Stability of Coulomb systems in a magnetic field.

I study N electrons and M protons in a magnetic field. It is shown that the total energy per particle is bounded below by a constant independent of M and N, provided the fine structure constant is small. Here, the total energy includes the energy of the magnetic field.

Broken symmetry and strangeness of the semiconductor impurity band metal-insulator transition

The filamentary model of the metal-insulator transition in randomly doped semiconductor impurity bands is geometrically equivalent to similar models for continuous transitions in dilute antiferromagnets and even to the λ transition in liquid He, but the critical behaviors are different. The origin of these differences lies in two factors: quantum statistics and the presence of long range Coulomb forces on both sides of the transition in the electrical case. In the latter case, in addition to the main transition, there are two satellite transitions associated with disappearance of the filamentary structure in both insulating and metallic phases. These two satellite transitions were first identified by Fritzsche in 1958, and their physical origin is explained here in geometrical and topological terms that facilitate calculation of critical exponents.

Structure-based conformational preferences of amino acids

Proteins can be very tolerant to amino acid substitution, even within their core. Understanding the factors responsible for this behavior is of critical importance for protein engineering and design. Mutations in proteins have been quantified in terms of the changes in stability they induce. For example, guest residues in specific secondary structures have been used as probes of conformational preferences of amino acids, yielding propensity scales. Predicting these amino acid propensities would be a good test of any new potential energy functions used to mimic protein stability. We have recently developed a protein design procedure that optimizes whole sequences for a given target conformation based on the knowledge of the template backbone and on a semiempirical potential energy function. This energy function is purely physical, including steric interactions based on a Lennard-Jones potential, electrostatics based on a Coulomb potential, and hydrophobicity in the form of an environment free energy based on accessible surface area and interatomic contact areas. Sequences designed by this procedure for 10 different proteins were analyzed to extract conformational preferences for amino acids. The resulting structure-based propensity scales show significant agreements with experimental propensity scale values, both for α-helices and β-sheets. These results indicate that amino acid conformational preferences are a natural consequence of the potential energy we use. This confirms the accuracy of our potential and indicates that such preferences should not be added as a design criterion.

A “midinfrared” scenario for cuprate superconductivity

I conjecture that the mechanism of superconductivity in the cuprates is a saving, due to the improved screening resulting from Cooper pair formation, of the part of the Coulomb energy associated with long wavelengths and midinfrared frequencies. This scenario is shown to provide a plausible explanation of the trend of transition temperature with layering structure in the Ca-spaced compounds and to predict a spectacularly large decrease in the electron-energy-loss spectroscopy cross-section in the midinfrared region on transition to the superconducting state, as well as less spectacular but still surprisingly large changes in the optical behavior. Existing experimental results appear to be consistent with this picture.

Room-temperature single-electron junction.

The design, realization, and test performances of an electronic junction based on single-electron phenomena that works in the air at room temperature are hereby reported. The element consists of an electrochemically etched sharp tungsten stylus over whose tip a nanometer-size crystal was synthesized. Langmuir-Blodgett films of cadmium arachidate were transferred onto the stylus and exposed to a H2S atmosphere to yield CdS nanocrystals (30-50 angstrom in diameter) imbedded into an organic matrix. The stylus, biased with respect to a flat electrode, was brought to the tunnel distance from the film and a constant gap value was maintained by a piezo-electric actuator driven by a feedback circuit fed by the tunneling current. With this set-up, it is possible to measure the behavior of the current flowing through the quantum dot when a bias voltage is applied. Voltage-current characteristics measured in the system displayed single-electron trends such as a Coulomb blockade and Coulomb staircase and revealed capacitance values as small as 10(-19) F.

Protonation dynamics of the extracellular and cytoplasmic surface of bacteriorhodopsin in the purple membrane.

The dynamics of proton binding to the extracellular and the cytoplasmic surfaces of the purple membrane were measured by laser-induced proton pulses. Purple membranes, selectively labeled by fluorescein at Lys-129 of bacteriorhodopsin, were pulsed by protons released in the aqueous bulk from excited pyranine (8-hydroxy-1,3,6-pyrenetrisulfonate) and the reaction of protons with the indicators was measured. Kinetic analysis of the data imply that the two faces of the membrane differ in their buffer capacities and in their rates of interaction with bulk protons. The extracellular surface of the purple membrane contains one anionic proton binding site per protein molecule with pK = 5.1. This site is within a Coulomb cage radius (approximately 15 A) from Lys-129. The cytoplasmic surface of the purple membrane bears 4-5 protonable moieties (pK = 5.1) that, due to close proximity, function as a common proton binding site. The reaction of the proton with this cluster is at a very fast rate (3.10(10) M-1.s-1). The proximity between the elements is sufficiently high that even in 100 mM NaCl they still function as a cluster. Extraction of the chromophore retinal from the protein has a marked effect on the carboxylates of the cytoplasmic surface, and two to three of them assume positions that almost bar their reaction with bulk protons. The protonation dynamics determined at the surface of the purple membrane is of relevance both for the vectorial proton transport mechanism of bacteriorhodopsin and for energy coupling, not only in halobacteria, but also in complex chemiosmotic systems such as mitochondrial and thylakoid membranes.