The peptide–urea interactions in water are relevant for the understanding of the conformational stability of polypeptides and the denaturation of proteins. Interesting calorimetric results on ternary and quaternary aqueous solutions containing urea and some N-acetylamides of simple amino acids (glycine, D- and L-alanine and L-leucine) are reported, discussed and compared with the properties of similar amide–urea–water and cyclic dipeptide–urea–water systems. The results were used to calculate the excess enthalpies and their virial coefficients. The usual treatment of the data was enlarged to cover solution containing racemates. The second virial coefficients of the excess enthalpies are discussed on the basis of the McMillan–Mayer theory and additivity of groups. However, the increasing amount of data at our disposal, allows us to refine the conclusions of preceding works from both our own and other laboratories. The additivity of the water-mediated interactions between urea and alkylic groups is confirmed, but evidence is given that extra contributions appear for the peptide–urea interaction, when more than one peptidic or amidic group is present on the same molecule.
The data of dielectric normal mode process (αI) and the segmental mode process (αII) reported in parts 1 and 2 of this series were analysed to find a relationship between the friction coefficient ζ for the normal mode process and the relaxation time τs for the segmental mode process. Assuming τs proportional to ζ, we attempted to superpose the τnvs. molecular weight Mw plots for solutions with different concentrations C. From the vertical and horizontal shift factors, we found that the monomeric friction coefficient ζm is approximately proportional to C2τs. To explain this result, we assumed that the effective size for the segmental motions expands with decreasing C. The molecular weight M0 for the unit of the segmental motion was estimated based on the computer simulation reported by Verdier and Stockmayer. The M0 was found to be 77 for bulk cis-PI and increased with decreasing C approximately in proportion to C–1.
Dielectric measurements were carried out on concentrated toluene solutions of narrow molecular-weight distribution cis-polyisoprenes (cis-PI) with molecular weight of 1.6 × 103 to 5 × 104. Two loss maxima were observed in the temperature range 200–400 K. The low-frequency process was assigned to the normal mode process due to the fluctuation of the end-to-end distance of the cis-PI molecules; and the high-frequency process to the segmental mode process related to the glass transition of the polymer. The molecular-weight dependence of the relaxation time τn for the normal mode process at a fixed concentration was found to be similar to the viscoelastic relaxations: below the characteristic molecular weight Mc, τn∝M2 but in the range above Mc, τn∝M4.3. The Mc varied in proportion to the inverse of concentration. The high-frequency segmental mode process exhibited the relaxation time τs almost independent of the molecular weight. The time–temperature superposition principle was applied to construct master curves of the dielectric loss factor ε″ over a wide frequency range. The ε″ curves broadened with increasing molecular weight and concentration in the high frequency side of the curves. However, the half width of the loss curves was almost independent of the molecular weight and concentration.
Surface-enhanced Raman spectra (SERS) are reported for nicotinamide adenine dinucleotide (NAD+, oxidised form) adsorbed on colloidal silver at concentrations between 10–3 and 10–5 mol dm–3, and using various excitation wavelengths. Adenosine diphosphate (ADP) gave an SER spectrum similar to that of NAD+, except for the absence of the nicotinamide SER band observed for NAD+ at 1030 cm–1. NADH (reduced form of NAD+) did not give an SER spectrum at the alkaline pHs necessary to prevent decomposition. No SER spectrum was obtained for NAD+ when bound to the enzyme GAPDH; the NAD+ is too deeply buried in the protein for effective surface enhancement. The applicability of the surface selection rules arising from the electromagnetic enhancement mechanism to determine NAD+ orientation at the silver surface is discussed in the light of the probable contribution to SERS intensity from a charge-transfer mechanism. While the selection rules do not, in this case, appear useful for determining NAD+ orientation at silver, wavenumber shifts do indicate surface interaction with N1 of adenine. The general surface geometry of NAD+ adsorbed on silver at low ( < 10–4 mol dm–3) concentrations is deduced as being extended, with phosphate groups binding directly to silver, and ribose, nicotinamide and adenine moieties all in close proximity to the surface.
Chlorided Pt/Al2O3 catalysts have been subjected to a series of consecutive oxidation / reduction cycles and the quality of Pt dispersion after each reduction stage monitored by infrared study of adsorbed CO. Low-temperature (613 or 723 K) oxidation treatments failed to improve Pt dispersion after subsequent reduction. Platinum dispersion was enhanced to an increasing extent with the increasing oxidation temperatures 793, 843 and 893 K. However, this effect was reversed after several oxidation/reduction cycles during which chlorine was lost from the sample. High oxidation temperature (993 K) led to drastic sintering of dispersed platinum. The results are discussed in relation to the essential role of mobile PtIVOxCly species on the support surface during the redispersion process. Enhancement in platinum dispersion is discussed in terms of not only increases in the number of exposed platinum atoms but also the redistribution of platinum to provide stronger interactions with the alumina support.
The translational mobility of methane sorbed on an NaZSM-5 sample of proven crystallinity has been studied by quasi-elastic neutron scattering (QENS) and an n.m.r. pulsed field gradient technique (n.m.r. PFGT) at different loadings and temperatures. In both methods, long-range self-diffusion is detected. However, owing to the different timescales of the two experimental methods applied, the mean diffusion paths followed are of different magnitude: in QENS, molecular translation is measured up to 6nm; in n.m.r. PFGT, the mean molecular displacements amount to some µm. Nevertheless, since the r.m.s. molecular displacements followed by both methods considerably exceed the distances between the pore intersections of the ZSM-5 channel network, in both techniques the translational self-diffusion coefficient of guest molecules inside the zeolite pores is detected. For the intracrystalline self-diffusion coefficient, D, as well as for the activation energy of self-diffusion, Ea, the values determined by the two independent methods agree very well. In the temperature region 200–250 K, the intracrystalline self-diffusion coefficients of methane in ZSM-5 are found to be of the order of 10–5–10–4 cm2 s–1, with only slight concentration and temperature dependence. The activation energy determined by both methods amounts to 4–5 kJ mol–1. Further agreement between QENS and n.m.r. PFGT is obtained by comparing the mean molecular jump lengths, which by both techniques are found to be ca. 1 nm for light hydrocarbons in ZSM-5, slightly decreasing with increasing loading.
Infrared spectra are reported of ethanoic acid and trifluoroethanoic acid adsorbed on barium sulphate. Ethanoic acid was non-dissociatively adsorbed as a monomer, either by liganding to Ba2+ surface sites or by hydrogen-bond donation to sulphate ions, and as a dimer. Dissociative adsorption led to adsorbed ethanoate anions and the concomitant protonation of sulphate ions to give HSO–4. The enhanced Brønsted acidity and depleted Lewis basicity of trifluoroethanoic acid resulted in a much greater extent of dissociative adsorption at the expense of non-dissociative adsorption of acid monomer. The extent of dimer formation on the barium sulphate surface was similar for the two acids.
Exposure of very dilute solutions of CF3COCH3 in CFCl3 to 60Co γ-rays at 77 K gave ˙CF3 radicals, which were clearly identified by their e.s.r. spectra. There was no indication of the presence of the parent cations, CF3(CH3)CO+, which were expected to form under these conditions, and it is concluded that these decompose at 77 K to give ˙CF3+CH3CO+. Methyl radicals were not detected. These results are compared with recent gas-phase studies which suggest that ˙CH3 should have been formed with far higher probability than ˙CF3. Whilst acetone cations also fail to give ˙CH3 radicals, cyclobutanone cations undergo ring-opening at 77 K to give H2ĊCCH2CH2CO+ radical cations. Under these circumstances, (CCl3)2CO gave only ˙CCl3 radicals at 77 K, also well characterised by their e.s.r. spectra. Again there was no sign of spectra for the parent cations, so decomposition into ˙CCl3+CCl3CO+ is postulated. In contrast, CCl3CHO gave a novel radical at 77 K, characterised by a 40 G doublet, together with a large positive g-shift. This species, identified as the parent radical cation, decomposed in the 120 K region to give in ˙CCl3 radical.
The excess volumes, VE, of several binary mixtures of simple molecular fluids have been measured as a function of composition and temperature. The results cannot be explained qualitatively using perturbation theories based on mixtures of spherical molecules and multipoles. The interplay of different kinds of anisotropy in the intermolecular potential seems to display complicated behaviour. A generalized van der Waals equation, a model based on a lattice with holes and a semiempirical equation of state have been used for fitting the experimental results satisfactorily. However, none of the models manages to predict other excess functions even for these relatively simple fluids.
The fluorescence of cyclotetrasilanes with an extremely large Stokes shift (up to 13700 cm–1) has been observed in rigid matrices at 77 K. The large Stokes shift is interpreted in terms of variation in potential energy for the S1 and S0 states as a function of the ring Si–Si bond distance. Fluorescence properties are dependent on the molecular structures (planar and bent forms) of the compounds; the fluorescence quantum yield, lifetime, and Stokes shift for the former are greater than those for the latter.
The relation between isobaric and isochoric equilibrium parameters has been examined from a new viewpoint based on specifically designed partial molar properties at constant temperature and solvent concentration. These are defined by text-decoration:overlineXB(T, cA)=(∂X/∂nB)T, cA, n′ and solutes in the corresponding ideal solution conform to text-decoration:overlineAidB(T, cA)=text-decoration:overlineA°B(T, cA)+RTln (rB/r°B) where A is the Helmholtz energy and rB=nB/nA. On this basis, standard equilibrium constants are introduced for reactions at constant T and cA not involving the solvent stoichiometrically. These equilibrium constants are related to an ideal process consisting of mixing standard solutions containing the reactants, complete transformation of reactants into products and separation to standard solutions containing the products. This ideal process at constant T and cA is rendered isochoric (in the sense of constant total volume) by an appropriate selection of standard solution compositions. A whole set of standard molar quantities of reaction ΔrX°(T, cA) is defined in terms of text-decoration:overlineX°B(T, cA). Exact equations linking ΔrX°(T, cA) where X=A, U, S and Cv, with the more usual Δr, X°(T, p) quantities where X=G, H, S and Cp, respectively, are derived. Particular attention has been paid to the thermodynamics of the quasi-equilibrium of activation at constant (T, p) and at constant (T, cA), and to their interrelationship. The results are compared with those of previous approaches and shown to be generally equivalent in the limit of infinitely dilute solutions. This approach is unique in its interpretative capabilities and in giving a rationale of the 'pressure of activation'. The quantities text-decoration:overlineXB(T, cA) and text-decoration:overlineXidB(T, cA) are suitable for studying mineralogically and metallurgically important interstitial solid solutions.
A set of equations is developed for the thermodynamic properties of 1:1 electrolytes using as its starting point the non-linearized version of the Poisson–Boltzmann radial distribution function and Kirkwood–Buff theory to forge the connection with the salt chemical potential. It is shown that the Debye–Hückel (DH) limiting law arises not from the linear term in the exponential expansion of the distribution but from the square term. The extended DH equation arises as a direct consequence of the treatment, although the interpretation of the distance of closest approach term is slightly different. By including only two parameters, one representing the distance of closest approach and a constant to represent ion–solvent interactions, excluded-volume effects and/or higher-order ion–ion interactions, the theory is able to model the activity coefficients of aqueous KCl accurately from infinite dilution to 4 mol kg–1. The theory is compared with other approaches.
Photoeffects, revealed during the growth of anodic oxide films on niobium under illumination, confirm the existence of interference effects at wavelengths well below the absorption edge of the films. Such effects were observed for films grown in different ways at various growth rates and vanish only at very short wavelengths. The experimental results suggest an efficiency of carrier generation close to one due to the very high electric fields existing inside the growing film. The fitting of the experimental curves (photocurrent or photovoltage vs. thickness) at long wavelengths allowed the estimation of the mobility times lifetime products for both photocarriers. Experiments performed at very short wavelength indicate that the hole is the slow photocarrier in anodic Nb2O5 films. The influence of illumination and of the ionic fluxes on the electron lifetime is reported. Useful information on the kinetics of film growth can be obtained from photoelectrochemical measurements.
Consecutive oxidation–reduction cycles involving successive oxidation treatments of 0.3 % Pt/ Al2O3 at various temperatures, each followed by a reduction treatment in hydrogen to produce active catalyst, lead to changes in catalytic activity for the reactions of n-hexane with hydrogen. Decreased activity is paralleled by changes in selectivity brought about by the increased tendency of large platinum particles towards coking. Changes in selectivity are not observed where an increase in activity occurs. Results show that the reforming reactions of n-hexane can be used to gain information about the state of metal particles following sintering/redispersion of Pt on alumina.
Exposure of aqueous xanthine oxidase to 60Co γ-rays at 77 K resulted in electron addition to an iron–sulphur centre at low doses. At higher doses, addition to the MoVI unit was also observed, using ESR detection. The results are interpreted in terms of rapid electron transfer from MoV to the nearest Fe/S cluster, with permanent trapping at molybdenum only when a second electron is generated in a given molecule. The ESR spectrum for the primary MoV centre closely resembles that for a centre previously only detected in the presence of a substrate molecule, such as xanthine, and known as the ‘very rapid’ centre. Hence we conclude that there is no major covalent bonding between MoV and the substrate in this complex.On annealing, the first change involved conversion of the primary MoV centre to a secondary MoV centre, exhibiting a 13 G proton hyperfine coupling. The ESR parameters closely resemble those for a centre previously described as the ‘rapid’ centre. This is the first detectable species in rapid-freeze experiments in the absence of specific substrates. When D2O was used, the proton coupling was lost. The ESR parameters are compatible with the postulate that the initial species has a sulphide ligand (Mo—S)– and the second species is protonated on sulphur (Mo—SH).Further annealing results in irreversible loss of the MoV signal and concomitant growth of a second Fe/S cluster signal. These results are discussed in terms of the remarkable selectivity of electrons ejected within the protein by γ-rays, and the very different rates of electron transfers between the different centres.There was also evidence for electron capture at an RS—SR unit giving, initially, an RS—SR– radical anion, followed by an irreversible conversion into a characteristic centre, probably formed from the anion by protonation. This centre is of interest since it is also formed from solvated electrons in pulse-radiolysis studies.
Infrared spectral data are presented for the adsorption of acetaldehyde on pure and acid-modified dispersed silica at various temperatures. New species are found, and it is shown that an oligomerisation takes place during adsorption. The interaction of adsorbed acetaldehyde with oxygen results in adsorbed oxidation products only on silica modified with acetic acid but not on pure silica. The role of adsorbed acid in formation of oligomeric acetaldehydes as a possible intermediate in the oxidation process is discussed.
The complexation of cobalt(II) with chloride, bromide and iodide ions has been studied by spectrophotometry and calorimetry in hexamethyl-phosphoric triamide (HMPA) containing 0.1 mol dm–3(n-C4H9)4NCIO4 as a constant ionic medium at 25 °C. The formation of [CoXn](2–n)(n= 1–4 with X as CI, n= 1–3 with X as Br and n= 1 and 2 with X as I) is proposed, and their formation constants, enthalpies and entropies are determined. Electronic spectra of individual cobalt(II) halogeno complexes are also extracted, indicating that the cobalt(II) ion is four-coordinated in HMPA(unlike other oxygen-donor solvents which show six-coordination), and four-coordinate tetrahedral [CoXn(HMPA)4–n](2–n)+(n= 1–4) are formed stepwise. In spite of the remarkable strong donicity of HMPA compared to other aprotic donor solvents, the ΔH°1values of formation of [CoX]+ are even smaller, or more negative, in HMPA is observed in contrast to endothermicity in N,N- dimethylformamide or dimethyl sulphoxide. This suggests that, although being sterically allowed, the four-coordination of [co(HMPA)4]2+is considerable crowded for bulky HMPA molecules, and Co—O(HMPA) bonds are thus elongated to some extent.
Yields of CO2 resulting from the oxidation of formate induced by the reaction of copper(I) 1: 10 phenanthroline [(OP)2Cu+] with H2O2 have been studied (a) by radiolysis of (OP)2Cu2+–H2O2–formate mixtures in which (OP)2Cu+ is produced at low steady-state concentrations and (b) by mixing initially high concentrations of (OP)2Cu+ with H2O2 and formate. The CO2 yields per mole of (OP)2Cu+ were independent of [H2O2], but increased with increasing [HCO–2] and with decreasing [(OP)2Cu+] to a limiting value of 1.75, which was independent of these concentrations, and hence of dose rate during radiolysis. It is concluded that Fenton-like reactions of (OP)2Cu+ produce both (OP)2Cu3+ and a peroxo complex, (OP)2CuH2O+2, but not OH˙. The peroxo complex oxidises formate with a rate constant in the region of 3 × 107 dm3 mol–1 s–1, but (OP)2Cu3+ does not. Instead it is consumed by reacting quantitatively with (OP)2Cu+, thus terminating what would otherwise be an efficient chain mechanism. The peroxo complex decomposes to (OP)2Cu3+, and differs from OH˙ in its reactivity with formate, (OP)2Cu2+, and phenanthroline.
Proton fluxes at the disc of a ring–disc electrode can be measured by potentiometric measurement with a bismuth oxide ring electrode. When the bulk pH is close to 7, fluxes as low as 10–12 mol s–1 cm–2 can be measured. Theory is developed to describe the relation between the disc flux and the ring pH, allowing for the fact that the diffusion layer may be divided into an H+ region and an OH– region with a titration reaction taking place on the interface between the regions. Analytical expressions, allowing for the different diffusion coefficients of H+ and OH–, are derived for four separate cases. A case diagram is constructed to show the interrelation between the different cases and to show the relation between the disc flux, the bulk pH and the pH at the ring.
Three methods to evaluate apparent activation energies for reduction from temperature-programmed reduction (TPR) measurements are discussed. Application of the well known method of Gentry results in values of the experimental activation energy, Ee, which are dependent on the heating rate in the region 8–15 K min–1. Two new approaches are described: the reference method and the curve-fitting method.Application of a curve-fitting method, by integration of an appropriate nucleation equation, results in an activation energy for the growth of nuclei, Ea, of 60 ± 3 kJ mol–1 for both bulk and supported V2O5.In the reference method, the onset of a single TPR peak of a sample under investigation is compared to the onset of the TPR peak of a reference sample with a known value of Ee. Activation energies obtained by the reference method are in good agreement for almost all heating rates applied (8–20 K min–1). Agreement with the activation energy obtained by Gentry's method exists only in a limited region of heating rates.
