Influence of the Length of the Alanine Spacer on the Acidic–Basic Properties of the Ac–Lys–(Ala) n –Lys–NH2 Peptides (n?=?0, 1, 2, …, 5)

By using the potentiometric titration method, we have determined the pK _a values of the two terminal lysine groups in six alanine-based peptides differing in the length of the alanine chain: Ac?CLys?CLys?CNH₂ (KK), Ac?CLys?CAla?CLys?CNH₂ (KAK), Ac?CLys?CAla?CAla?CLys?CNH₂ (KAK2), Ac?CLys?CAla?CAla?CAla?CLys?CNH₂ (KAK3), Ac?CLys?CAla?CAla?CAla?CAla?CLys?CNH₂ (KAK4), and Ac?CLys?CAla?CAla?CAla?CAla?CAla?CLys?CNH₂ (KAK5) in aqueous solution. For each compound, the model of two stepwise acid?Cbase equilibria was fitted to the potentiometric-titration data. As expected, the pK _a values of the lysine groups increase with increasing length of the alanine spacer, which means that the influence of the electrostatic field between one charged group on the other decreases with increasing length of the alanine spacer. However, for KAK3, the pK _a1 value (8.20) is unusually small and pK _a2 (11.41) is remarkably greater than pK _a1, suggesting that the two groups are close to each other and, in turn, that a chain-reversal conformation is present for this peptide. Starting with KAK3, the differences between pK _a1 and pK _a2 decrease; however, for the longest peptide (KAK5), the values of pK _a1 and pK _a2 still differ by about 1 unit, i.e., by more than the value of log₁₀ (4)?=?0.60 that is a limiting value for the pK _a difference of dicarboxylic acids with increasing methylene-spacer length. Consequently, some interactions between the two charged groups are present and, in turn, a bent shape occurs even for the longest of the peptides studied.     

Influence of phosphorus concentration on the state of the surface layer of Pd–P hydrogenation catalysts

Phase composition and surface layer state of the Pd–P hydrogenation catalyst formed at various P/Pd ratios from Pd(acac)₂ and white phosphorus in a hydrogen atmosphere were determined. Palladium on the catalyst surface is mainly in two chemical states: as Pd(0) clusters and as palladium phosphides. As the P/Pd ratio increases, the fraction and size of palladium clusters decrease, and also the phase composition of formed palladium phosphides changes: Pd₃P_0.8 → Pd₅P₂ → PdP₂. The causes of the modifying action of phosphorus on the properties of palladium catalysts for hydrogenation of unsaturated compounds were considered.     

The Effect of Concentration Parameters on Complexation in the Iron(0)–Iron(II)–Glycine–Water System

The processes of formation of iron(II) complexes in aqueous glycine solutions in the pH range of 1.0–8.0 at 298 K and ionic strength of 1 mol/L (NaClO₄) are studied using Clark and Nikolskii’s oxidation potential method. The type and number of coordinated ligands, the nuclearity, and the total composition of the resulting complexes are determined. The following complex species are formed in the investigated system: [Fe(OH)(H₂O)₅]⁺, [FeHL(H₂O)₅]²⁺, [Fe(HL)(OH)(H₂O)₄]⁺, [Fe(OH)₂(H₂O)₄]⁰, [Fe₂(HL)₂(OH)₂(H₂O)₈]²⁺, and [Fe(HL)₂(H₂O)₄]²⁺. Their formation constants are calculated by the successive iterations method using Yusupov’s theoretical and experimental oxidation function. The model parameters of the resulting coordination compounds are determined.     

Thermodynamic properties of tetraphenylantimony benzophenoxymate in the region of 0–450 K

Heat capacity of tetraphenylantimony benzophenoxymate Ph₄SbONCPh₂ is measured for first time using adiabatic calorimeter in the range from 6K to 350K and differential scanning calorimeter in the range from 330 K to 450 K. In the range of 400–450 K is revealed a melting accompanied with partial decomposition of the substance. Standard thermodynamic functions of crystalline Ph₄SbONCPh₂ in the range from T → 0 K to 440 K are calculated. Enthalpy of combustion of this compound is measured in a combustion calorimeter with isothermal cover and static bomb. Standard thermodynamic formation functions of crystalline Ph₄SbONCPh₂ at 298.15 K are calculated. Fractal dimension D is revealed.     

Effect of the State of a Surface Layer on the Properties of Pd–P Catalysts in the Hydrogenation of Alkylanthraquinones

Kinetics and Catalysis - X-ray photoelectron spectroscopy (XPS) was used to study the state of the surface layer of a Pd–P catalyst, which exhibits high selectivity in the hydrogenation of...     

Model studies of the chemisorption of hydrogen and oxygen on the Au (1 0 0) surface

 The convergence of chemisorption energy for hydrogen and oxygen on gold clusters is studied. Two theoretical approaches have been employed; wavefunction methods at the self-consistent-field second–order M?ller–Plesset level and density functional theory and the two methods are compared. Relativistic effective core potentials exploited in the former approach were developed in this work. Received: 25 October 1999 / Accepted: 21 February 2001 / Published online: 11 October 2001     

Radiationless decay of excited states of tetrahydrocannabinol through the S 1–S 0 (conical) intersection

The ground and electronically excited singlet states of tetrahydrocannabinol have been studied theoretically using density functional and time-dependent density functional methods. The vertical excitation energies, the equilibrium geometries as well as the adiabatic excitation energies have been determined. Opening of the six-membered ring between the oxygen and carbon atoms has been considered as photochemical reaction path. This mechanism leads to a typical excited-state intramolecular hydrogen-transfer process and produces low-lying S ₀–S ₁ intersection (possible conical intersection, CI) which provides a channel for effective radiationless deactivation of the electronically excited state.     

Andirolides H–P from the flower of andiroba (Carapa guianensis,Meliaceae)

Three new gedunins, an andirobin, three mexicanolides, and two phragmalin-type limonoids named andirolides H (1), I (2), J (3), K (4), L (5), M (6), N (7), O (8), and P (9) were isolated from an oil of the flower of Carapa guianensis Aublet (Meliaceae). Their structures including the absolute configurations were determined by means of the NMR and CD spectra as well as FABMS. Andirolide H (1) showed antimalarial activity against Plasmodium falciparum in vitro.     

Structures of three molybdenum(0)–tetracarbonyl–Schiff base compounds

The structures of three molybdenum(0)–tetracarbonyl–Schiff base compounds, determined by X-ray diffraction techniques, are reported. The Schiff bases are those derived from 2-acetylpyridine and 4-methoxyaniline, 4-trifluoromethoxyaniline, or 3-fluoro-4-methylaniline. The structure of the Schiff base ligand has also been determined for the ligand derived from 4-trifluoromethoxyaniline. Comparisons are made between metal–nitrogen bond distances, and carbon–nitrogen and carbon–carbon bond distances in the chelate rings, for these and a number of related molybdenum(0)–tetracarbonyl-diimine compounds. Comparisons are also made between ligand geometry in the free and coordinated states.     

Thermodynamics of alternating copolymer of propylene and CO in the 0–550 K region

The temperature dependences of the heat capacity and the temperatures and enthalpies of physical transitions of the alternating copolymer of propylene and CO were studied in the 5–550 K region by adiabatic vacuum and dynamic calorimetry techniques. The heat of combustion of the copolymer was measured at 298.15 K in a calorimeter with a static bomb and an isothermal shield. The thermodynamic parameters of glass transition and fusing were estimated. The thermodynamic functions in the 0–450 K region and the thermodynamic characteristics of the formation of the copolymer from simple substances atT=298.15 K andp=101.325 kPa were calculated. The thermodynamic parameters of the alternating copolymerization of bulk propylene and CO were calculated in the 0–450 K region at standard pressure.     

The heat capacity of the ethylene glycol-tetrahydrofuran system over the range of tetrahydrofuran concentrations 0–20 mol %

The heat capacity of the ethylene glycol (EG)-tetrahydrofuran (THF) system was measured over the range of THF concentrations 0–20 mol % and the temperature range 0–50°C using a DSC204FI differential scanning calorimeter. It was found that the heat capacity passed through a maximum at a THF concentration of 5 mol %; the maximum became more pronounced as the temperature increased. It was suggested that the maximum was caused by microphase separation at THF concentrations of 5–10 mol %. We were unable to observe phase separation with the lower critical point visually over this concentration range up to 95°C. Original Russian Text ? M.N. Rodnikova, D.B. Kayumova, L.A. Tsvetkova, I.A. Solonina, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 6, pp. 1196–1198.     

Electronic transition moment for the 0(0)(0) band of the à ← X̃ transition in the ethyl peroxy radical

The electronic transition moment for the G-conformer of ethyl peroxy was determined from the experimentally measured value of the peak absorption cross-section and the simulation of its rovibronic spectrum using the results of the high resolution spectroscopy of this molecule. The resulting value is |μ(e)(G)| = 2.55(6) × 10(-2) Debye, which is compared to values from electronic structure calculations.     

Application of heating microscopy to the study of thermal behaviour of ZnO–P2O5–WO3 glasses

The effect of WO₃ on thermal behaviour and thermal stability of ZnO–P₂O₅–WO₃ glasses prepared in compositional series (100 ? x)[0.5ZnO–0.5P₂O₅] ? xWO₃ (x = 0–60) was investigated by heating microscopy and the results were correlated with the results determined by conventional thermodilatometry and differential thermal analysis. Thermoanalytical studies showed that the glass transformation temperature and dilatation softening temperature increase with increasing WO₃ content while thermal expansion coefficient decreases. The highest stability towards crystallization possess glasses containing 20–30 mol% WO₃. Major compounds formed by the crystallization of the glasses were Zn(PO₃)₂, WO₃ and W₁₈P₂O₅₉. The values of sphere temperature, hemisphere temperature and flow temperature obtained using heating microscopy were strongly influenced by the degree of crystallization process at the sintering.     

Aging of the paint palette of Valerio Castello (1624–1659) in different paintings of the same age (1650–1655)

It is well known, from ancient Egypt, that some pigments and colourants can change with time for light effect or chemical attack. Cennino Cennini in the fifteenth century in his book “Il libro dell’arte o trattato della pittura” describes the use of many pigments and their degradation. He was aware of the problems and was able to suggest the answers in the use of pigments on several supports, but he could not understand the physical–chemical reason of the alteration processes. In this study, we point out the aging effects in seven paintings, practically of the same period (1650–1655). We considered in particular green, white and blue pigments of the palette of Valerio Castello. About 150 spots were selected on works painted on four different supports, canvas, wood panel, copper and slate. For each point, several determinations were carried on the pigments and decomposition products, aiming to determine the state of conservation of the paintings, the nature of the pigments, their alteration and if the support can affect the kinetics of degradation.     

Liquid–liquid equilibrium of the ternary system water + acetic acid + sec-butyl acetate

Experimental liquid–liquid equilibrium (LLE) of the water–acetic acid–sec-butyl acetate ternary system was investigated at 298.15, 303.15, 308.15 and 313.15 K and at atmospheric pressure. Separation factors were also evaluated for the immiscibility region. The NRTL and UNIQUAC models were applied to fit the experimental data for the ternary system. The binary interaction parameters obtained from both models were found to be successfully correlated with the equilibrium compositions. The UNIFAC group contribution method was employed to predict the observed ternary LLE data. It was found that four types of the UNIFAC model (UNIFAC, UNIFAC-LL, UNIFAC-DMD, and UNIFAC-LBY) did not provide a good prediction of the LLE data for this ternary system.