Hennie Valkenier | Université libre de Bruxelles (original) (raw)
Papers by Hennie Valkenier
Nature Nanotechnology, 2012
Physical Chemistry Chemical Physics, 2014
As the dimensions of a conductor approach the nanoscale, quantum effects begin to dominate, and i... more As the dimensions of a conductor approach the nanoscale, quantum effects begin to dominate, and it becomes possible to control the conductance through direct manipulation of the electron wavefunction. Such control has been demonstrated in various mesoscopic devices at cryogenic temperatures, but it has proved to be difficult to exert control over the wavefunction at higher temperatures. Molecules have typical energy level spacings (~eV) that are much larger than the thermal energy at 300 K (~25 meV), and are therefore natural candidates for such experiments. Previously, phenomena such as giant magnetoresistance, Kondo effects and conductance switching have been observed in single molecules, and theorists have predicted that it should also be possible to observe quantum interference in molecular conductors, but until now all the evidence for such behaviour has been indirect. Here, we report the observation of destructive quantum interference in charge transport through two-terminal molecular junctions at room temperature. We studied five different rigid π-conjugated molecular wires, all of which form self-assembled monolayers on a gold surface, and find that the degree of interference can be controlled by simple chemical modifications of the molecular wire.
Chemical communications (Cambridge, England), Jan 11, 2015
Tris-N-arylthioureas derived in one step from 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene are r... more Tris-N-arylthioureas derived in one step from 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene are remarkably effective anion carriers. With optimised aryl substituents their activities come close to the best currently known, suggesting that they might find use as readily available standards in anion transport research.
Nano Letters, 2015
It is understood that molecular conjugation plays an important role in charge transport through s... more It is understood that molecular conjugation plays an important role in charge transport through single-molecule junctions. Here, we investigate electron transport through an anthraquinone based single-molecule three-terminal device. With the use of an electric-field induced by a gate electrode, the molecule is reduced resulting into a 10-fold increase in the off-resonant differential conductance. Theoretical calculations link the change in differential conductance to a reduction-induced change in conjugation, thereby lifting destructive interference of transport pathways.
Journal of the American Chemical Society, Jan 8, 2015
Biotin[6]uril hexaesters represent a new class of anionophores which operate solely through C-H··... more Biotin[6]uril hexaesters represent a new class of anionophores which operate solely through C-H···anion interactions. The use of soft H-bond donors favors the transport of less hydrophilic anions (e.g., Cl(-), NO3(-)) over hard, stongly hydrated anions (e.g., HCO3(-) and SO4(2-)). Especially relevant is the selectivity between chloride and bicarbonate, the major inorganic anions in biological systems.
Angewandte Chemie International Edition, 2015
Accounts of chemical research, Jan 17, 2013
The natural product Valinomycin is a well-known transmembrane cation carrier. Despite being uncha... more The natural product Valinomycin is a well-known transmembrane cation carrier. Despite being uncharged, this molecule can extract potassium ions from water without counterions and ferry them through a membrane interior. Because it only transports positive ions, it is electrogenic, mediating a flow of charge across the membrane. Equivalent agents for anions would be valuable research tools and may have therapeutic applications, especially in the treatment of "channelopathies" such as cystic fibrosis. However, no such molecules have been found in nature. In this Account, we describe our research toward synthetic and rationally designed "anti-Valinomycins". As our core approach to this problem, we used the steroid nucleus, provided by cholic acid, as a scaffold for the assembly of anion receptors. By positioning H-bond donors on this framework, especially urea and thiourea groups in conformationally constrained axial positions, we created binding sites capable of exceptionally high affinities (up to 10(11) M(-1) for R4N(+)Cl(-) in chloroform). The extended hydrocarbon surface of the steroid helped to maintain compatibility with nonpolar media. When we tested these "cholapods" for chloride transport in vesicles, they provided the first evidence for electrogenic anion transport mediated by electroneutral organic carriers: in other words, they are the first authenticated anti-Valinomycins. They also proved active in live cells that we grew and assayed in an Ussing chamber. In subsequent work, we have shown that the cholapods can exhibit very high activities, with transport observed down to carrier/lipid ratios of 1:250,000. We also understand some of the effects of structure on the activity of these molecules. For example, in most cases, powerful transporters also act as powerful receptors. On the other hand, some modifications which favor binding do not promote transport. We gained functional advantages by cyclizing the cholapod architecture, which encloses the anion binding site. We could also simplify the structure without compromising function. A steroid-inspired trans-decalin framework has proved highly effective and may lead to agents with practical advantages. Changing an ester side-chain in this system revealed a surprising effect, whereby increased length and/or lipophilicity resulted in substantially raised activity. Although much remains to be discovered about these anionophores, their high activities and intrinsic tuneabilities bode well for applications. In future work, we plan to develop and exploit these molecules as tools for biophysical research and to explore the possibility of useful biological activity.
Journal of the American Chemical Society, Jan 3, 2014
Transmembrane anion carriers (anionophores) have potential in biological research and medicine, p... more Transmembrane anion carriers (anionophores) have potential in biological research and medicine, provided high activities can be obtained. There is particular interest in treating cystic fibrosis (CF), a genetic illness caused by deficient anion transport. Previous work has found that anionophore designs featuring axial ureas on steroid and trans-decalin scaffolds can be especially effective. Here we show that replacement of ureas by thioureas yields substantial further enhancements. Six new bis-thioureas have been prepared and tested for Cl(-)/NO3(-) exchange in 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol large unilamellar vesicles (LUVs). The bis-thioureas are typically >10 times more effective than the corresponding ureas and are sufficiently active that transport by molecules acting singly in LUVs is readily detected. The highest activity is shown by decalin 9, which features N-(3,5-bis(trifluoromethyl)phenyl)thioureido and octyl ester substituents. A single molecule of transporter 9 in a 200 nm vesicle promotes Cl(-)/NO3(-) exchange with a half-life of 45 s and an absolute rate of 850 chloride anions per second. Weight-for-weight, this carrier is only slightly less effective than CFTR, the natural anion channel associated with CF.
Angewandte Chemie International Edition, 2014
Transmembrane ion transporters (ionophores) are widely investigated as supramolecular agents with... more Transmembrane ion transporters (ionophores) are widely investigated as supramolecular agents with potential for biological activity. Tests are usually performed in synthetic membranes that are assembled into large unilamellar vesicles (LUVs). However transport must be followed through bulk properties of the vesicle suspension, because LUVs are too small for individual study. An alternative approach is described whereby ion transport can be revealed and quantified through direct observation. The method employs giant unilamellar vesicles (GUVs), which are 20-60 μm in diameter and readily imaged by light microscopy. This allows characterization of individual GUVs containing transporter molecules, followed by studies of transport through fluorescence emission from encapsulated indicators. The method provides new levels of certainty and relevance, given that the GUVs are similar in size to living cells. It has been demonstrated using a highly active anion carrier, and should aid the development of compounds for treating channelopathies such as cystic fibrosis.
Journal of the American Chemical Society, 2011
This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers ... more This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga(2)O(3) as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10(-1)A/cm(2) at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 < |V| < 0.4 V. The calculations predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calculated precisely. In this sense, the theoretical predictions and experimental conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theoretical predictions on single molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.
Nature Nanotechnology, 2012
Nature Nanotechnology, 2012
Physical Chemistry Chemical Physics, 2014
As the dimensions of a conductor approach the nanoscale, quantum effects begin to dominate, and i... more As the dimensions of a conductor approach the nanoscale, quantum effects begin to dominate, and it becomes possible to control the conductance through direct manipulation of the electron wavefunction. Such control has been demonstrated in various mesoscopic devices at cryogenic temperatures, but it has proved to be difficult to exert control over the wavefunction at higher temperatures. Molecules have typical energy level spacings (~eV) that are much larger than the thermal energy at 300 K (~25 meV), and are therefore natural candidates for such experiments. Previously, phenomena such as giant magnetoresistance, Kondo effects and conductance switching have been observed in single molecules, and theorists have predicted that it should also be possible to observe quantum interference in molecular conductors, but until now all the evidence for such behaviour has been indirect. Here, we report the observation of destructive quantum interference in charge transport through two-terminal molecular junctions at room temperature. We studied five different rigid π-conjugated molecular wires, all of which form self-assembled monolayers on a gold surface, and find that the degree of interference can be controlled by simple chemical modifications of the molecular wire.
Chemical communications (Cambridge, England), Jan 11, 2015
Tris-N-arylthioureas derived in one step from 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene are r... more Tris-N-arylthioureas derived in one step from 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene are remarkably effective anion carriers. With optimised aryl substituents their activities come close to the best currently known, suggesting that they might find use as readily available standards in anion transport research.
Nano Letters, 2015
It is understood that molecular conjugation plays an important role in charge transport through s... more It is understood that molecular conjugation plays an important role in charge transport through single-molecule junctions. Here, we investigate electron transport through an anthraquinone based single-molecule three-terminal device. With the use of an electric-field induced by a gate electrode, the molecule is reduced resulting into a 10-fold increase in the off-resonant differential conductance. Theoretical calculations link the change in differential conductance to a reduction-induced change in conjugation, thereby lifting destructive interference of transport pathways.
Journal of the American Chemical Society, Jan 8, 2015
Biotin[6]uril hexaesters represent a new class of anionophores which operate solely through C-H··... more Biotin[6]uril hexaesters represent a new class of anionophores which operate solely through C-H···anion interactions. The use of soft H-bond donors favors the transport of less hydrophilic anions (e.g., Cl(-), NO3(-)) over hard, stongly hydrated anions (e.g., HCO3(-) and SO4(2-)). Especially relevant is the selectivity between chloride and bicarbonate, the major inorganic anions in biological systems.
Angewandte Chemie International Edition, 2015
Accounts of chemical research, Jan 17, 2013
The natural product Valinomycin is a well-known transmembrane cation carrier. Despite being uncha... more The natural product Valinomycin is a well-known transmembrane cation carrier. Despite being uncharged, this molecule can extract potassium ions from water without counterions and ferry them through a membrane interior. Because it only transports positive ions, it is electrogenic, mediating a flow of charge across the membrane. Equivalent agents for anions would be valuable research tools and may have therapeutic applications, especially in the treatment of "channelopathies" such as cystic fibrosis. However, no such molecules have been found in nature. In this Account, we describe our research toward synthetic and rationally designed "anti-Valinomycins". As our core approach to this problem, we used the steroid nucleus, provided by cholic acid, as a scaffold for the assembly of anion receptors. By positioning H-bond donors on this framework, especially urea and thiourea groups in conformationally constrained axial positions, we created binding sites capable of exceptionally high affinities (up to 10(11) M(-1) for R4N(+)Cl(-) in chloroform). The extended hydrocarbon surface of the steroid helped to maintain compatibility with nonpolar media. When we tested these "cholapods" for chloride transport in vesicles, they provided the first evidence for electrogenic anion transport mediated by electroneutral organic carriers: in other words, they are the first authenticated anti-Valinomycins. They also proved active in live cells that we grew and assayed in an Ussing chamber. In subsequent work, we have shown that the cholapods can exhibit very high activities, with transport observed down to carrier/lipid ratios of 1:250,000. We also understand some of the effects of structure on the activity of these molecules. For example, in most cases, powerful transporters also act as powerful receptors. On the other hand, some modifications which favor binding do not promote transport. We gained functional advantages by cyclizing the cholapod architecture, which encloses the anion binding site. We could also simplify the structure without compromising function. A steroid-inspired trans-decalin framework has proved highly effective and may lead to agents with practical advantages. Changing an ester side-chain in this system revealed a surprising effect, whereby increased length and/or lipophilicity resulted in substantially raised activity. Although much remains to be discovered about these anionophores, their high activities and intrinsic tuneabilities bode well for applications. In future work, we plan to develop and exploit these molecules as tools for biophysical research and to explore the possibility of useful biological activity.
Journal of the American Chemical Society, Jan 3, 2014
Transmembrane anion carriers (anionophores) have potential in biological research and medicine, p... more Transmembrane anion carriers (anionophores) have potential in biological research and medicine, provided high activities can be obtained. There is particular interest in treating cystic fibrosis (CF), a genetic illness caused by deficient anion transport. Previous work has found that anionophore designs featuring axial ureas on steroid and trans-decalin scaffolds can be especially effective. Here we show that replacement of ureas by thioureas yields substantial further enhancements. Six new bis-thioureas have been prepared and tested for Cl(-)/NO3(-) exchange in 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol large unilamellar vesicles (LUVs). The bis-thioureas are typically >10 times more effective than the corresponding ureas and are sufficiently active that transport by molecules acting singly in LUVs is readily detected. The highest activity is shown by decalin 9, which features N-(3,5-bis(trifluoromethyl)phenyl)thioureido and octyl ester substituents. A single molecule of transporter 9 in a 200 nm vesicle promotes Cl(-)/NO3(-) exchange with a half-life of 45 s and an absolute rate of 850 chloride anions per second. Weight-for-weight, this carrier is only slightly less effective than CFTR, the natural anion channel associated with CF.
Angewandte Chemie International Edition, 2014
Transmembrane ion transporters (ionophores) are widely investigated as supramolecular agents with... more Transmembrane ion transporters (ionophores) are widely investigated as supramolecular agents with potential for biological activity. Tests are usually performed in synthetic membranes that are assembled into large unilamellar vesicles (LUVs). However transport must be followed through bulk properties of the vesicle suspension, because LUVs are too small for individual study. An alternative approach is described whereby ion transport can be revealed and quantified through direct observation. The method employs giant unilamellar vesicles (GUVs), which are 20-60 μm in diameter and readily imaged by light microscopy. This allows characterization of individual GUVs containing transporter molecules, followed by studies of transport through fluorescence emission from encapsulated indicators. The method provides new levels of certainty and relevance, given that the GUVs are similar in size to living cells. It has been demonstrated using a highly active anion carrier, and should aid the development of compounds for treating channelopathies such as cystic fibrosis.
Journal of the American Chemical Society, 2011
This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers ... more This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga(2)O(3) as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10(-1)A/cm(2) at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 < |V| < 0.4 V. The calculations predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calculated precisely. In this sense, the theoretical predictions and experimental conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theoretical predictions on single molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.
Nature Nanotechnology, 2012