2,854 research outputs found
Nonequilibrium transport through a quantum dot weakly coupled to Luttinger liquids
We study the nonequlibrium transport through a quantum dot weakly coupled to
Luttinger liquids (LL). A general current expression is derived by using
nonequilibrium Green function method. Then a special case of the dot with only
a single energy level is discussed. As a function of the dot's energy level, we
find that the current as well as differential conductance is strongly
renormalized by the interaction in the LL leads. In comparison with the system
with Fermi liquid (FL) leads, the current is suppressed, consistent with the
suppression of the electron tunneling density of states of the LL; and the
outset of the resonant tunneling is shifted to higher bias voltages. Besides,
the linear conductance obtained by Furusaki using master equation can be
reproduced from our result.Comment: 8 pages, 3 figures, Late
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THE DISCOVERY AND STUDY OF FLUVIRUCIN B1 POLYKETIDE SYNTHASE
Rapidly decreasing numbers of viable therapeutic leads in the pharmaceutical pipeline demand new, sustainable methods for improved drug discovery and development. Despite vast improvements in de novo drug design and target recognition, Nature remains the richest source of small molecule therapeutics. Among many natural products, polyketides are not only the most promising ones for developing new antibiotic leads, but also exhibit unusually high therapeutic value ranging from clinical use as anticancer, antiviral, and immunosuppressant drugs.
Modular polyketide synthases (PKSs) are dedicated nano-machinery that can be manipulated to produce a structurally diverse library for drug discovery programs. The ability to manipulate these natural systems to produce novel metabolites rests largely on increased mechanistic understanding of how these molecules are generated and how these processes can be manipulated. As impressive as their pharmaceutical properties are, the biosynthetic engineering potential of these compounds continues to draw widespread attention from the research community. Although some success has been realized in terms of polyketide structure diversification, severe limitations in engineered product output continue to impede efforts toward practical combinatorial biosynthesis. This thesis is focused on understanding and exploiting a new biosynthetic enzyme assembly and overcoming the engineering hurdles for making novel polyketide metabolites.
Fluvirucin B1, produced by Actinomadura vulgaris, is a 14-membered macrolactam active against a variety of infectious fungi as well as influenza A. Despite considerable interest from the synthetic community, very little information is available regarding the biosynthetic origins of the fluvirucins. Herein, we report the identification and initial characterization of the fluvirucin B1 polyketide synthase and related enzymes.
The cluster consists of five extender modules flanked by an N-terminal acyl carrier protein and C-terminal thioesterases domain. All but one of the synthase modules contain the full complement of tailoring domains (ketoreductase, dehydratase, and enoyl reductase) as determined by sequence homology with known polyketide synthases. Active site analyses of several key components of the cluster are performed to further verify that this gene cluster is associated with production of fluvirucin B1. This work will both open doors toward a better understanding of macrolactam formation and provide an avenue to genetics based diversification of fluvirucin structure
A Rate-Compatible Sphere-Packing Analysis of Feedback Coding with Limited Retransmissions
Recent work by Polyanskiy et al. and Chen et al. has excited new interest in
using feedback to approach capacity with low latency. Polyanskiy showed that
feedback identifying the first symbol at which decoding is successful allows
capacity to be approached with surprisingly low latency. This paper uses Chen's
rate-compatible sphere-packing (RCSP) analysis to study what happens when
symbols must be transmitted in packets, as with a traditional hybrid ARQ
system, and limited to relatively few (six or fewer) incremental transmissions.
Numerical optimizations find the series of progressively growing cumulative
block lengths that enable RCSP to approach capacity with the minimum possible
latency. RCSP analysis shows that five incremental transmissions are sufficient
to achieve 92% of capacity with an average block length of fewer than 101
symbols on the AWGN channel with SNR of 2.0 dB.
The RCSP analysis provides a decoding error trajectory that specifies the
decoding error rate for each cumulative block length. Though RCSP is an
idealization, an example tail-biting convolutional code matches the RCSP
decoding error trajectory and achieves 91% of capacity with an average block
length of 102 symbols on the AWGN channel with SNR of 2.0 dB. We also show how
RCSP analysis can be used in cases where packets have deadlines associated with
them (leading to an outage probability).Comment: To be published at the 2012 IEEE International Symposium on
Information Theory, Cambridge, MA, USA. Updated to incorporate reviewers'
comments and add new figure
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