Effects of antimicrobial peptides on membrane dynamics: A comparison of fluorescence and NMR experiments

Antimicrobial peptides (AMPs) represent a promising class of compounds to fight resistant infections. They are commonly thought to kill bacteria by perturbing the permeability of their cell membranes. However, bacterial killing requires a high coverage of the cell surface by bound peptides, at least in the case of cationic and amphipathic AMPs. Therefore, it is conceivable that peptide accumulation on the bacterial membranes might interfere with vital cellular functions also by perturbing bilayer dynamics, a hypothesis that has been termed "sand in the gearbox". Here we performed a systematic study of such possible effects, for two representative peptides (the cationic cathelicidin PMAP-23 and the peptaibol alamethicin), employing fluorescence and NMR spectroscopies. These approaches are commonly applied to characterize lipid order and dynamics, but sample different time-scales and could thus report on different membrane properties. In our case, fluorescence anisotropy measurements on liposomes labelled with probes localized at different depths in the bilayer showed that both peptides perturb membrane fluidity and order. Pyrene excimer-formation experiments showed a peptideinduced reduction in lipid lateral mobility. Finally, laurdan fluorescence indicated that peptide binding reduces water penetration below the headgroups region. Comparable effects were observed also in fluorescence experiments performed directly on live bacterial cells. By contrast, the fatty acyl chain order parameters detected by deuterium NMR spectroscopy remained virtually unaffected by addition of the peptides. The apparent discrepancy between the two techniques confirms previous sporadic observations and is discussed in terms of the different characteristic times of the two approaches. The perturbation of membrane dynamics in the ns timescale, indicated by the multiple fluorescence approaches reported here, could contribute to the antimicrobial activity of AMPs, by affecting the function of membrane proteins, which is strongly dependent on the physicochemical properties of the bilayer

Field emission of silicon nanowires: conditioning and stability

We report the low-pressure chemical vapor deposition growth and field emission characterization of silicon nanowires SiNWs . Our field emission results show the importance of the so called conditioning process on the reproducibility of the emission performance itself; this effect has proven to be reversible for the investigated current regime. We explained this behavior by invoking a current-driven desorption of residual adsorbed gases. A highly reproducible turn-on electric field of 27 V/ m is found for a diode-connected SiNW planar sample. Furthermore, stability analysis is performed showing the technologically promising field emission behavior of the samples

Towards the realization of multielectrode field emission devices : controlled growth of single walled carbon nanotube arrays

We reported the design and realization of a carbon nanotube-based integrated multielectrode device. Patterned Si/SiO2/Nb/Nb2O5 multilayer was successfully realized by means of a few, common photolithographic processes with the minimum number of mask alignment steps. Such structure constitutes the patterned substrate of successive Hot Filament Chemical Vapour Deposition (HFCVD) process. Selective growth of highly oriented SWCNT arrays was obtained in the predefined locations while survival of the entire structure was achieved. Field emission measurements of such materials were carried out. Good and reproducible field emission behaviour has been observed in several realized structures

Towards the realization of multielectrode field emission devices : controlled growth of single walled carbon nanotube arrays

We reported the design and realization of a carbon nanotube-based integrated multielectrode device. Patterned Si/SiO2/Nb/Nb2O5 multilayer was successfully realized by means of a few, common photolithographic processes with the minimum number of mask alignment steps. Such structure constitutes the patterned substrate of successive Hot Filament Chemical Vapour Deposition (HFCVD) process. Selective growth of highly oriented SWCNT arrays was obtained in the predefined locations while survival of the entire structure was achieved. Field emission measurements of such materials were carried out. Good and reproducible field emission behaviour has been observed in several realized structures

Structural MRI correlates of cognitive impairment in patients with multiple sclerosis: A Multicenter Study

In a multicenter setting, we applied voxel-based methods to different structural MR imaging modalities to define the relative contributions of focal lesions, normal-appearing white matter (NAWM), and gray matter (GM) damage and their regional distribution to cognitive deficits as well as impairment of specific cognitive domains in multiple sclerosis (MS) patients. Approval of the institutional review boards was obtained, together with written informed consent from all participants. Standardized neuropsychological assessment and conventional, diffusion tensor and volumetric brain MRI sequences were collected from 61 relapsing-remitting MS patients and 61 healthy controls (HC) from seven centers. Patients with ≥2 abnormal tests were considered cognitively impaired (CI). The distribution of focal lesions, GM and WM atrophy, and microstructural WM damage were assessed using voxel-wise approaches. A random forest analysis identified the best imaging predictors of global cognitive impairment and deficits of specific cognitive domains. Twenty-three (38%) MS patients were CI. Compared with cognitively preserved (CP), CI MS patients had GM atrophy of the left thalamus, right hippocampus and parietal regions. They also showed atrophy of several WM tracts, mainly located in posterior brain regions and widespread WM diffusivity abnormalities. WM diffusivity abnormalities in cognitive-relevant WM tracts followed by atrophy of cognitive-relevant GM regions explained global cognitive impairment. Variable patterns of NAWM and GM damage were associated with deficits in selected cognitive domains. Structural, multiparametric, voxel-wise MRI approaches are feasible in a multicenter setting. The combination of different imaging modalities is needed to assess and monitor cognitive impairment in MS