropy, but increases the Jensen–Shannon complexity of magnetic fluctuations; these changes are weak and anisotropic with respect to the ambient magnetic field and it is not clear if they can be used to indirectly discern the presence of instability-driven waves. [ABSTRACT FROM AUTHOR]