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 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)).

Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HC C(CH 2 ) n CH 3 , n = 5, 7, 9, and 11) with Au(111) at 60°Cwere characterized using scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and contact angles of water. In contrast to previous spectroscopic studies of this type of SAMs, these combined microscopic and spectroscopic experiments confirm formation of highly ordered SAMs having packing densities and molecular chain orientations very similar to those of alkanethiolates on Au(111). Physical properties, hydrophobicity, high surface order, and packing density, also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The formation of high-quality SAMs from alkynes requires careful preparation and manipulation of reactants in an oxygen-free environment; trace quantities of O 2 lead to oxidized contaminants and disordered surface films. The oxidation process occurs during formation of the SAM by oxidation of the −CC− group (most likely catalyzed by the gold substrate in the presence of O 2 ).T hin organic films based on self-assembled monolayers (SAMs) 1 are ubiquitous in surface science. The reaction of organic thiols (RSH) with group Ib metals (Au and Ag) to generate SAMs with composition Au/AgSR is the reaction most commonly used to prepare SAMs, 1 although reactions that generate organosilanes on silicon 2 (SiR) and organic carboxylates on silver 3 (AgO 2 CR) have attractive properties, and a number of other precursors have been surveyed. There have also been scattered descriptions of SAMs formed on gold from solutions of alkynes 4 (HCC(CH 2 ) n CH 3 , n = 3, 5, 7, 9, 11, and 13), ethynylbenzene 5 (HCCC 6 H 5 ) or nalkylmercury(II) tosylates 6 (CH 3 (CH 2 ) n HgOTs, n = 4 and 18) on Au(111). Although the potential interest of SAMs having metal−CCR bonds is high, since they offer a new type of metal−organic bond, most of these studies have used preparations analogous to those employed with n-alkanethiols and have generated SAMs that do not seem to be highly ordered and, thus, are perhaps unsuitable for detailed studies of the physical chemistry of the surface. In particular, there are no procedures that describe the formation of SAMs that are highly ordered in two dimensionsa key requirement for high-quality surface science. The most recent analyses of n-alkyl-based SAMs on Au(111) indicate a "liquid-like" structure of the monolayer, 6 and XPS analyses of SAMs formed from alkynes 4,5 suggest that these SAMs are sensitive to oxidation at an undefined point in their formation; that is, oxidation occurs either during or after SAM formation (for example, by reaction of the AuCCR bond with O 2 ). Contact angle analyses of increasing lengths of alkynes (HCC(CH 2 ) n CH 3 , n = 5, 7, 9, and 11) also suggest 4 that the quality of these SAMs is lower than those based on n-alkanethiols.Although SAMs have enabled studies of wetting, 7,8 adhesion, 9,10 and charge transport 3,11−13 (...

This paper describes a method to detect the presence of bacteria in aqueous samples, based on the capture of bacteria on a syringe filter, and the infection of targeted bacterial species with a bacteriophage (phage). The use of phage as a reagent provides two opportunities for signal amplification: (i) the replication of phage inside a live bacterial host and (ii) the delivery and expression of the complementing gene that turns on enzymatic activity and produces a colored or fluorescent product. Here we demonstrate a phage-based amplification scheme with an M13KE phage that delivers a small peptide motif to an F(+), α-complementing strain of Escherichia coli K12, which expresses the ω-domain of β-galactosidase (β-gal). The result of this complementation-an active form of β-gal-was detected colorimetrically, and the high level of expression of the ω-domain of β-gal in the model K12 strains allowed us to detect, on average, five colony-forming units (CFUs) of this strain in 1 L of water with an overnight culture-based assay. We also detected 50 CFUs of the model K12 strain in 1 L of water (or 10 mL of orange juice, or 10 mL of skim milk) in less than 4 h with a solution-based assay with visual readout. The solution-based assay does not require specialized equipment or access to a laboratory, and is more rapid than existing tests that are suitable for use at the point of access. This method could potentially be extended to detect many different bacteria with bacteriophages that deliver genes encoding a full-length enzyme that is not natively expressed in the target bacteria.