Key interaction patterns in proteins revealed by cluster expansion of the partition function

The European Physical Journal E -     

Charging of silver bromide aqueous interface: Evaluation of enthalpy and entropy of interfacial reactions from surface potential data

Dependence of surface potential (electrostatic potential at the inner Helmholtz plane, Ψ(0)) at the silver bromide aqueous electrolyte interface was measured as a function of the activities of Br(-) and Ag(+) by using a single crystal silver bromide electrode (SCr-AgBr). Absolute values of surface potentials were obtained from electrode potentials of SCr-AgBr and isoelectric points. Measurements were performed at different temperatures in the range from 10 to 50°C. Corresponding equilibrium constants of interfacial reactions were obtained using the surface complexation model and interpreted via the van't Hoff equation. As a result of the interpretation for the binding of bromide ions leading to a negative surface charge, the thermodynamic parameters obtained were Δ(n)H(°)=-33kJmol(-1) and Δ(n)S(°)=-31Jmol(-1)K(-1); and for the binding of silver ions leading to a positive surface charge, Δ(p)H(°)=-72kJmol(-1) and Δ(p)S(°)=-196Jmol(-1)K(-1). Association of counterions (CI) with oppositely charged surface sites partially compensates the surface charge. Assuming approximately the same affinities for anions (NO(3)(-)) and cations (K(+)) thermodynamic parameters for their binding were obtained as Δ(CI)H(°)≈7kJmol(-1) and Δ(CI)S(°)≈105Jmol(-1)K(-1).     

Conformational analysis of hydrazones. 1H dynamic nuclear magnetic resonance and solvent effects in aryl- and 2-furylaldehyde ethylaminoacetylhydrazones

Aryl- and 2-furylaldehyde ethylaminoacetylhydrazones were examined in different solvents and over a wide temperature range with ¹H NMR in order to study their conformational properties. Nearly equal amounts of the E/Z isomers, relative to the C?N bond, are present, even when the solvents and the substituents on the aldimino carbon produce small changes in the isomeric mixture. The activation parameters of the thermal isomerization process were measured, and the results are in the line with a lateral-shift type mechanism, also supported by theoretical calculations on a model compound. No other internal process was noted from the low-temperature spectral behaviour, and this was interpreted in terms of a highly biased equilibrium concerning the rotation around the C(O)? N bond. Chemical shifts obtained in different solvents also enable the most stable arrangement of the whole molecule of these compounds to be postulated.     

Effect of Graphene Characteristics on Morphology and Performance of Composite Noble Metal-Reduced Graphene Oxide SERS Substrate

Graphene/noble metal substrates for surface enhanced RAMAN scattering (SERS) possess synergistically improved performance, due to the strong chemical enhancement mechanism accounted to graphene and the electromagnetic mechanism raised from the metal nanoparticles. However, only the effect of noble metal nanoparticles characteristics on the SERS performance was studied so far. In attempts to bring a light to the effect of quality of graphene, in this work, two different graphene oxides were selected, slightly oxidized GOS (20%) with low aspect ratio (1000) and highly oxidized (50%) GOG with high aspect ratio (14,000). GO and precursors for noble metal nanoparticles (NP) simultaneous were reduced, resulting in rGO decorated with AgNPs and AuNPs. The graphene characteristics affected the size, shape, and packing of nanoparticles. The oxygen functionalities actuated as nucleation sites for AgNPs, thus GOG was decorated with higher number and smaller size AgNPs than GOS. Oppositely, AuNPs preferred bare graphene surface, thus GOS was covered with smaller size, densely packed nanoparticles, resulting in the best SERS performance. Fluorescein in concentration of 10⁻⁷ M was detected with enhancement factor of 82 × 10⁴. This work demonstrates that selection of graphene is additional tool toward powerful SERS substrates.     

Heterocyclic compounds from 3,3-dimercapto-1-aryl-2-propen-1-ones. Note 3. Condensation with N-methyl-o-phenylenediamine and N-benzyl-o-phenylenediamine

Two seven-membered ring compounds and three five-membered ring compounds were obtained by reaction in hot xylene of 3,3-dimercapto-l-phenyl-2-propen-1-one (1) with N-alkyl-o-phenylenediamines (2) . Compounds isolated were the 4-phenyl-5-alkyl-1,5-dihydro-2H-1,5-benzodiazepine-2-thiones (3) the 1-alkyl-4-phenyl-1,3-dihydro-2H-1,5-benzodiazepine-2-thiones (4) , the 1-alkyl-2-phenacylbenzimidazoles (5) , the 1-alkyl-2-phenylbenzimidazoles (6) and the 1-alkyl-2-methylbenzimidazoles (7) . The structures of these compounds were elucidated from their chemical reactivity and their nmr and ir spectra.     

Measurement of surface potential at silver chloride aqueous interface with single-crystal AgCl electrode

The surface potential at the silver chloride aqueous interface was measured by means of a single-crystal silver chloride electrode (SCr-AgCl). The measurements were conducted by titration of the KCl solution with AgNO3, and vice versa. The SCr-AgCl electrode potentials were converted to surface potentials psi(0) by setting zero at the point of zero charge at pCl = 5.2. The psi(0)(pCl) function was linear, with a slope 12% lower with respect to the Nernst equation. It was demonstrated that the surface potential at the silver halide aqueous interface could be interpreted by means of the surface complexation model, originally developed for metal oxides.     

Variable temperature n.m.r. behaviour of 1,5-benzodiazepinones and 1,5-benzodiazepinthiones

The n.m.r. variable temperature behaviour of the 7-membered ring in 1,3-dihydro-2H-1,5-benzodiazepin-2-one derivatives and in the corresponding 2-thiones has been investigated. The results indicate that the ring inversion barrier is higher in the thiones and dependent on substituents bonded at N-1. The activation parameters are obtained and discussed.     

Heterocyclic compounds from 3,3-dimercapto-1-aryl-2-propen-1-ones. Note 1. 4-Ary1-1,3-dihydro-2H-1,5-benzodiazepine-2-thiones

A series of new 4-aryl-1,3-dihydro-2H-1,5-benzodiazepine-2-thiones ( 3 ) has been synthesized by condensing the 3,3-dimercapto-1-aryl-2-propen-1-ones with o-phenylenediamine. The structure was established by the results of acid cleavage and by nmr spectra. The alkylation of compounds 3 gave 2-alkylthio-4-aryl-3H-1,5 benzodiazepines ( 10 ).     

Thermodynamics of Complexation of Alkali Metal Cations by a Lower-Rim Calix[4]arene Amino Acid Derivative

Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-d-α-phenylglycylcarbonylmethoxy)calix[4]arene (L) in methanol and acetonitrile was studied by means of direct and competitive microcalorimetric titrations at 25 °C. The thermodynamic parameters of complexation reactions showed that all the reactions investigated were enthalpically controlled. In both solvents the reaction enthalpy was most favorable for Na⁺ binding with L leading to the highest affinity of the examined calix[4]arene derivative towards this cation. The solubilities (and consequently the solution Gibbs energies) of the ligand were determined, as were the corresponding solution enthalpies and entropies. No significant difference was observed between the solution thermodynamic quantities of L in the two solvents, whereas the transfer of complex species from methanol to acetonitrile was found to be quite favorable. The interactions of solvent molecules with the free and the complexed ligand were investigated by ¹H NMR spectroscopy. It was concluded that in both cases inclusion of an acetonitrile molecule into the hydrophobic cavity of L occurred, which significantly affected the cation complexation in this solvent. The thermodynamic data were discussed regarding the structural properties of the ligand, the free and the complexed cations as well as the solvation abilities of the solvents examined. In this respect, the specific solvent-solute interactions and the intramolecular NH⋅⋅⋅O=C hydrogen bonds at the lower rim of L were particularly addressed.