@article{3304, abstract = {Complex traits often involve interactions between different genetic loci. This can lead to sign epistasis, whereby mutations that are individually deleterious or neutral combine to confer a fitness benefit. In order to acquire the beneficial genotype, an asexual population must cross a fitness valley or plateau by first acquiring the deleterious or neutral intermediates. Here, we present a complete, intuitive theoretical description of the valley-crossing process across the full spectrum of possible parameter regimes. We calculate the rate at which a population crosses a fitness valley or plateau of arbitrary width, as a function of the mutation rates, the population size, and the fitnesses of the intermediates. We find that when intermediates are close to neutral, a large population can cross even wide fitness valleys remarkably quickly, so that valley-crossing dynamics may be common even when mutations that directly increase fitness are also possible. Thus the evolutionary dynamics of large populations can be sensitive to the structure of an extended region of the fitness landscape — the population may not take directly uphill paths in favor of paths across valleys and plateaus that lead eventually to fitter genotypes. In smaller populations, we find that below a threshold size, which depends on the width of the fitness valley and the strength of selection against intermediate genotypes, valley-crossing is much less likely and hence the evolutionary dynamics are less influenced by distant regions of the fitness landscape.}, author = {Daniel Weissman and Desai, Michael M and Fisher, Daniel S and Feldman, Marcus W}, journal = {Theoretical Population Biology}, number = {4}, pages = {286 -- 300}, publisher = {Academic Press}, title = {{The rate at which asexual populations cross fitness valleys}}, doi = {10.1016/j.tpb.2009.02.006}, volume = {75}, year = {2009}, } @article{3309, abstract = { Mastitis, a worldwide endemic disease of dairy cows, is an important cause of decreased efficiency in milk production. Early medical treatment can reduce the nonreversible losses in milk production caused by this infection. Various diagnostic tests for mastitis are available, including a test measuring the electrical conductivity of milk (MEC test), the industry standard of somatic cell counting (SCC test), a bacteriological test, and a recently developed test measuring mammary associated amyloid A (MAA test). None of these tests is considered a gold standard, however. The aim of the present study was to determine which of these tests provides the best results, and at what cost, to improve the efficiency of milk production. For this study, 25 cows were tested at all four quarters of the udder with each of the aforementioned mastitis diagnostic tests. Based on the data, the disease prevalence as well as the sensitivity and the specificity of the four tests were estimated with a Bayesian approach by extending the Hui and Walter model with two independent tests and two populations to a model with four partially dependent tests and one population. This model was further combined with a receiver operating characteristics analysis to estimate the overall test accuracy.}, author = {Caroline Uhler}, journal = {Journal of Agricultural Biological and Environmental Statistics}, number = {1}, pages = {79 -- 98}, publisher = {Springer}, title = {{Mastitis in dairy production: Estimation of sensitivity, specificity and disease prevalence in the absence of a gold standard}}, doi = {10.1198/jabes.2009.0005}, volume = {14}, year = {2009}, } @article{337, abstract = {The formation of hollow vs solid particles by means of the oxidation reaction of solid metal particles depends on the differential self-diffusivities of the reactants through the composite shell, the reaction probabilities at each interface, and the concentration and diffusivity of the element in solution. By means of a kinetic model of the oxidation process, we determine the phase diagrams for the geometry of the oxidized particles and propose four shell growth regimes. We experimentally illustrate the different growth scenarios by changing the conditions of oxidation of cadmium spherical crystals using different chalcogen precursors. }, author = {Cabot, Andreu and Ibáñez, Maria and Guardia, Pablo and Alivisatos, Paul}, journal = {Journal of the American Chemical Society}, number = {32}, pages = {11326 -- 11328}, publisher = {ACS}, title = {{Reaction regimes on the synthesis of hollow particles by the Kirkendall effect}}, doi = {10.1021/ja903751p}, volume = {131}, year = {2009}, } @article{3398, abstract = {Why is a particular architecture for a pathway chosen over seemingly equivalent alternatives? Çağatay et al. (2009) use a synthetic biology approach to show that fluctuations—or noise—in protein levels may play a key role in determining which network design is selected during evolution.}, author = {Bollenbach, Tobias and Kishony, Roy}, journal = {Cell}, number = {3}, pages = {460 -- 461}, publisher = {Cell Press}, title = {{Quiet gene circuit more fragile than its noisy peer}}, doi = {10.1016/j.cell.2009.10.005}, volume = {139}, year = {2009}, } @phdthesis{3400, abstract = {Invasive fungal infections pose a serious threat to immunocompromised people. Most of these infections are caused by either Candida or Aspergillus species, with A. fumigatus being the predominant causative agent of Invasive Aspergillosis. Affected people comprise mainly haematopoietic stem cell or solid organ transplant patients who receive either high-dose corticosteroids or immunosuppressants. These risk factors predispose to the development of Invasive Aspergillosis which is lethal in 20 to 80 % of the cases, largely due to insufficient efficacy of current antifungal therapy. Thus one major aim in current mycological research is the identification of new drug targets. The polysaccharide-based fungal cell wall is both essential to fungi and absent from human cells which makes it appear an attractive new target. Notably, many components of the A. fumigatus cell wall, including the polysaccharide galactomannan, glycoproteins, and glycolipids, contain the unusual sugar galactofuranose (Galf). In contrast to the other cell wall monosaccharides, Galf does not occur on human cells but is known as component of cell surface molecules of many pathogenic bacteria and protozoa, such as Mycobacterium tuberculosis or Leishmania major. These molecules are often essential for virulence or viability of these organisms which suggested a possible role of Galf in the pathogenicity of A. fumigatus. To address the importance of Galf in A. fumigatus, the key biosynthesis gene glfA, encoding UDPgalactopyranose mutase (UGM), was deleted. In different experimental approaches it was demonstrated that the absence of the glfA gene led to a complete loss of Galf-containing glycans. Analysis of the DeltaglfA phenotype revealed growth and sporulation defects, reduced thermotolerance and an increased susceptibility to antifungal drugs. Electron Microscopy indicated a cell wall defect as a likely cause for the observed impairments. Furthermore, the virulence of the DeltaglfA mutant was found to be severely attenuated in a murine model of Invasive Aspergillosis. The second focus of this study was laid on further elucidation of the galactofuranosylation pathway in A. fumigatus. In eukaryotes, a UDP-Galf transporter is likely required to transport UDP-Galf from the cytosol into the organelles of the secretory pathway, but no such activity had been described. Sixteen candidate genes were identified in the A. fumigatus genome of which one, glfB, was found in close proximity to the glfA gene. In vitro transport assays revealed specificity of GlfB for UDP-Galf suggesting that glfB encoded indeed a UDP-Galf transporter. The influence of glfB on galactofuranosylation was determined by a DeltaglfB deletion mutant, which closely recapitulated the DeltaglfA phenotype and was likewise found to be completely devoid of Galf. It could be concluded that all galactofuranosylation processes in A. fumigatus occur in the secretory pathway, including the biosynthesis of the cell wall polysaccharide galactomannan whose subcellular origin was previously disputed. Thus in the course of this study the first UDP-Galf specific nucleotide sugar transporter was identified and its requirement for galactofuranosylation in A. fumigatus demonstrated. Moreover, it was shown that blocking the galactofuranosylation pathway impaired virulence of A. fumigatus which suggests the UDP-Galf biosynthesis enzyme UGM as a target for new antifungal drugs.}, author = {Philipp Schmalhorst}, pages = {1 -- 72}, publisher = {Gottfried Wilhelm Leibniz Universität Hannover}, title = {{Biosynthesis of Galactofuranose Containing Glycans and Their Relevance for the Pathogenic Fungus Aspergillus fumigatus}}, year = {2009}, } @article{3408, abstract = {Mechanical forces govern physiological processes in all living organisms. Many cellular forces, for example, those generated in cyclic conformational changes of biological machines, have repetitive components. In apparent contrast, little is known about how dynamic protein structures respond to periodic mechanical information. Ubiquitin is a small protein found in all eukaryotes. We developed molecular dynamics simulations to unfold single and multimeric ubiquitins with periodic forces. By using a coarse-grained representation, we were able to model forces with periods about 2 orders of magnitude longer than the protein's relaxation time. We found that even a moderate periodic force weakened the protein and shifted its unfolding pathways in a frequency- and amplitude-dependent manner. A complex dynamic response with secondary structure refolding and an increasing importance of local interactions was revealed. Importantly, repetitive forces with broadly distributed frequencies elicited very similar molecular responses compared to fixed-frequency forces. When testing the influence of pulling geometry on ubiquitin's mechanical stability, it was found that the linkage involved in the mechanical degradation of cellular proteins renders the protein remarkably insensitive to periodic forces. We also devised a complementary kinetic energy landscape model that traces these observations and explains periodic-force, single-molecule measurements. In turn, this analytical model is capable of predicting dynamic protein responses. These results provide new insights into ubiquitin mechanics and a potential mechanical role during protein degradation, as well as first frameworks for dynamic protein stability and the modeling of repetitive mechanical processes.}, author = {Szymczak, Piotr and Harald Janovjak}, journal = {Journal of Molecular Biology}, number = {3}, pages = {443 -- 456}, publisher = {Elsevier}, title = {{Periodic forces trigger a complex mechanical response in ubiquitin}}, doi = {10.1016/j.jmb.2009.04.071}, volume = {390}, year = {2009}, } @article{13414, abstract = {Supraspherical aggregates of crosslinked metal nanoparticles are transformed into pancakes and nanorods by mechanical stresses and shears imparted by macroscopic objects (see image). The dimensions of both types of nanostructures can be controlled by the pressures applied.}, author = {Browne, Kevin P. and Klajn, Rafal and Villa, JulieAnn and Grzybowski, Bartosz A.}, issn = {1613-6829}, journal = {Small}, keywords = {Biomaterials, Biotechnology, General Materials Science, General Chemistry}, number = {23}, pages = {2656--2658}, publisher = {Wiley}, title = {{Mechanofabrication of pancake and rodlike nanostructures from deformable nanoparticle aggregates}}, doi = {10.1002/smll.200900902}, volume = {5}, year = {2009}, } @article{13415, abstract = {Systems in which nanoscale components of different types can be captured and/or released from organic scaffolds provide a fertile basis for the construction of dynamic, exchangeable functional materials. In such heterogeneous systems, the components interact with one another by means of programmable, noncovalent bonding interactions. Herein, we describe polymers that capture and release functionalized nanoparticles selectively during redox-controlled aggregation and disaggregation, respectively. The interactions between the polymer and the NPs are mediated by the reversible formation of polypseudorotaxanes, and give rise to architectures ranging from short chains composed of few nanoparticles to extended networks of nanoparticles crosslinked by the polymer. In the latter case, the polymer/nanoparticle aggregates precipitate from solution such that the polymer acts as a selective ‘sponge’ for the capture/release of the nanoparticles of different types.}, author = {Klajn, Rafal and Olson, Mark A. and Wesson, Paul J. and Fang, Lei and Coskun, Ali and Trabolsi, Ali and Soh, Siowling and Stoddart, J. Fraser and Grzybowski, Bartosz A.}, issn = {1755-4349}, journal = {Nature Chemistry}, keywords = {General Chemical Engineering, General Chemistry}, pages = {733--738}, publisher = {Springer Nature}, title = {{Dynamic hook-and-eye nanoparticle sponges}}, doi = {10.1038/nchem.432}, volume = {1}, year = {2009}, } @article{13416, abstract = {The reversible molecular template-directed self-assembly of gold nanoparticles (AuNPs), a process which relies solely on noncovalent bonding interactions, has been demonstrated by high-resolution transmission electron microscopy (HR-TEM). By employing a well-known host−guest binding motif, the AuNPs have been systemized into discrete dimers, trimers, and tetramers. These nanoparticulate twins, triplets, and quadruplets, which can be disassembled and reassembled either chemically or electrochemically, can be coalesced into larger, permanent polygonal structures by thermal treatment using a focused HR-TEM electron beam.}, author = {Olson, Mark A. and Coskun, Ali and Klajn, Rafal and Fang, Lei and Dey, Sanjeev K. and Browne, Kevin P. and Grzybowski, Bartosz A. and Stoddart, J. Fraser}, issn = {1530-6992}, journal = {Nano Letters}, keywords = {Mechanical Engineering, Condensed Matter Physics, General Materials Science, General Chemistry, Bioengineering}, number = {9}, pages = {3185--3190}, publisher = {American Chemical Society}, title = {{Assembly of polygonal nanoparticle clusters directed by reversible noncovalent bonding interactions}}, doi = {10.1021/nl901385c}, volume = {9}, year = {2009}, } @article{13417, abstract = {Mission Impossible: Metal nanoparticles (NPs) coated with photoresponsive ligands are used as “inks” for self-erasing “paper” whereby light-induced self-assembly of the NPs is transduced into local color changes (see picture). Depending on the degree of self-assembly, multicolor images can be written using only one type of NP ink. Duration of image erasure is regulated by the surface concentration of photoactive groups and can range from seconds to days.}, author = {Klajn, Rafal and Wesson, Paul J. and Bishop, Kyle J. M. and Grzybowski, Bartosz A.}, issn = {1521-3773}, journal = {Angewandte Chemie International Edition}, keywords = {General Chemistry, Catalysis}, number = {38}, pages = {7035--7039}, publisher = {Wiley}, title = {{Writing self-erasing images using metastable nanoparticle “inks”}}, doi = {10.1002/anie.200901119}, volume = {48}, year = {2009}, } @article{13418, abstract = {In traditional photoconductors1,2,3, the impinging light generates mobile charge carriers in the valence and/or conduction bands, causing the material’s conductivity to increase4. Such positive photoconductance is observed in both bulk and nanostructured5,6 photoconductors. Here we describe a class of nanoparticle-based materials whose conductivity can either increase or decrease on irradiation with visible light of wavelengths close to the particles’ surface plasmon resonance. The remarkable feature of these plasmonic materials is that the sign of the conductivity change and the nature of the electron transport between the nanoparticles depend on the molecules comprising the self-assembled monolayers (SAMs)7,8 stabilizing the nanoparticles. For SAMs made of electrically neutral (polar and non-polar) molecules, conductivity increases on irradiation. If, however, the SAMs contain electrically charged (either negatively or positively) groups, conductivity decreases. The optical and electrical characteristics of these previously undescribed inverse photoconductors can be engineered flexibly by adjusting the material properties of the nanoparticles and of the coating SAMs. In particular, in films comprising mixtures of different nanoparticles or nanoparticles coated with mixed SAMs, the overall photoconductance is a weighted average of the changes induced by the individual components. These and other observations can be rationalized in terms of light-induced creation of mobile charge carriers whose transport through the charged SAMs is inhibited by carrier trapping in transient polaron-like states9,10. The nanoparticle-based photoconductors we describe could have uses in chemical sensors and/or in conjunction with flexible substrates.}, author = {Nakanishi, Hideyuki and Bishop, Kyle J. M. and Kowalczyk, Bartlomiej and Nitzan, Abraham and Weiss, Emily A. and Tretiakov, Konstantin V. and Apodaca, Mario M. and Klajn, Rafal and Stoddart, J. Fraser and Grzybowski, Bartosz A.}, issn = {1476-4687}, journal = {Nature}, keywords = {Multidisciplinary}, number = {7253}, pages = {371--375}, publisher = {Springer Nature}, title = {{Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles}}, doi = {10.1038/nature08131}, volume = {460}, year = {2009}, } @article{13419, abstract = {Reaction-diffusion (RD) processes initiated from the surfaces of mesoscopic particles can fabricate complex core-and-shell structures. The propagation of a sharp RD front selectively removes metal colloids or nanoparticles from the supporting gel or polymer matrix. Once fabricated, the core structures can be processed “remotely” via galvanic replacement reactions, and the composite particles can be assembled into open-lattice crystals.}, author = {Wesson, Paul J. and Soh, Siowling and Klajn, Rafal and Bishop, Kyle J. M. and Gray, Timothy P. and Grzybowski, Bartosz A.}, issn = {1521-4095}, journal = {Advanced Materials}, keywords = {Mechanical Engineering, Mechanics of Materials, General Materials Science}, number = {19}, pages = {1911--1915}, publisher = {Wiley}, title = {{“Remote” fabrication via three-dimensional reaction-diffusion: Making complex core-and-shell particles and assembling them into open-lattice crystals}}, doi = {10.1002/adma.200802964}, volume = {21}, year = {2009}, } @article{13420, abstract = {Weakly protected metal nanoparticles (MNPs) are used as precursors for the preparation of catenane- and pseudorotaxane-decorated NPs of various compositions (gold, palladium, platinum). When attached to the surface of MNPs, the molecular switches retain their switching abilities. The redox potentials of these switches depend on and can be regulated by the composition of the mixed self-assembled monolayers covering the MNPs.}, author = {Klajn, Rafal and Fang, Lei and Coskun, Ali and Olson, Mark A. and Wesson, Paul J. and Stoddart, J. Fraser and Grzybowski, Bartosz A.}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, keywords = {Colloid and Surface Chemistry, Biochemistry, General Chemistry, Catalysis}, number = {12}, pages = {4233--4235}, publisher = {American Chemical Society}, title = {{Metal nanoparticles functionalized with molecular and supramolecular switches}}, doi = {10.1021/ja9001585}, volume = {131}, year = {2009}, } @article{13421, abstract = {Side-chain poly[2]catenanes at the click of a switch! A bistable side-chain poly[2]catenane has been synthesized and found to form hierarchical self-assembled hollow superstructures of nanoscale dimensions in solution. Molecular electromechanical switching (see picture) of the material is demonstrated, and the ground-state equilibrium thermodynamics and switching kinetics are examined as the initial steps towards processible molecular-based electronic devices and nanoelectromechanical systems.}, author = {Olson, Mark A. and Braunschweig, Adam B. and Fang, Lei and Ikeda, Taichi and Klajn, Rafal and Trabolsi, Ali and Wesson, Paul J. and Benítez, Diego and Mirkin, Chad A. and Grzybowski, Bartosz A. and Stoddart, J. Fraser}, issn = {1521-3773}, journal = {Angewandte Chemie International Edition}, keywords = {General Chemistry, Catalysis}, number = {10}, pages = {1792--1797}, publisher = {Wiley}, title = {{A bistable poly[2]catenane forms nanosuperstructures}}, doi = {10.1002/anie.200804558}, volume = {48}, year = {2009}, } @article{17809, abstract = {Supermassive black hole binaries (SMBHBs) in galactic nuclei are thought to be a common by-product of major galaxy mergers. We use simple disk models for the circumbinary gas and for the binary–disk interaction to follow the orbital decay of SMBHBs with a range of total masses (M) and mass ratios (q), through physically distinct regions of the disk, until gravitational waves (GWs) take over their evolution. Prior to the GW-driven phase, the viscous decay is generically in the stalled "secondary-dominated" regime. SMBHBs spend a non-negligible fraction of a fiducial time of 107 yr at orbital periods between days ≲torb≲ yr, and we argue that they may be sufficiently common to be detectable, provided they are luminous during these stages. A dedicated optical or X-ray survey could identify coalescing SMBHBs statistically, as a population of periodically variable quasars, whose abundance obeys the scaling Nvar ∝ tαvar within a range of periods around tvar∼ tens of weeks. SMBHBs with M ≲ 107 M☉, with 0.5 ≲ α ≲ 1.5, would probe the physics of viscous orbital decay, whereas the detection of a population of higher-mass binaries, with α = 8/3, would confirm that their decay is driven by GWs. The lowest-mass SMBHBs (M ≲ 105–6 M☉) enter the GW-driven regime at short orbital periods, when they are already in the frequency band of the Laser Interferometric Space Antenna (LISA). While viscous processes are negligible in the last few years of coalescence, they could reduce the amplitude of any unresolved background due to near-stationary LISA sources. We discuss modest constraints on the SMBHB population already available from existing data, and the sensitivity and sky coverage requirements for a detection in future surveys. SMBHBs may also be identified from velocity shifts in their spectra; we discuss the expected abundance of SMBHBs as a function of their orbital velocity.}, author = {Haiman, Zoltán and Kocsis, Bence and Menou, Kristen}, issn = {0004-637X}, journal = {The Astrophysical Journal}, number = {2}, pages = {1952--1969}, publisher = {American Astronomical Society}, title = {{The population of viscosity- and gravitational wave-driven supermassive black hole binaries among luminous active galactic nuclei}}, doi = {10.1088/0004-637x/700/2/1952}, volume = {700}, year = {2009}, } @article{1798, abstract = {The mammalian brain is assembled from thousands of neuronal cell types that are organized in distinct circuits to perform behaviorally relevant computations. Transgenic mouse lines with selectively marked cell types would facilitate our ability to dissect functional components of complex circuits. We carried out a screen for cell type-specific green fluorescent protein expression in the retina using BAC transgenic mice from the GENSAT project. Among others, we identified mouse lines in which the inhibitory cell types of the night vision and directional selective circuit were selectively labeled. We quantified the stratification patterns to predict potential synaptic connectivity between marked cells of different lines and found that some of the lines enabled targeted recordings and imaging of cell types from developing or mature retinal circuits. Our results suggest the potential use of a stratification-based screening approach for characterizing neuronal circuitry in other layered brain structures, such as the neocortex.}, author = {Sandra Siegert and Scherf, Brigitte G and Del Punta, Karina and Didkovsky, Nick and Heintz, Nathaniel M and Roska, Botond M}, journal = {Nature Neuroscience}, number = {9}, pages = {1197 -- 1204}, publisher = {Nature Publishing Group}, title = {{Genetic address book for retinal cell types}}, doi = {10.1038/nn.2370}, volume = {12}, year = {2009}, } @article{1799, abstract = {The detection of approaching objects, such as looming predators, is necessary for survival. Which neurons and circuits mediate this function? We combined genetic labeling of cell types, two-photon microscopy, electrophysiology and theoretical modeling to address this question. We identify an approach-sensitive ganglion cell type in the mouse retina, resolve elements of its afferent neural circuit, and describe how these confer approach sensitivity on the ganglion cell. The circuit's essential building block is a rapid inhibitory pathway: it selectively suppresses responses to non-approaching objects. This rapid inhibitory pathway, which includes AII amacrine cells connected to bipolar cells through electrical synapses, was previously described in the context of night-time vision. In the daytime conditions of our experiments, the same pathway conveys signals in the reverse direction. The dual use of a neural pathway in different physiological conditions illustrates the efficiency with which several functions can be accommodated in a single circuit.}, author = {Münch, Thomas A and Da Silveira, Ravá A and Sandra Siegert and Viney, Tim J and Awatramani, Gautam B and Roska, Botond M}, journal = {Nature Neuroscience}, number = {10}, pages = {1308 -- 1316}, publisher = {Nature Publishing Group}, title = {{Approach sensitivity in the retina processed by a multifunctional neural circuit}}, doi = {10.1038/nn.2389}, volume = {12}, year = {2009}, } @article{18028, abstract = {We measure the conductance and current–voltage characteristics of two amine-terminated molecular wires— 4,4'-diaminostilbene and bis-(4-aminophenyl)acetylene—by breaking Au point contacts in a molecular solution at room temperature. Histograms compiled from thousands of measurements show a slight increase in the molecular junction conductance (I/V) as the bias is increased to nearly 450 mV. Comparatively, similar conductance measurements made with 1,6-diaminohexane, a saturated molecule, demonstrate almost no bias dependence. We also present a new technique to measure a statistically defined current–voltage (I–V) curve. Application to all three molecules shows that 4,4'-diaminostilbene exhibits the largest increase in differential conductance as a function of applied bias. This indicates that the predominant transport channel for 4,4'-diaminostilbene (the highest occupied molecular orbital) is closer to the Fermi level of the metal than that of the other molecules, consistent with the trends observed in the molecular ionization potential. We find that junctions constructed with the conjugated molecules show greater noise in individual junctions and less structural stability, on average, at biases greater than 450 mV. In contrast, junctions formed with the alkane can sustain a bias of up to 900 mV. This significantly affects the statistically averaged I–V characteristic measured for the conjugated molecules at higher bias.}, author = {Widawsky, J R and Kamenetska, M and Klare, J and Nuckolls, C and Steigerwald, M L and Hybertsen, M S and Venkataraman, Latha}, issn = {1361-6528}, journal = {Nanotechnology}, number = {43}, publisher = {IOP Publishing}, title = {{Measurement of voltage-dependent electronic transport across amine-linked single-molecular-wire junctions}}, doi = {10.1088/0957-4484/20/43/434009}, volume = {20}, year = {2009}, } @article{18029, abstract = {We compare the conductance of 1,4-bis(methylthio)benzene with that of 2,3,6,7-tetrahydrobenzo[1,2-b:4,5-b′]dithiophene and the conductance of 1,4-bis(methylseleno)benzene with that of 2,3,6,7-tetrahydrobenzo[1,2-b:4,5-b′]diselenophene and show explicitly that the orientation of an Au−S or Au−Se bond relative to the aromatic π system controls electron transport through conjugated molecules. Specifically, we have found that the conduction pathway connects the Au electrodes to the aromatic π-system via the chalcogen p lone pairs, and greater overlaps among these components lead to higher conductivity through the molecular junction.}, author = {Park, Young S. and Widawsky, Jonathan R. and Kamenetska, Maria and Steigerwald, Michael L. and Hybertsen, Mark S. and Nuckolls, Colin and Venkataraman, Latha}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {31}, pages = {10820--10821}, publisher = {American Chemical Society}, title = {{Frustrated rotations in single-molecule junctions}}, doi = {10.1021/ja903731m}, volume = {131}, year = {2009}, } @article{18030, abstract = {We analyze the formation and evolution statistics of single-molecule junctions bonded to gold electrodes using amine, methyl sulfide, and dimethyl phosphine link groups by measuring conductance as a function of junction elongation. For each link, the maximum elongation and formation probability increase with molecular length, strongly suggesting that processes other than just metal-molecule bond breakage play a key role in junction evolution under stress. Density functional theory calculations of adiabatic trajectories show sequences of atomic-scale changes in junction structure, including shifts in the attachment point, that account for the long conductance plateau lengths observed.}, author = {Kamenetska, M. and Koentopp, M. and Whalley, A. C. and Park, Y. S. and Steigerwald, M. L. and Nuckolls, C. and Hybertsen, M. S. and Venkataraman, Latha}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {12}, publisher = {American Physical Society}, title = {{Formation and evolution of single-molecule junctions}}, doi = {10.1103/physrevlett.102.126803}, volume = {102}, year = {2009}, }