Excitonic Instability and Origin of the Mid-Gap States

In the framework of the two-band model of a doped semiconductor the self-consistent equations describing the transition into the excitonic insulator state are obtained for the 2D case. It is found that due to the exciton-electron interactions the excitonic phase may arise with doping in a semiconductor stable initially with respect to excitonic transition in the absence of doping. The effects of the strong interactions between electron (hole) Fermi-liquid (FL) and excitonic subsystems can lead to the appearance of the states lying in the middle of the insulating gap.Comment: 2 pages with 2 figures available upon request, LaTex Version 3.0 (PCTeX), to appear in the Proceedings of the M2S-HTSC IV Conferenc

Electron Transfer Across A Peptide-peptide Interface Within A Designed Metalloprotein

Mechanistic studies of biological electron-transfer (ET) reactions have involved the use of surface-derivatized proteins, protein−protein complexes, and polypeptide-bridged donor−acceptor compounds. These latter studies seek to use well-defined model systems to better define the role of the intervening protein matrix in mediating biological electron transfers. However, whereas many in vivo ET reactions occur across a noncovalent protein−protein interface, the primary role of the peptide spacers found in current model systems is to provide a covalent link between the donor and acceptor sites. As such, these systems are poorly suited to probe the mechanisms of ET reactions occurring across a peptide−peptide interface

From bubbles to foam: dilute to dense evolution of hadronic wave function at high energy

We derive the evolution of a hadronic light cone wave function with energy at weak coupling. Our derivation is valid both in the high and the low partonic density limit, and thus encompasses both the JIMWLK and the KLWMIJ evolution. The hadronic wave function is shown to evolve by the action of the Bogoliubov-type operator, which diagonalizes on the soft gluon sector the light-cone hamiltonian in the presence of an arbitrary valence charge density. We find explicitly the action of this operator on the soft as well as the valence degrees of freedom of the theory.Comment: 30 page

Phase boundaries in mixtures of membrane-forming amphiphiles and micelle-forming amphiphiles

AbstractThe phase behavior of mixtures of phospholipids and detergents in aqueous solutions is an issue of basic importance for understanding the solubilization and reconstitution of biological membranes. We review the existing knowledge on the compositionally induced reversible transformation of phospholipid bilayers into lipid-detergent mixed micelles. First, we describe the experimental protocols used for preparation of such mixtures and emphasize the scope and limitations of the various techniques used for evaluation of the microstructures of the self-assembled amphiphiles in the mixture. Subsequently, we interpret the existing data in terms of the spontaneous curvature of the amphiphiles and the finite size of the mixed micelles. These considerations lead to a general description of the phase behavior, which forms the basis for a rational approach to solubilization and reconstitution experiments

How lipid flippases can modulate membrane structure

AbstractPhospholipid flippases, are proteins able to translocate phospholipids from one side of a membrane to the other even against a gradient of concentration and thereby able to establish, or annihilate, a transmembrane asymmetrical lipid distribution. This lipid shuttling forms new membrane structures, in particular vesicles, which are associated with diverse physiological functions in eukaryotic cells such as lipid and protein traffic via vesicles between organelles or towards the plasma membrane, and the stimulation of fluid phase endocytosis. The transfer of lipids is also responsible for the triggering of membrane associated events such as blood coagulation, the recognition and elimination of apoptotic or aged cells, and the regulation of phosphatidylserine dependent enzymes. Exposure of new lipid-head groups on a membrane leaflet by rapid flip-flop can serve as a specific signal and, upon recognition, can be the cause of physiological modifications. Membrane bending is one of the mechanisms by which such activities can be triggered. We show that the lateral membrane tension is an important physical factor for the regulation of the size of the membrane invaginations. Finally, we suggest in this review that this diversity of functions benefits from the diversity of the lipids existing in a cell and the ability of proteins to recognize specific messenger molecules

Front-to-Rear Membrane Tension Gradient in Rapidly Moving Cells

AbstractMembrane tension is becoming recognized as an important mechanical regulator of motile cell behavior. Although membrane-tension measurements have been performed in various cell types, the tension distribution along the plasma membrane of motile cells has been largely unexplored. Here, we present an experimental study of the distribution of tension in the plasma membrane of rapidly moving fish epithelial keratocytes. We find that during steady movement the apparent membrane tension is ∼30% higher at the leading edge than at the trailing edge. Similar tension differences between the front and the rear of the cell are found in keratocyte fragments that lack a cell body. This front-to-rear tension variation likely reflects a tension gradient developed in the plasma membrane along the direction of movement due to viscous friction between the membrane and the cytoskeleton-attached protein anchors embedded in the membrane matrix. Theoretical modeling allows us to estimate the area density of these membrane anchors. Overall, our results indicate that even though membrane tension equilibrates rapidly and mechanically couples local boundary dynamics over cellular scales, steady-state variations in tension can exist in the plasma membranes of moving cells

Distance Dependence Of Electron Transfer Along Artificial Beta-strands At 298 And 77 K

Photoinduced electron-transfer rate constants were measured for a series of binuclear metallopeptides consisting of a [Ru(bpy)(2)(cmbpy)](2+) electron donor tethered to a Co-III(NH3)(5) electron acceptor by an oligovaline peptide chain (bpy = 2,2\u27-bipyridine, cmbpy = 4-carboxy-4\u27-methyl-2,2\u27-bipyridine). These compounds were shown by H-1 NMR to adopt the conformational properties found within the individual strands of a beta-pleated sheet in both aqueous and methanol solutions. Emission lifetime measurements and HPLC product analysis show that the binuclear donor/acceptor compounds undergo photoinduced electron transfer (ET). The values of k(et) decrease with increasing donor/acceptor distance according to the expression k(et) = k\u27 exp[-beta(r(-)r(0))]. A distance attenuation factor of beta = 1.1 +/- 0.4 Angstrom(-1) is seen both in H2O at 298 K and in an ethanol-methanol glass at 77 K. The ET kinetics obtained at 77 K for 1-3 were single exponential, indicating that the compounds maintain a unique donor/acceptor separation and do nor exist within multiple conformations. The similarity in behavior obtained under very different solvent conditions indicates that the electronic coupling term dominates the distance dependence of k(et)