The structures of E. coli NfsA bound to the antibiotic nitrofurantoin; to 1,4-benzoquinone and to FMN

NfsA is a dimeric flavoprotein that catalyses the reduction in nitroaromatics and quinones by NADPH. This reduction is required for the activity of nitrofuran antibiotics. The crystal structure of free Escherichia coli NfsA and several homologues have been determined previously, but there is no structure of the enzyme with ligands. We present here crystal structures of oxidised E. coli NfsA in the presence of several ligands, including the antibiotic nitrofurantoin. Nitrofurantoin binds with the furan ring, rather than the nitro group that is reduced, near the N5 of the FMN. Molecular dynamics simulations show that this orientation is only favourable in the oxidised enzyme, while potentiometry suggests that little semiquinone is formed in the free protein. This suggests that the reduction occurs by direct hydride transfer from FMNH(−) to nitrofurantoin bound in the reverse orientation to that in the crystal structure. We present a model of nitrofurantoin bound to reduced NfsA in a viable hydride transfer orientation. The substrate 1,4-benzoquinone and the product hydroquinone are positioned close to the FMN N5 in the respective crystal structures with NfsA, suitable for reaction, but are mobile within the active site. The structure with a second FMN, bound as a ligand, shows that a mobile loop in the free protein forms a phosphate-binding pocket. NfsA is specific for NADPH and a similar conformational change, forming a phosphate-binding pocket, is likely to also occur with the natural cofactor

Identifying the coiled-coil triple helix structure of β-peptide nanofibers at atomic resolution

Peptide self-assembly represents a powerful bottom-up approach to the fabrication of new nanomaterials. β3-peptides are non-natural peptides composed entirely of β-amino acids, which have an extra methylene in the backbone and we reported the first fibers derived from the self-assembly of β3-peptides that adopt unique 14-helical structures. β3-peptide assemblies represent a class of stable nanomaterials that can be used to generate bio- and magneto-responsive materials with proteolytic stability. However, the three-dimensional structure of many of these materials remains unknown. In order to develop structure-based criteria for the design of new β3-peptide-based biomaterials with tailored function, we investigated the structure of a tri-β3-peptide nanoassembly by molecular dynamics simulations and X-ray fiber diffraction analysis. Diffraction data was collected from aligned fibrils formed by Ac-β3[LIA] in water and used to inform and validate the model structure. Models with threefold radial symmetry resulted in stable fibers with a triple-helical coiled-coil motif and measurable helical pitch and periodicity. The fiber models revealed a hydrophobic core and twist along the fiber axis arising from a maximization of contacts between hydrophobic groups of adjacent tripeptides on the solvent-exposed fiber surface. These atomic structures of macro-scale fibers derived from β3-peptide-based materials provide valuable insight into the effects of the geometric placement of the side-chains and the influence of solvent on the core fiber structure which is perpetuated in the superstructure morphology

Surface dynamics and ligand-core interactions of quantum sized photoluminescent gold nanoclusters

Quantum-sized metallic clusters protected by biological ligands represent a new class of luminescent materials; yet the understanding of structural information and photoluminescence origin of these ultrasmall clusters remains a challenge. Herein we systematically study the surface ligand dynamics and ligand–metal core interactions of peptide-protected gold nanoclusters (AuNCs) with combined experimental characterizations and theoretical molecular simulations. We show that the peptide sequence plays an important role in determining the surface peptide structuring, interfacial water dynamics and ligand–Au core interaction, which can be tailored by controlling peptide acetylation, constituent amino acid electron donating/withdrawing capacity, aromaticity/hydrophobicity and by adjusting environmental pH. Specifically, emission enhancement is achieved through increasing the electron density of surface ligands in proximity to the Au core, discouraging photoinduced quenching, and by reducing the amount of surface-bound water molecules. These findings provide key design principles for understanding the surface dynamics of peptide-protected nanoparticles and maximizing the photoluminescence of metallic clusters through the exploitation of biologically relevant ligand properties

Low Background Signal Readout Electronics for the MAJORANA DEMONSTRATOR

The MAJORANA DEMONSTRATOR is a planned 40 kg array of Germanium detectors intended to demonstrate the feasibility of constructing a tonne-scale experiment that will seek neutrinoless double beta decay ($0\nu\beta\beta$) in $^{76}\mathrm{Ge}$. Such an experiment would require backgrounds of less than 1 count/tonne-year in the 4 keV region of interest around the 2039 keV Q-value of the $\beta\beta$ decay. Designing low-noise electronics, which must be placed in close proximity to the detectors, presents a challenge to reaching this background target. This paper will discuss the MAJORANA collaboration's solutions to some of these challenges