We show that reproductively mature male sea lampreys release a bile acid that acts as a potent sex pheromone, inducing preference and searching behavior in ovulated female lampreys. The secreted bile acid 7alpha,12alpha,24-trihydroxy-5alpha-cholan-3-one 24-sulfate was released in much higher amounts relative to known vertebrate steroid pheromones and may be secreted through the gills. Hence, the male of this fish species signals both its reproductive status and location to females by secreting a pheromone that can act over long distances.

In this work, we bridge the gap between short-range tunneling in molecular junctions and activated hopping in bulk organic films, and greatly extend the distance range of charge transport in molecular electronic devices. Three distinct transport mechanisms were observed for 4.5-22-nm-thick oligo(thiophene) layers between carbon contacts, with tunneling operative when d < 8 nm, activated hopping when d > 16 nm for high temperatures and low bias, and a third mechanism consistent with field-induced ionization of highest occupied molecular orbitals or interface states to generate charge carriers when d = 8-22 nm. Transport in the 8-22-nm range is weakly temperature dependent, with a field-dependent activation barrier that becomes negligible at moderate bias. We thus report here a unique, activationless transport mechanism, operative over 8-22-nm distances without involving hopping, which severely limits carrier mobility and device lifetime in organic semiconductors. Charge transport in molecular electronic junctions can thus be effective for transport distances significantly greater than the 1-5 nm associated with quantum-mechanical tunneling.all-carbon molecular junction | attenuation coefficient | field ionization | strong electronic coupling C harge transport mechanisms in organic and molecular electronics underlie the ultimate functionality of a new generation of electronic devices. Understanding, controlling, and designing molecular devices for use as practical components requires an intimate knowledge of the system energy levels and operative transport mechanisms, and how key variables such as molecule length, identity, temperature, etc., affect device performance parameters. Especially interesting in this context is the relationship between organic electronic devices, which typically have active layer thicknesses of tens to hundreds of nanometers, and molecular electronic devices reported to date, in which at least one dimension for charge propagation is below 10 nm. Indeed, many types of functional organic electronic devices have been demonstrated, including thinfilm transistors, organic light-emitting diodes, and memory cells (1, 2). Bridging the gap between organic and molecular devices may therefore reveal pathways for improving the performance of such devices, or even lead to new types of devices based on alternative transport mechanisms.The great majority of molecular electronic devices investigated to date have transport distances of <5 nm between the contacts, where the prevalent transport mechanism is quantum-mechanical tunneling. For this distance range, there is general agreement that the conductance scales exponentially with length, with an attenuation coefficient (β), defined as the slope of ln J vs. thickness (d), equal to 8 to 9 nm −1 for aliphatic molecules (3-6) and 2-3 nm −1 for aromatic molecules (7)(8)(9)(10)(11)(12)(13)(14). A few molecular electronic systems have been investigated beyond 5 nm (15, 16), some of which exhibit a decrease in β to less than 1 nm −1 . Such small values of β ar...

Molecular junctions are essentially modified electrodes familiar to electrochemists where the electrolyte is replaced by a conducting "contact." It is generally hypothesized that changing molecular structure will alter system energy levels leading to a change in the transport barrier. Here, we show the conductance of seven different aromatic molecules covalently bonded to carbon implies a modest range (<0.5 eV) in the observed transport barrier despite widely different free molecule HOMO energies (>2 eV range). These results are explained by considering the effect of bonding the molecule to the substrate. Upon bonding, electronic inductive effects modulate the energy levels of the system resulting in compression of the tunneling barrier. Modification of the molecule with donating or withdrawing groups modulate the molecular orbital energies and the contact energy level resulting in a leveling effect that compresses the tunneling barrier into a range much smaller than expected. Whereas the value of the tunneling barrier can be varied by using a different class of molecules (alkanes), using only aromatic structures results in a similar equilibrium value for the tunnel barrier for different structures resulting from partial charge transfer between the molecular layer and the substrate. Thus, the system does not obey the Schottky-Mott limit, and the interaction between the molecular layer and the substrate acts to influence the energy level alignment. These results indicate that the entire system must be considered to determine the impact of a variety of electronic factors that act to determine the tunnel barrier.energy alignment | molecular electronics | electronic coupling | charge transport | Fermi-level pinning T he conductance of electrical charge through and across molecular entities is the basis of molecular and organic electronics (1, 2). Understanding, controlling, and designing electronic circuits using organic molecules as components is a major goal of molecular electronics (3); however, it has been a challenge to identify all of the factors that govern the conductance of a molecular junction. Rather than being a simple property of the molecule itself, many circumstances contribute to the measured electronic properties of the junction. Some of the important features include the nature of the molecule-contact bonding (4), the properties of the contact materials (5, 6), the orientation of the molecules relative to the contacts (7), and the structure of the molecule (5,8,9). Although there is no general consensus on exactly how each of these features affects the conductance of the junction, it is generally agreed that the alignment of the molecular and contact energy levels is an important factor (10-13). The offset between the substrate Fermi energy (E f ) and the molecular orbital closest in energy to E f is often used to estimate charge transport barriers in the context of tunneling or charge injection models; however, it is increasingly clear that the situation is complex and that there is no simple meth...

HPPK-HP-MgAMPCPP mimics most closely the natural ternary complex of HPPK and provides details of protein-substrate interactions. The coordination of the two Mg(2+) ions helps create the correct geometry for the one-step reaction of pyrophosphoryl transfer, for which we suggest an in-line single displacement mechanism with some associative character in the transition state. The rigidity of the adenine-binding pocket and hydrogen bonds are responsible for adenosine specificity. The nonconserved residues that interact with the substrate might be responsible for the species-dependent properties of an isozyme.

The promise of molecular electronic devices stems from the possibilities offered by the rich electronic structure of organic molecules. The use of molecules as functional components in microelectronic devices has long been envisioned to augment or even replace silicon. However, the understanding of what controls charge transport in these devices involves complexities stemming from numerous variables that are often interactive and exert a controlling influence on transport, confounding the role of the molecular component. This perspective discusses various aspects of molecular electronics, from the initial "vision quests" of single molecule, functional electronic elements, to the molecular tunnel junctions that have been studied and characterized in-depth. Aspects of energy level alignment are discussed in the context of charge transport mechanisms, as are important electronic interactions when molecules are bonded to conducting "contacts". In addition, integration of molecular components with microelectronic processing is considered, as are the prospects for functional, real-world devices.

Background:The methylerythritol phosphate (MEP) pathway is required for the biosynthesis of plastid-derived isoprenoids from plants. Results: Deoxyxylulose-5-phosphate synthase (DXS) was cloned from Populus trichocarpa, and metabolic regulation was tested. Conclusion: Both isopentenyl diphosphate and dimethylallyl diphosphate inhibit DXS by competing with thiamine pyrophosphate. Significance: Prediction of isoprene emission from trees and bioengineering of MEP pathway will be aided by these results.

We report a "running start, two-bond" protocol to analyze elongation by human RNA polymerase II (RNAP II). In this procedure, the running start allowed us to measure rapid rates of elongation and provided detailed insight into the RNAP II mechanism. Formation of two bonds was tracked to ensure that at least one translocation event was analyzed. By using this method, RNAP II is stalled briefly at a defined template position before restoring the next NTP. Significantly, slow reaction steps are identified both before and after phosphodiester bond synthesis, and both of these steps can be highly dependent on the next templated NTP. The initial and final NTP-driven events, however, are not identical, because the slow step after chemistry, which includes translocation and pyrophosphate release, is regulated differently by elongation factors hepatitis ␦ antigen and transcription factor IIF. Because recovery from a stall and the processive transition from one bond to the next can be highly NTP-dependent, we conclude that translocation can be driven by the incoming substrate NTP, a model fully consistent with the RNAP II elongation complex structure.Pre-steady state kinetic analysis allows the progress of an enzymatic reaction to be tracked in real time (1, 2), and coupling enzyme functional dynamics to the structure provides the clearest insight into the mechanism. In this paper, we compare the first transient state kinetic studies of human (Homo sapiens) RNAP II 1 to the x-ray structure of the yeast (Saccharomyces cerevisiae) RNAP II elongation complex (EC) (3). These studies give new insight into the RNAP II mechanism and demonstrate the feasibility of a detailed kinetic study of a highly regulated enzyme that is at the hub of gene control in human cells.There is increasing recognition that transcriptional elongation is highly regulated in eukaryotes (4 -8). As an example, hepatitis ␦ antigen (HDAg) strongly stimulates RNAP II elongation in vitro (6, 9). HDAg is the sole gene product of the small RNA genome of hepatitis ␦ virus, which is maintained as a satellite particle by hepatitis B virus. The role of HDAg in elongation may be clinically significant because hepatitis ␦ virus often complicates severe and chronic presentations of human hepatitis B virus infection. The general cellular transcription factor IIF (TFIIF) has been shown to stimulate RNAP II elongation 5-10-fold in vitro, by suppressing transcriptional pausing (10 -16). The role of TFIIF in elongation may be of particular importance during the promoter escape phase of the transcription cycle (17, 18). Here viral HDAg and cellular TFIIF are used as probes of H. sapiens RNAP II elongation.In this work, we use rapid quench kinetics to demonstrate critical NTP-dependent steps during RNA synthesis. First, we analyzed recovery from a stall at a defined template position, in the presence of TFIIF or HDAg. During stall recovery, two fractions of EC were clearly observed on the active pathway, and most significantly, these ECs had different requirements for bindin...

This Article explores the idea of using nonmetallic contacts for molecular electronics. Metal-free, all-carbon molecular electronic junctions were fabricated by orienting a layer of organic molecules between two carbon conductors with high yield (>90%) and good reproducibility (rsd of current density at 0.5 V <30%). These all-carbon devices exhibit current density-voltage (J-V) behavior similar to those with metallic Cu top contacts. However, the all-carbon devices display enhanced stability to bias extremes and greatly improved thermal stability. Completed carbon/nitroazobenzene(NAB)/carbon junctions can sustain temperatures up to 300 °C in vacuum for 30 min and can be scanned at ±1 V for at least 1.2 × 10(9) cycles in air at 100 °C without a significant change in J-V characteristics. Furthermore, these all-carbon devices can withstand much higher voltages and current densities than can Cu-containing junctions, which fail upon oxidation and/or electromigration of the copper. The advantages of carbon contacts stem mainly from the strong covalent bonding in the disordered carbon materials, which resists electromigration or penetration into the molecular layer, and provides enhanced stability. These results highlight the significance of nonmetallic contacts for molecular electronics and the potential for integration of all-carbon molecular junctions with conventional microelectronics.