569 research outputs found
Identifying Proteins of High Designability via Surface-Exposure Patterns
Using an off-lattice model, we fully enumerate folded conformations of
polypeptide chains of up to N = 19 monomers. Structures are found to differ
markedly in designability, defined as the number of sequences with that
structure as a unique lowest-energy conformation. We find that designability is
closely correlated with the pattern of surface exposure of the folded
structure. For longer chains, complete enumeration of structures is
impractical. Instead, structures can be randomly sampled, and relative
designability estimated either from designability within the random sample, or
directly from surface-exposure pattern. We compare the surface-exposure
patterns of those structures identified as highly designable to the patterns of
naturally occurring proteins.Comment: 17 pages, 12 figure
Electrical Conductance of Molecular Wires
Molecular wires (MW) are the fundamental building blocks for molecular
electronic devices. They consist of a molecular unit connected to two continuum
reservoirs of electrons (usually metallic leads). We rely on Landauer theory as
the basis for studying the conductance properties of MW systems. This relates
the lead to lead current to the transmission probability for an electron to
scatter through the molecule. Two different methods have been developed for the
study of this scattering. One is based on a solution of the Lippmann-Schwinger
equation and the other solves for the {\bf t} matrix using Schroedinger's
equation. We use our methodology to study two problems of current interest. The
first MW system consists of 1,4 benzene-dithiolate (BDT) bonded to two gold
nanocontacts. Our calculations show that the conductance is sensitive to the
chemical bonding between the molecule and the leads. The second system we study
highlights the interesting phenomenon of antiresonances in MW. We derive an
analytic formula predicting at what energies antiresonances should occur in the
transmission spectra of MW. A numerical calculation for a MW consisting of
filter molecules attached to an active molecule shows the existence of an
antiresonance at the energy predicted by our formula.Comment: 14 pages, 5 figure
The role of energy productivity in the U.S. agriculture
This paper investigates the role of energy on U.S. agricultural productivity using panel data at the state level for the period 1960-2004. We first provide a historical account of energy use in U.S. agriculture. To do this we rely on the Bennet cost indicator to study how the price and volume components of energy costs have developed over time. We then proceed to analyze the contribution of energy to productivity in U.S. agriculture employing the Bennet-Bowley productivity indicator. An important feature of the Bennet-Bowley indicator is its direct association with the change in (normalized) profits. Thus our study is also able to analyze the link between profitability and productivity in U.S. agriculture. Panel regression estimates indicate that energy prices have a negative effect on profitability in the U.S. agricultural sector. We also find that energy productivity has generally remained below total farm productivity following the 1973-1974 global energy crisis
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