Baghbanzadeh, Mostafa

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Baghbanzadeh, Mostafa

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Baghbanzadeh

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Mostafa

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Baghbanzadeh, Mostafa

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Odd–Even Effects in Charge Transport across n -Alkanethiolate-Based SAMs
(American Chemical Society (ACS), 2014) Baghbanzadeh, Mostafa; Simeone, Felice C.; Bowers, Carleen; Liao, Kung-ching; Thuo, Martin; Baghbanzadeh, Mahdi; Miller, Michael S.; Carmichael, Tricia Breen; Whitesides, George
This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions with the structure MTS/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on AuTS and AgTS demonstrated a statistically significant odd–even effect on AuTS, but not on AgTS, that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on AuTS with nodd and neven belonging to two separate sets, and while there is a significant difference between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0 ± 0.3 and log|J0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, β, for nodd and neven alkyl chains are indistinguishable (βAu,even = 0.73 ± 0.02 Å–1, and βAu,odd= 0.74 ± 0.02 Å–1). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of β and J0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates.
Interactions between Hofmeister Anions and the Binding Pocket of a Protein
(American Chemical Society (ACS), 2015) Fox, Jerome Michael; Kang, Kyungtae; Sherman, Woody; Héroux, Annie; Sastry, G. Madhavi; Baghbanzadeh, Mostafa; Lockett, Matthew; Whitesides, George
This paper uses the binding pocket of human carbonic anhydrase II (HCAII, EC 4.2.1.1) as a tool to examine the properties of Hofmeister anions that determine (i) where, and how strongly, they associate with concavities on the surfaces of proteins and (ii) how, upon binding, they alter the structure of water within those concavities. Results from X-ray crystallography and isothermal titration calorimetry show that most anions associate with the binding pocket of HCAII by forming inner-sphere ion pairs with the Zn2+ cofactor. In these ion pairs, the free energy of anion–Zn2+ association is inversely proportional to the free energetic cost of anion dehydration; this relationship is consistent with the mechanism of ion pair formation suggested by the “law of matching water affinities”. Iodide and bromide anions also associate with a hydrophobic declivity in the wall of the binding pocket. Molecular dynamics simulations suggest that anions, upon associating with Zn2+, trigger rearrangements of water that extend up to 8 Å away from their surfaces. These findings expand the range of interactions previously thought to occur between ions and proteins by suggesting that (i) weakly hydrated anions can bind complementarily shaped hydrophobic declivities, and that (ii) ion-induced rearrangements of water within protein concavities can (in contrast with similar rearrangements in bulk water) extend well beyond the first hydration shells of the ions that trigger them. This study paints a picture of Hofmeister anions as a set of structurally varied ligands that differ in size, shape, and affinity for water and, thus, in their ability to bind to—and to alter the charge and hydration structure of—polar, nonpolar, and topographically complex concavities on the surfaces of proteins.
Charge Tunneling along Short Oligoglycine Chains
(Wiley-Blackwell, 2015) Baghbanzadeh, Mostafa; Bowers, Carleen; Rappoport, Dmitrij; Zaba, Tomasz; Gonidec, Mathieu; Al-Sayah, Mohammad; Cyganik, Piotr; Aspuru-Guzik, Alan; Whitesides, George
This work examines charge transport (CT) through self-assembled monolayers (SAMs) of oligoglycines having an N-terminal cysteine group that anchors the molecule to a gold substrate, and demonstrate that CT is rapid (relative to SAMs of n-alkanethiolates). Comparisons of rates of charge transport-using junctions with the structure AuTS /SAM//Ga2 O3 /EGaIn (across these SAMs of oligoglycines, and across SAMs of a number of structurally and electronically related molecules) established that rates of charge tunneling along SAMs of oligoglycines are comparable to that along SAMs of oligophenyl groups (of comparable length). The mechanism of tunneling in oligoglycines is compatible with superexchange, and involves interactions among high-energy occupied orbitals in multiple, consecutive amide bonds, which may by separated by one to three methylene groups. This mechanistic conclusion is supported by density functional theory (DFT).
Characterizing the Metal–SAM Interface in Tunneling Junctions
(American Chemical Society (ACS), 2015) Bowers, Carleen; Liao, Kung-ching; Zaba, Tomasz; Rappoport, Dmitrij; Baghbanzadeh, Mostafa; Breiten, Benjamin; Krzykawska, Anna; Cyganik, Piotr; Whitesides, George
his paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag)TS/A(CH2)nH//Ga2O3/EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O2CR), and Au/acetylene (Au/C≡CR), where R is an n-alkyl group. Values of J0 and β (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).
Tunneling across SAMs Containing Oligophenyl Groups
(American Chemical Society (ACS), 2016) Bowers, Carleen; Rappoport, Dmitrij; Baghbanzadeh, Mostafa; Simeone, Felice; Liao, Kung-ching; Semenov, Sergey; Zaba, Tomasz; Cyganik, Piotr; Aspuru-Guzik, Alan; Whitesides, George
This paper describes rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga2O3/EGaIn as the top electrode. It compares the injection current, J0, and the attenuation constant, β, of the simplified Simmons equation, across oligophenyl groups (R = Phn; n = 1, 2, 3), with three different anchoring groups (thiol, HSR; methanethiol, HSCH2R; and acetylene, HC≡CR) that attach R to the template-stripped gold and silver substrates. The results demonstrate that the structure of the molecules between the anchoring group (-S- or -C≡C-) and the oligophenyl moiety significantly influences charge transport. SAMs of SPhn, and C≡CPhn on gold show similar values of β and log|J0| (β = 0.28 ± 0.03 Å-1 and log|J0| = 2.7 ± 0.1 for Au/SPhn; β = 0.30 ± 0.02 Å-1 and log|J0| = 3.0 ± 0.1 for Au/C≡CPhn). The introduction of a single intervening methylene (CH2) group, between the anchoring sulfur atom and the aromatic units to generate SAMs of SCH2Phn, increases β to ~0.6 Å-1 on both gold and silver substrates. (For n-alkanethiolates on gold the corresponding values are β = 0.76 Å-1 and log|J0| = 4.2). As a generalization, based on this and other work, it seems that increasing the height of the tunneling barrier in the region of the interfaces increases β, and may decrease J0; by contrast, it appears that lowering the height of the barrier at these interfaces has little influence on β or J0.
Autocatalytic, bistable, oscillatory networks of biologically relevant organic reactions
(Springer Nature, 2016) Semenov, Sergey; Kraft, Lewis J.; Ainla, Alar; Zhao, Mengxia; Baghbanzadeh, Mostafa; Campbell, Victoria; Kang, Kyungtae; Fox, Jerome Michael; Whitesides, George
Networks of organic chemical reactions are centrally important in life, and were likely to have played a central role in its origins. Network dynamics regulate cell division, circadian rhythms, nerve impulses, chemotaxis, and guide development of organisms. Although out-of-equilibrium networks of chemical reactions have the potential to display emergent network dynamics such as spontaneous pattern formation, bistability, and periodic oscillations, the principles that enable networks of organic reactions to develop complex behaviors are incompletely understood. Here we describe a network of biologically relevant organic reactions (amide formation, thiolate-thioester exchange, thiolate-disulfide interchange, and conjugate addition) that displays bistability and oscillations in concentrations of organic thiols and amides. Oscillations arise from the interaction between three subcomponents of the network: (i) an autocatalytic cycle that generates thiols and amides from thioesters and dialkyl disulfides; (ii) a trigger that controls autocatalytic growth; and (iii) inhibitory processes that remove activating thiol species produced during the autocatalytic cycle. In contrast to previous studies demonstrating oscillations and bistability using highly evolved biomolecules (i.e., enzymes and DNA) or inorganic molecules of questionable biochemical relevance (e.g. those used in Belousov-Zhabotinsky-type reactions), the organic molecules used in our network are relevant to current metabolism and similar to those that might have existed on early Earth. By using small organic molecules to build a network of organic reactions with autocatalytic, bistable, and oscillatory behavior, we identified principles that clarify how dynamic networks relevant to life might possibly have developed. In the future, modifications of this network will clarify the influence of molecular structure on the dynamics of reaction networks, and may enable the design of biomimetic networks, and of synthetic self-regulating and evolving chemical systems.
Introducing Ionic and/or Hydrogen Bonds into the SAM//Ga 2 O 3 Top-Interface of Ag TS /S(CH 2 ) n T//Ga 2 O 3 /EGaIn Junctions
(American Chemical Society (ACS), 2014) Bowers, Carleen; Liao, Kung-ching; Yoon, Hyo Jae; Rappoport, Dmitrij; Baghbanzadeh, Mostafa; Simeone, Felice C.; Whitesides, George
Junctions with the structure AgTS/S(CH2)nT//Ga2O3/EGaIn (where S(CH2)nT is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga2O3. Introducing a series of Lewis acidic/basic functional groups (T = −OH, −SH, −CO2H, −CONH2, and −PO3H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm2, that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen- or ionic bonds at the T//Ga2O3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J0, for T = −CO2H, and T = −CH2CH3−two groups having similar extended lengths (in Å, or in numbers of non-hydrogen atoms)−suggests that both groups make indistinguishable contributions to the height of the tunneling barrier.
The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in Ag TS S(CH 2 ) n M(CH 2 ) m T//Ga 2 O 3 /EGaIn Junctions
(American Chemical Society (ACS), 2014) Yoon, Hyo; Bowers, Carleen; Baghbanzadeh, Mostafa; Whitesides, George
This paper describes a physical‐organic studyof the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self‐assembled monolayer (SAM)‐based large‐area junctions of the form AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn. Here AgTS is a template‐stripped silver substrate, ‐M‐ and ‐T are “middle” and “terminal” functional groups, and EGaIn is eutectic galliumindium alloy. A range of uncharged polar groups (‐T), having permanent dipole moments in the range 0.5 < μ <4.5, were incorporated into the SAM. A comparison of the electrical characteristics of these junctions with junctions formed from n‐alkanethiolates led to the conclusion that the rates of charge tunneling are insensitive to the replacement of terminal alkyl groups with terminal polar groups. The current densities measured in this work suggest that the tunneling decay parameter (β) and injection current (Jo) for SAMs terminated in non‐polar n‐alkyl groups, and polar groups, are statistically indistinguishable.
Rectification in Tunneling Junctions: 2,2′-Bipyridyl-Terminated n -Alkanethiolates
(American Chemical Society (ACS), 2014) Yoon, Hyo Jae; Liao, Kung-ching; Lockett, Matthew R.; Kwok, Sen Wai; Baghbanzadeh, Mostafa; Whitesides, George
Molecular rectification is a particularly attractive phenomenon to examine in studying structure–property relationships in charge transport across molecular junctions, since the tunneling currents across the same molecular junction are measured, with only a change in the sign of the bias, with the same electrodes, molecule(s), and contacts. This type of experiment minimizes the complexities arising from measurements of current densities at one polarity using replicate junctions. This paper describes a new organic molecular rectifier: a junction having the structure AgTS/S(CH2)11-4-methyl-2,2′-bipyridyl//Ga2O3/EGaIn (AgTS: template-stripped silver substrate; EGaIn: eutectic gallium–indium alloy) which shows reproducible rectification with a mean r+ = |J(+1.0 V)|/|J(−1.0 V)| = 85 ± 2. This system is important because rectification occurs at a polarity opposite to that of the analogous but much more extensively studied systems based on ferrocene. It establishes (again) that rectification is due to the SAM, and not to redox reactions involving the Ga2O3 film, and confirms that rectification is not related to the polarity in the junction. Comparisons among SAM-based junctions incorporating the Ga2O3/EGaIn top electrode and a variety of heterocyclic terminal groups indicate that the metal-free bipyridyl group, not other features of the junction, is responsible for the rectification. The paper also describes a structural and mechanistic hypothesis that suggests a partial rationalization of values of rectification available in the literature.
Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)
(American Chemical Society (ACS), 2017) Baghbanzadeh, Mostafa; Bowers, Carleen M.; Rappoport, Dmitrij; ?aba, Tomasz; Yuan, Li; Kang, Kyungtae; Liao, Kung-Ching; Gonidec, Mathieu; Rothemund, Philipp Josef Michael; Cyganik, Piotr; Aspuru-Guzik, Alan; Whitesides, George
This paper describes charge transport by tunneling across self-assembled monolayers (SAMs) of thiol-terminated derivatives of oligo(ethylene glycol) (HS(CH2CH2O)nCH3; HS(EG)nCH3); these SAMs are positioned between gold bottom electrodes and Ga2O3/EGaIn top electrodes and are of the form: AuTS/S(EG)nCH3//Ga2O3/EGaIn. Comparison of the attenuation factor (β of the simplified Simmons equation) across these SAMs with the corresponding value obtained with length–matched SAMs of n-alkanethiols demonstrates, surprisingly, that SAMs of oligoethylene glycol have values of β (β = 0.29 ± 0.02 natom-1 and β = 0.24 ± 0.01 Å-1) lower than those of SAMs of n-alkanethiolates (β = 0.94 ± 0.02 natom-1 and β = 0.77 ± 0.03 Å-1). The value of β for tunneling across oligoethylene glycols is comparable to that across oligophenylenes (β = 0.28 ± 0.03 Å-1). There are two possible origins for the unexpectedly low value of β for (EG)n-derived SAMs. The more probable involves a mechanism for tunneling based on the superexchange model. This model accounts for the rapid hole tunneling across SAMs of oligo(ethylene glycol)s using interactions among the high-energy, occupied orbitals associated with the lone-pair electrons on oxygen. According to calculations using density functional theory (DFT), these orbitals—localized orbitals predominately on the backbone oxygen atoms—are lower in energy (EMO = -6.8– -7.2 eV), but more delocalized (due to interactions between orbitals on neighboring oxygen atoms), than the highest occupied molecular orbital (HOMO, EMO : ~ -5.7 eV) localized on sulfur. Nonetheless, the existence of these high-energy, delocalized occupied orbitals, which are not present in analogous n-alkanethiols (EMO < -8.5 eV for orbitals associated with CH2), rationalize the low value of β. SAMs of oligo(ethylene glycol)s (and of oligomers of glycine). SAMs based on S(EG)nCH3 are, in this mechanism, good conductors (by hole tunneling), but good insulators (by electron and/or hole drift conduction)—an unexpected observation that suggests SAMs derived from these or electronically similar molecules as a new class of electronic materials. A second but less probable mechanism for this unexpectedly low value of β for SAMs of S(EG)nCH3 rests on the possibility of disorder in the SAM, and a systematic discrepancy between different estimates of the thickness of these SAMs.