Antimicrobial cationic amphiphiles derived from aminoglycoside pseudo-oligosaccharide antibiotics interfere with the structure and function of bacterial membranes and offer a promising direction for the development of novel antibiotics. Herein, we report the design and synthesis of cationic amphiphiles derived from the pseudo-trisaccharide aminoglycoside tobramycin and its pseudo-disaccharide segment nebramine. Antimicrobial activity, membrane selectivity, mode of action, and structure-activity relationships were studied. Several cationic amphiphiles showed marked antimicrobial activity, and one amphiphilic nebramine derivative proved effective against all of the tested strains of bacteria; furthermore, against several of the tested strains, this compound was well over an order of magnitude more potent than the parent antibiotic tobramycin, the membrane-targeting antimicrobial peptide mixture gramicidin D, and the cationic lipopeptide polymyxin B, which are in clinical use.
We show that the normal state transport properties of nano-scale granular Aluminum films, near the metal to insulator transition, present striking similarities with those of Kondo systems. Those include a negative magneto-resistance, a minimum of resistance R at a temperature Tm in metallic films, a logarithmic rise at low temperatures and a negative curvature of R (T ) at high temperatures. These normal state properties are interpreted in terms of spin-flip scattering of conduction electrons by local magnetic moments, possibly located at the metal/oxide interfaces. Their co-existence with the enhanced superconductivity seen in these films is discussed.PACS numbers: 74.81. Bd, 72.15.Qm Granular Al films have been known for many years to have an enhanced superconducting critical temperature. In this paper we show that in such films, conduction electrons interact with localized magnetic moments. This new finding is surprising since coexistence of an enhanced superconductivity with magnetic moments is unexpected.We present new transport measurements on aluminum films consisting of nano-scale Al grains, about 2 nm in size, weakly coupled through thin Al oxide barriers [1]. We find that near the metal to insulator transition (MIT) their magneto-resistance is increasingly negative and scales with (H/T), with an exponent close to 2, up to about 100 K. Additionally, samples having a positive resistance temperature coefficient (metallic behavior) present a minimum of resistance at a temperature T m of several 10 K depending on the film's resistivity and a temperature dependence of the resistance compatible with a logarithmic increase below T m . This logarithmic increase is more clearly seen in films whose resistance increases continuously with decreasing temperature. All metallic films near the MIT display a negative curvature of the R(T) curves. These transport properties point out to spin scattering of conducting electrons, as occurs in Kondo systems [2,3]. We discuss possible origins of localized magnetic moments and the compatibility of spin scattering of conduction electrons with the enhanced superconductivity seen in these films.Samples were prepared by thermal evaporation of 99.999% pure Al pellets from ceramic crucibles under a reduced pressure of oxygen in the range of 1 ÷ 3.5 × 10 −5 Torr. Substrates of Si − Si 2 O were cooled by liquid nitrogen during evaporation. The normal state resistivity, ρ RT , of the films was controlled by the oxygen pressure used during evaporation and by the evaporation rate.
The presence of free spins in granular Al films is directly demonstrated by µSR measurements. A Mott transition is observed by probing the increase of the spin-flip scattering rate of conduction electrons as the nano-size metallic grains are being progressively decoupled. Analysis of the magnetoresistance in terms of an effective Fermi energy shows that it becomes of the order of the grains electrostatic charging energy at a room temperature resistivity ρ ≈ 50, 000 µΩ cm, at which a metal to insulator transition is known to exist. As this transition is approached the magneto-resistance exhibits a Heavy-Fermion like behavior, consistent with an increased electron effective mass.PACS numbers: 74.81. Bd, 71.30.+h, 72.15.Qm, 74.25.Ha Thanks to advances in the development of the Density functional Mean Field Theory (DMFT) [1], considerable advances have been made in recent years towards a detailed understanding of the Mott metal to insulator transition, predicted to occur when the electron-electron interaction is of the order of the bandwidth [2]. However the experimental observation of this transition has remained a challenge in three dimensional systems. This is because in a homogeneous metal the Coulomb interaction is by several orders of magnitude smaller than the bandwidth, even in the presence of a relatively high concentration of impurities [3,4].We show here that a Mott transition takes place in granular metals, as nano-size grains are being decoupled from each other by a progressive reduction of the intergrain tunneling probability. Two of the main features of this transition predicted by DMFT theory, an increase of the electron effective mass and a non-critical behavior of the electronic density of states as the transition is approached, have been observed. These observations have been made possible by the presence of free spins in granular Aluminum films, which we confirm here by direct µSR measurements. Interaction of these spins with conduction electrons results in a negative magneto-resistance [5]. We have used it as a tool to follow changes of the effective Fermi energy of the granular medium as the transition is being approached. When it occurs, at a room temperature resistivity of about 50,000 µΩcm, we find that the effective Fermi energy is of the order of the grain's charging electrostatic energy. The superconducting critical temperature of the films remains relatively high up to close to the transition, indicating there is no drastic reduction of the density of states up to the transition.Low energy muon spin rotation/relaxation (LE µSR) experiments [6] were performed on film not too close to the MIT transition. The measurements were performed at the Swiss Muon Source on the µE4 beam-line, at the Paul Scherrer Institute, in Switzerland. With implantation energy of 10 keV all the muons stop in the 100 nm FIG. 1. Temperature dependence of the muon spin relaxation rate of electronic origin λ for a sample with ρ300K ≈ 140 µΩcm. λ appears to saturate around a temperature where ρ(T ) starts to increa...
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