Structure and Mode-of-Action of the Two-Peptide (Class-IIb) Bacteriocins

This review focuses on the structure and mode-of-action of the two-peptide (class-IIb) bacteriocins that consist of two different peptides whose genes are next to each other in the same operon. Optimal antibacterial activity requires the presence of both peptides in about equal amounts. The two peptides are synthesized as preforms that contain a 15–30 residue double-glycine-type N-terminal leader sequence that is cleaved off at the C-terminal side of two glycine residues by a dedicated ABC-transporter that concomitantly transfers the bacteriocin peptides across cell membranes. Two-peptide bacteriocins render the membrane of sensitive bacteria permeable to a selected group of ions, indicating that the bacteriocins form or induce the formation of pores that display specificity with respect to the transport of molecules. Based on structure–function studies, it has been proposed that the two peptides of two-peptide bacteriocins form a membrane-penetrating helix–helix structure involving helix–helix-interacting GxxxG-motifs that are present in all characterized two-peptide bacteriocins. It has also been suggested that the membrane-penetrating helix–helix structure interacts with an integrated membrane protein, thereby triggering a conformational alteration in the protein, which in turn causes membrane-leakage. This proposed mode-of-action is similar to the mode-of-action of the pediocin-like (class-IIa) bacteriocins and lactococcin A (a class-IId bacteriocin), which bind to a membrane-embedded part of the mannose phosphotransferase permease in a manner that causes membrane-leakage and cell death

Repeatability of Corticospinal and Spinal Measures during Lengthening and Shortening Contractions in the Human Tibialis Anterior Muscle

Elements of the human central nervous system (CNS) constantly oscillate. In addition, there are also methodological factors and changes in muscle mechanics during dynamic muscle contractions that threaten the stability and consistency of transcranial magnetic stimulation (TMS) and perpherial nerve stimulation (PNS) measures

Optical resonances of indium islands on GaAs(001) observed by reflectance anisotropy spectroscopy

The optical properties of indium islands on GaAs(001) surfaces have been studied by reflectance anisotropy spectroscopy as a function of metal coverage. A large optical anisotropy is observed, which shows an oscillatory behavior and scales with the island size: mean island sizes determined by scanning electron microscope correspond to the wavelengths where extremes in the optical anisotropy arise. We explain this behavior by surface plasmon resonances of the island structure which induce a huge optical anisotropy related to the anisotropic shape and distribution of the In islands. Model calculations of the reflectance anisotropy spectroscopy signal based on a layer system where the island film is represented by an effective medium consisting of ellipsoidal metal particles in a vacuum matrix reproduce the main oscillation and support our conclusion

Ge growth on GaAs(001) surfaces studied by reflectance anisotropy spectroscopy

Surface modifications induced by germanium deposition onto clean GaAs(001) substrates have been monitored by reflectance anisotropy spectroscopy (RAS) and low energy electron diffraction (LEED). The clean GaAs(001) surfaces onto which Ge has been evaporated were c(4 x 4), (2 x 4), and (4 x 1) reconstructed. Regardless of the initial surface reconstruction, after deposition of 0.5 monolayers of Ge and further annealing at 850 K, we have always observed a (1 x 2) LEED pattern and the same characteristic RAS spectrum. On the contrary, overlayer structures obtained at intermediate stages between the clean surface and this (1 x 2) phase depend upon the initial surface reconstruction. Modifying the (1 x 2) reconstructed surface by deposition of additional monolayers of Ge or exposure to atmosphere, we have separated the surface, interface, and bulk contributions to the RAS spectra. Finally, monitoring the characteristic linear-electro-optical feature appearing at E1 and E1 + Δ1 bulk critical points, we discuss how its change in amplitude and sign could be connected to a variation of the substrate doping induced by annealing

Ge/GaAs(001) interface formation investigated by reflectance anisotropy spectroscopy

The formation of the Ge/GaAs(001) interface has been investigated following the transformation of an As-dimer terminated GaAs(001)(2 X 4) surface into a Ge-Ga-dimer terminated (1 X 2) reconstruction and the subsequent deposition up to 10 ML of Ge. The modification of the surface atomic geometry and the related electronic structure has been monitored by reflectance anisotropy spectroscopy (RAS) and low-energy electron diffraction. Experimental results are compared to density-functional-theory-local-density-approximation and tight-binding calculations of the surface structure and optical response, respectively. The comparison between calculated and measured RAS spectra allows us to show that the (2 X 4) structure transforms into a well-ordered (1 X 2) passing through a disordered (2 X 4) phase while a previously proposed intermediate (2 X 1) structure is ruled out. At higher Ge coverages, surface and Ge/GaAs-interface contributions to the optical spectra are separated by surface modification through exposure to atmosphere. A interface contribution is identified between 1.5 eV and 2.5 eV, almost identical in line shape and amplitude to the RAS features on the Ge-Ga-dimer terminated GaAs surface. This finding demonstrates that the backbonds of the Ge-Ga-dimers, present at the Ge-Ga-dimer terminated surface as well as at the Ge/GaAs interface, determine the optical anisotropy, whereas the Ge-Ga-dimer bond itself does not contribute significantly. [S0163-1829(99)12315-4]

Optical characterization of indium-terminated GaAs(001) surfaces

We have investigated the growth of thin indium layers on As- and Ga-terminated GaAs(001) surfaces by reflectance anisotropy spectroscopy (RAS), low energy electron diffraction (LEED), and auger electron spectroscopy. Room temperature deposition of indium on the (2 X 4)/c(2 X 8) surface and subsequent annealing at 450 degrees C leads to the formation of an In-terminated surface showing a (4X2) LEED pattern, accompanied with strong changes in the measured surface optical anisotropy. When indium is deposited onto the (4X2)/c(8 X2) surface, on the contrary, the (4X2) In-terminated surface is already formed at room temperature deposition without needing annealing, as demonstrated by the RAS spectra. The finding that almost identical RAS spectra and (4X2) LEED patterns are obtained in both cases shows that the same final atomic structure is achieved. Finally, we conclude that the structure of the In-terminated surface is similar to that of the clean Ga-rich surface, although a more detailed model would need accurate calculations of the microscopic origin of the measured anisotropy

Surfactant-mediated growth of indium on GaAs(001)

The influence of antimony as surfactant on the growth mode of indium on GaAs(001) has been investigated for deposition at various temperatures. Clean surfaces of (2 × 4) reconstruction were prepared in UHV by thermal desorption of a protective arsenic cap deposited on top of homoepitaxially grown MBE layers. Before In deposition, 2 ML Sb were pre-deposited on top of the clean GaAs surface of part of the samples. The In growth mode was then monitored by AES and compared with that occurring during In growth on GaAs without an Sb interlayer. After removal of the samples from UHV, SEM investigations show regularly shaped, flat In islands oriented along the 〈110〉 substrate directions. In comparison, In islands grown without an Sb interlayer also show a preferential orientation along the 〈110〉 substrate directions, but differ in shape and size. Low-temperature In deposition (120 K) leads to the formation of a continuous In layer, which tears during warm up to room temperature. An exchange between surfactant material and indium during growth is found for both low- and room-temperature In deposition. X-ray diffraction measurements show a strong influence of the Sb surfactant on the crystallinity of the In film

Detection of surface states anisotropies at GaAs(001)(2 × 4) decapped surfaces

The surface or bulk origin of the optical anisotropies detected by reflectance anisotropy spectroscopy (RAS) at GaAs(001)(2 × 4) surfaces has been extensively investigated in the last years and a quite general agreement has been reached that the dominating character would be bulk‐like. Nevertheless, a very recent paper [F. Arciprete et al., Phys. Rev. B 69, 081308(R) (2004)] has again issued the presence of surface states contributions in optical anisotropies, revealing a structure at 2.5 eV due to surface states, in addition to the well known features around 2.9 eV and 4.5 eV related to the bulk critical points E1 and E0′. We have carried out a new experiment to prove this conclusion by following the changes in the optical anisotropy of a GaAs(001)(2 × 4) surface in the range 2.0–5.0 eV induced by Ag/Sb‐codeposition. The interface Ag/GaAs(001) is known to be not reactive. Due to its surfactant effect, codeposition of Sb leads to a nearly epitaxial growth of the Ag overlayer. We show that at the early stages of deposition (nominally at 0.25 monolayer) an evident modification of the RAS spectrum is detected at 2.5 eV, well below the photon energy (2.9 eV) where bulk‐like anisotropies appear. We relate this modification to the disappearance of surface states characteristic of the (2 × 4) reconstruction, in excellent agreement with previous conclusions and experiments. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim