ABSTRACT:The mechanism (or mechanisms) of enthalpy− entropy (H/S) compensation in protein−ligand binding remains controversial, and there are still no predictive models (theoretical or experimental) in which hypotheses of ligand binding can be readily tested. Here we describe a particularly well-defined system of protein and ligandshuman carbonic anhydrase (HCA) and a series of benzothiazole sulfonamide ligands with different patterns of fluorinationthat we use to define enthalpy/entropy (H/S) compensation in this system thermodynamically and structurally. The binding affinities of these ligands (with the exception of one ligand, in which the deviation is understood) to HCA are, despite differences in fluorination pattern, indistinguishable; they nonetheless reflect significant and compensating changes in enthalpy and entropy of binding. Analysis reveals that differences in the structure and thermodynamic properties of the waters surrounding the bound ligands are an important contributor to the observed H/S compensation. These results support the hypothesis that the molecules of water filling the active site of a protein, and surrounding the ligand, are as important as the contact interactions between the protein and the ligand for biomolecular recognition, and in determining the thermodynamics of binding.

This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (n odd and n even ) using junctions with the structure M TS /SAM//Ga 2 O 3 /EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on Au TS and Ag TS demonstrated a statistically significant odd−even effect on Au TS , but not on Ag TS , that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on Au TS with n odd and n even belonging to two separate sets, and while there is a significant difference between the values of injection current density, J 0 , for these two series (log|J 0Au,even | = 4.0 ± 0.3 and log|J 0Au,odd | = 4.5 ± 0.3), the values of tunneling decay constant, β, for n odd and n even alkyl chains are indistinguishable (β Au,even = 0.73 ± 0.02 Å ). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of β and J 0 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.

This paper describes a physical-organic study of the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across selfassembled monolayer (SAM)-based large-area junctions of the form Ag TS S(CH 2 ) n M(CH 2 ) m T//Ga 2 O 3 /EGaIn. Here Ag TS is a template-stripped silver substrate, -M-and -T are "middle" and "terminal" functional groups, and EGaIn is eutectic gallium−indium alloy. Twelve uncharged polar groups (-T = CN, CO 2 CH 3 , CF 3 , OCH 3 , N(CH 3 ) 2 , CON(CH 3 ) 2 , SCH 3 , SO 2 CH 3 , Br, P(O)(OEt) 2 , NHCOCH 3 , OSi(OCH 3 ) 3 ), 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 those of 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 the terminal polar groups in this set. The current densities measured in this work suggest that the tunneling decay parameter and injection current for SAMs terminated in nonpolar n-alkyl groups, and polar groups selected from common polar organic groups, are statistically indistinguishable.A central goal in the field of molecular electronics is to understand relationships between rates of charge transport and molecular structure.1−13 Using self-assembled monolayer (SAM)-based large-area junctions having the structure Ag TS S-(CH 2 ) n M(CH 2 ) m T//Ga 2 O 3 /EGaIn, where Ag TS is a templatestripped silver substrate and EGaIn is eutectic gallium−indium alloy, we explored the influence of the "terminal" group, T, and "middle" groups, M, of the SAM on the tunneling current. One current focus for our work is the importance of the two interfaces: Ag TS -SR and T//Ga 2 O 3 .14 This paper focuses on the latter interface and examines the influence of the group T on the rate of charge transport.Our results 7,15−17 (and those obtained using other types of junctions 9,11,18−26 ) have not led to a single, broad conclusion about this matter: a few groups T (e.g., ferrocene) 16,17 seem to change the rate of charge transport (relative to a methyl group), but some do not (Table S1 in the Supporting Information). We have, perhaps surprisingly, observed that the tunneling current is insensitive (within the precision of our measurements) to the structures of a range of nonpolar terminal aromatic and aliphatic groups with different geometries and electronic structures. 7This paper focuses on a specific physical-organic question: Do molecular dipoles, particularly when placed at the top interface between a thin, electrically insulating organic film (a SAM) and a conducting top electrode, influence the rates of charge transport by tunneling? 27,28 To answer this question, we examined the electrical characteristics of SAM-based large-area junctions of the form Ag TS S(CH 2 ) n M(CH 2 ) m T//Ga 2 O 3 / EGaIn, where -M-is either -CONH-or -CH 2 CH 2 -(depending on which was synthetically more accessible; replacing -CH 2 CH 2 -wi...

This 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(CH 2 ) n H//Ga 2 O 3 /EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O 2 CR),and Au/acetylene (Au/CtCR), where R is an n-alkyl group. Values of J 0 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, J 0 (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).

This paper investigates the influence of the atmosphere used in the fabrication of top electrodes from the liquid eutectic of gallium and indium (EGaIn) (the so-called "EGaIn" electrodes), and in measurements of current density, J(V) (A/cm 2 ), across selfassembled monolayers (SAMs) incorporated into Ag/SR//Ga 2 O 3 /EGaIn junctions, on values of J(V) obtained using these electrodes. A gas-tight measurement chamber was used to control the atmosphere in which the electrodes were formed, and also to control the environment in which the electrodes were used to measure current densities across SAM-based junctions. Seven different atmospheresair, oxygen, nitrogen, argon, and ammonia, as well as air containing vapors of acetic acid or waterwere surveyed using both "rough" conical-tip electrodes, and "smooth" hanging-drop electrodes. (The manipulation of the oxide film during the creation of the conical-tip electrodes leads to substantial, micrometer-scale roughness on the surface of the electrode, the extrusion of the drop creates a significantly smoother surface.) Comparing junctions using both geometries for the electrodes, across a SAM of n-dodecanethiol, in air, gave log |J| mean = −2.4 ± 0.4 for the conical tip, and log |J| mean = −0.6 ± 0.3 for the drop electrode (and, thus, Δlog |J| ≈ 1.8); this increase in current density is attributed to a change in the effective electrical contact area of the junction. To establish the influence of the resistivity of the Ga 2 O 3 film on values of J(V), junctions comprising a graphite electrode and a hanging-drop electrode were compared in an experiment where the electrodes did, and did not, have a surface oxide film; the presence of the oxide did not influence measurements of log |J(V)|, and therefore did not contribute to the electrical resistance of the electrode. However, the presence of an oxide film did improve the stability of junctions and increase the yield of working electrodes from ∼70% to ∼100%. Increasing the relative humidity (RH) in which J(V) was measured did not influence these values (across methyl (CH 3 )-or carboxyl (CO 2 H)-terminated SAMs) over the range typically encountered in the laboratory (20%−60% (RH)).

Abstract:This paper compares rates of charge transport by tunneling across junctions with the structures Ag TS X(CH 2 ) 2n CH 3 //Ga 2 O 3 /EGaIn (n = 1 -8 and X = -SCH 2 -and -O 2 C-); here Ag TS was template-stripped silver, and EGaIn is the eutectic alloy of gallium and indium. Its objective was to compare the tunneling decay coefficient (β, Å -1 ) and the injection current (J 0 , A/cm 2 ) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates.

This paper describes charge transport by tunneling across self-assembled monolayers (SAMs) of thiolterminated derivatives of oligo(ethylene glycol) (HS(CH 2 CH 2 O) n CH 3 ; HS(EG) n CH 3 ); these SAMs are positioned between gold bottom electrodes and Ga 2 O 3 /EGaIn top electrodes and are of the form: 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 (E MO = -6.8--7.2 eV), but more delocalized (due to interactions between orbitals on neighboring oxygen atoms), than the highest occupied molecular orbital (HOMO, E MO : ~-5.7 eV) localized on sulfur. Nonetheless, the existence of these high-energy, delocalized occupied orbitals, which are not present in analogous n-alkanethiols (E MO < -8.5 eV for orbitals associated with CH 2 ), rationalize the low value of β. SAMs of oligo(ethylene glycol)s (and of oligomers of glycine). SAMs based on S(EG) n CH 3 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) n CH 3 rests on the 3 possibility of disorder in the SAM, and a systematic discrepancy between different estimates of the thickness of these SAMs.4

This paper describes the relationship between the rates of charge transport (by tunneling) across self-assembled monolayers (SAMs) in a metal/SAM//Ga2O3/EGaIn junction and the geometric contact area (A g) between the conical Ga2O3/EGaIn top-electrode and the bottom-electrode. Measurements of current density, J(V), across SAMs of decanethiolate on silver demonstrate that J(V) increases with A g when the contact area is small (A g < 1000 μm2), but reaches a plateau between 1000 and 4000 μm2, where J(0.5 V) ≈ 10–0.52±0.10 A/cm2. The method used to fabricate Ga2O3/EGaIn electrodes generates a tip whose apex is thicker and rougher than its thin, smoother sides. When A g is small, the Ga2O3/EGaIn electrode contacts the bottom-electrode principally over this rough apex and forms irreproducible areas of electrical contact. When A g is large, the contact is through the smoother regions peripheral to the apex and is much more reproducible. Measurements of contact pressure between conical EGaIn electrodes and atomic force microscope cantilevers demonstrate that the nominal contact pressure (governed by the mechanical behavior of the oxide skin) decreases approximately inversely with the diameter of geometric contact. This self-regulation of pressure prevents damage to the SAM and makes the ratio of electrical contact area to geometric footprint approximately constant.