Attempts to infer DNA electron transfer from fluorescence quenching measurements on DNA strands doped with donor and acceptor molecules have spurred intense debate over the question of whether or not this important biomolecule is able to conduct electrical charges. More recently, first electrical transport measurements on micrometre-long DNA 'ropes', and also on large numbers of DNA molecules in films, have indicated that DNA behaves as a good linear conductor. Here we present measurements of electrical transport through individual 10.4-nm-long, double-stranded poly(G)-poly(C) DNA molecules connected to two metal nanoelectrodes, that indicate, by contrast, large-bandgap semiconducting behaviour. We obtain nonlinear current-voltage curves that exhibit a voltage gap at low applied bias. This is observed in air as well as in vacuum down to cryogenic temperatures. The voltage dependence of the differential conductance exhibits a peak structure, which is suggestive of the charge carrier transport being mediated by the molecular energy bands of DNA.

Ian M. Wasser was born in 1973 in the suburbs of Detroit, Michigan and graduated from Detroit Country Day School. He received a B. S. Chemistry degree in chemistry from the University of Michigan in 1995, where he conducted undergraduate research under the direction of Professor James E. Penner-Hahn. This was then followed in 1998 by an M. S. degree in chemistry from the University of California at Berkeley, where he studied organometallic chemistry and X-ray crystallography with Professor Peter Vollhardt. He is currently a doctoral candidate, working toward his Ph.D. in bioinorganic chemistry under the direction of Professor Kenneth D. Karlin at Johns Hopkins University, where his research focuses on the interactions of synthetic iron and copper complexes with nitric oxide.

Electrical transport measurements are reported for double-stranded DNA molecules located between nanofabricated electrodes. We observe the absence of any electrical conduction through these DNA-based devices, both at the single-molecule level as well as for small bundles of DNA. We obtain a lower bound of 10 TΩ for the resistance of a DNA molecule at length scales larger than 40 nm. It is concluded that DNA is insulating. This conclusion is based on an extensive set of experiments in which we varied key parameters such as the base-pair sequence [mixed sequence and homogeneous poly(dG)⋅poly(dC)], length between contacts (40–500 nm), substrate (SiO2 or mica), electrode material (gold or platinum), and electrostatic doping fields. Discrepancies with other reports in the literature are discussed.

Using a combination of electronic spectroscopies, electronic structural descriptions have been developed for a series of binuclear Cu A -type centers in Bacillus subtilis CcO and engineered into the blue copper proteins Pseudomonas aeruginosa azurin and Thiobacillus Versutus amicyanin. Parallel descriptions are developed for two structurally characterized mixed-valence (MV) and homovalent (II,II) synthetic copper thiolate dimers. Assignment of the excited-state spectral features allows the electronic structures of Cu A and the MV model to be understood and compared in relation to their copper coordination environments. These electronic structural descriptions are supported by SCF-XR-SW MO calculations, which are used to test systematically the effects of major structural perturbations linking the MV model geometry to that of Cu A . It is determined that both Cu-Cu compression and removal of the axial ligands are critical determinants of the orbital ground state in these dimers. The weakened axial interactions in Cu A appear to parallel the mechanism for protein control of electron transfer (ET) function observed in blue copper centers. The major geometric and electronic features of Cu A , including metal-ligand covalency, redox potentials, reorganization energies, valence delocalization, and the weakened axial bonding interactions, are discussed in relation to its ET function, and specific potential ET pathways are identified and compared.

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A new and relatively simple procedure to purify NO reductase from Paracoccus denitrificans by using the detergent lauryl maltoside has been developed. The purified enzyme consists of two subunits according to SDS polyacrylamide gel electrophoresis. Analysis of the content of prosthetic groups indicates the presence of non-haem iron in addition to the presence b and c cytochromes yielding a stoichiometry of haem b/haem c/non-haem iron = 2:1:1. The optical spectrum of reduced NO reductase shows bands of low-spin haem c and haem b with alpha-band absorbance maxima at 551 nm and 558 nm, respectively. The optical spectrum of oxidized NO reductase shows a broad absorbance hand around 590 nm which disappears upon reduction. This latter absorbance is ascribed to a high-spin haem b (charge-transfer) transition. The presence of high-spin haem b is also indicated by the shifts observed in the optical spectrum of oxidized NO reductase in the presence of NO or in the spectrum of reduced enzyme after addition of CO. The main features of the EPR spectrum of the oxidized enzyme are resonances from a highly anisotropic low-spin haem b (gz = 3.53) and from an anisotropic low-spin haem c with gz, y, x = 2.99, 2.28, 1.46, the two haems being present in an approximate 1:1 stoichiometry. Minor signals representing about 1% of the enzyme concentration due to high-spin haem b (g = 5.8-6.2) and a novel type of signal with g = 2.009 ascribed to high-spin non-haem ferric iron were also observed. The analysis of steady-state kinetic measurements of the NO reductase activity shows a sigmoidal relation between rate of NO reduction and NO concentration, consistent with a model describing sequential binding of two molecules of NO to the reduced enzyme. At high NO concentrations substrate inhibition occurs (Ki(apparent) = 13.5 microM) suggested to be due to binding of NO to oxidized enzyme. The absence from the EPR spectrum of signals originating from ferric non-haem iron and ferric high-spin haem b in stoichiometric amounts with respect to the enzyme concentration is suggested to be due to an antiferromagnetic coupling between these two centers. The steady-state kinetic behaviour and the optical and EPR spectroscopic properties of the NO reductase are incorporated into a tentative structural and mechanistic model.

In Saccharomyces cerevisiae, the NDI1 gene encodes a mitochondrial NADH dehydrogenase, the catalytic side of which projects to the matrix side of the inner mitochondrial membrane. In addition to this NADH dehydrogenase, S. cerevisiae exhibits another mitochondrial NADH-dehydrogenase activity, which oxidizes NADH at the cytosolic side of the inner membrane. To investigate whether open reading frames YMR145c/NDE1 and YDL 085w/NDE2, which exhibit sequence similarity with NDI1, encode the latter enzyme, NADH-dependent mitochondrial respiration was assayed in wild-type S. cerevisiae and nde deletion mutants. Mitochondria were isolated from aerobic, glucose-limited chemostat cultures grown at a dilution rate (D) of 0.10 h ؊1 , in which reoxidation of cytosolic NADH by wild-type cells occurred exclusively by respiration. Compared with the wild type, rates of mitochondrial NADH oxidation were about 3-fold reduced in an nde1⌬ mutant and unaffected in an nde2⌬ mutant. NADH-dependent mitochondrial respiration was completely abolished in an nde1⌬ nde2⌬ double mutant. Mitochondrial respiration of substrates other than NADH was not affected in nde mutants. In shake flasks, an nde1⌬ nde2⌬ mutant exhibited reduced specific growth rates on ethanol and galactose but not on glucose. Glucose metabolism in aerobic, glucose-limited chemostat cultures (D ‫؍‬ 0.10 h ؊1 ) of an nde1⌬ nde2⌬ mutant was essentially respiratory. Apparently, under these conditions alternative systems for reoxidation of cytosolic NADH could replace the role of Nde1p and Nde2p in S. cerevisiae.

Almost all organisms require iron for enzymes involved in essential cellular reactions. Aerobic microbes living at neutral or alkaline pH encounter poor iron availability due to the insolubility of ferric iron. Assimilatory ferric reductases are essential components of the iron assimilatory pathway that generate the more soluble ferrous iron, which is then incorporated into cellular proteins. Dissimilatory ferric reductases are essential terminal reductases of the iron respiratory pathway in iron-reducing bacteria. While our understanding of dissimilatory ferric reductases is still limited, it is clear that these enzymes are distinct from the assimilatory-type ferric reductases. Research over the last 10 years has revealed that most bacterial assimilatory ferric reductases are flavin reductases, which can serve several physiological roles. This article reviews the physiological function and structure of assimilatory and dissimilatory ferric reductases present in the Bacteria, Archaea and Yeast. Ferric reductases do not form a single family, but appear to be distinct enzymes suggesting that several independent strategies for iron reduction may have evolved.