The solubility of dodecane in water-amide mixtures

The solubility of dodecane has been determined in formamide, and in mixtures of water with formamide, N-methylformamide (NMF), and dimethylformamide (DMF) by a radiotracer method at 15°, 25° and 35°. The solubility of dodecane increases as the concentrations of the amides increase. The standard free energies (ΔG°), partial enthalpies (ΔH°), and standard entropies (ΔS°), of solution of dodecane have been calculated for the various systems. Positive values of ΔG° and negative values of ΔH° and ΔS° decrease numerically as the concentrations of amides increase.     

Binding of tolmetin and salicylic acid to human serum albumin as a function of temperature

When drug-protein binding data are evaluated thermodynamically standard free energy (ΔG°), standard enthalpy (ΔH°) and standard entropy (ΔS°) are usually estimated from association constants (K_a) derived from binding data obtained at only two temperatures. Estimation of ΔH° involves the assumption of its constancy in the temperature range studied and linearity of a van't Hoff plot of In K_a versus 1/T. Sometimes the assumption of such linearity is invalid for theoretical reasons and data obtained at only two temperatures contain no information concerning linearity of this plot. We present data for the binding of both tolmetin and salicylic acid to human serum albumin as a function of temperature which make doubtful the validity of using association constants of these drugs to derive thermodynamic constants other than ΔG° values.     

Temperature,medium and structural effects on the acid dissociation constants of certain Schiff bases derived from isatin with some amino acids and aroylhydrazines

The acid dissociatioin constants of certain Schiff bases derived from isatin with p-substituted benzoyl hydrazines and somee amino acids of glycine, β- and α-alanine, valine, β-phenyl-α-alanine and anthranilic acid have been determined potentiometrically at different temperatures in different aquo-organic solvent mixtures (ethanol, dioxane, ddimethyl formamide, methanol, acetone and tetrahydrofurance). The pKa values were demonstrated on the light of the different electronic and steric effects of the substituents and the solvent characteristics. In all the mixed media used, ionization of the compounds decreased by increasing the mole fraction of the organic cosolvent. Theermodynamic parameters (ΔH°, ΔG°, ΔS°) were evaluated. The structural effects of the investigated compounds on these parameters were reported and discussed.     

Aqueous nitrate ion adsorption/desorption by olive solid waste-based carbon activated using ZnCl2

ZnCl₂ activated carbon (ZnCl₂-AC, with specific surface area ~1480 m²/g) effectively adsorbs nitrate from water with uptake capacity (~5.5 mg/g) four-fold that for other commercial activated carbons (CACs). Kinetics show a small activation energy value (~7 kJ/mol) consistent with a diffusion-controlled process. This is further confirmed by intra-particle diffusion study. Negative thermodynamic parameters (ΔH° ?13.859 kJ/mol, ΔS° ?43.8 J/mol K, and ΔG° ?0.808 kJ/mol) indicate spontaneous nitrate adsorption. The high specific surface area rationalizes the high uptake capacity for the ZnCl₂-AC. A Langmuir type adsorption, is observed. ZnCl₂-AC effectively adsorbs the nitrate from water, even at high concentrations, and brings the concentration to allowed maximum values. Despite these advantages, the ZnCl₂-AC exhibits low desorption (~20%) of pre-adsorbed nitrate ions into concentrate solutions. Up to ~40% desorption is observed by grinding the pre-saturated ZnCl₂-AC. Developing other methods to completely recover the adsorbed nitrate and the adsorbent for multiple use is needed.     

Prediction of the solubility of griseofulvin in glycerides and other solvents of relatively low polarity from simple regular solution theory

Regular solution theory is applied to literature values of the solubility of griseofulvin in hydrocarbons and in other solvents of relatively low polarity. The solubility parameter of griseofulvin is calculated as $\text{10.2 ± 0.2}\text{cal}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{·}\text{cm}{}^{\mathrm{<mtext>?</mtext><mtext>3</mtext><mtext>2</mtext>}}\text{at 25 °}\text{C}$. The relatively high solubilities of griseofulvin in polar, non-hydrogen-bonded solvents are interpreted in terms of dipolar or hydrogen-bonding interactions between solute and solvent, whereas the sub-ideal solubilities in alcohols and water are attributed to solvent self-association. The solubilities of griseofulvin in the glycerides are predicted from regular solution theory in its simplest form, assuming that London dispersion forces are the most important interactions in solution. The solubility parameter of each solvent is calculated from its refractive index. The predicted values agree well with the experimental values of solubility in the triglycerides and diglycerides, except when the hydrocarbon component is so high that the solvent is behaving effectively as a hydrocarbon. The solubilities of griseofulvin in the monoglycerides are about one-fifth of the predicted values, and this is attributed to appreciable solvent self-association by hydrogen-bonding.     

A sustainably produced hydrochar from pomegranate peels for the purification of textile contaminants in an aqueous medium

Hydrothermal Carbonization presents an easy, inexpensive, and eco-friendly method to convert waste from natural resources into sustainable materials. This study used the Hydrothermal Carbonization (HTC) approach to make hydrochar from locally available pomegranate peels. Several analytical tools were employed for its characterization, including elemental analysis, X-ray Diffraction, micro-Raman, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Textual analysis, pH_pzc, and Boehm titration. The produced hydrochar performed well in removing cationic dye Basic Red 46 (BR46) from an aqueous medium, adsorbing 286.90 and 367.72 mg g^?1 within only 5 and 60 min, respectively, at ideal circumstances. The Freundlich model was the most appropriate for the equilibrium data, whilst the pseudo-second-order model was found closest to the kinetic data (R² = 0.980; Δqe = 7.934%). Per the thermodynamic study, the dye adsorption was spontaneous (ΔG° < 0) and endothermic (ΔH° = 32.777 kJ mol^?1). Furthermore, the prepared hydrochar (HCPP) has revealed good reusability, as it can be reused up to five times without losing its effectiveness. In summary, the outcomes obtained revealed that HCPP could be an effective and eco-friendly adsorbent for the treatment of colored wastewater.     

Synthesis and stabilization of amino and carboxy terminal constrained collagenous peptides

Short collagenous peptides cross-linked at their amino and carboxy termini with Lys-Lys-dimer template(s) were synthesized, and the effect of the cross-linking on their stabilities was investigated by thermal denaturation experiments. Two chemoselective ligations were used for the construction of the amino and the carboxy cross-linked peptides. The thermal transition temperature (T_m) and the standard free energies (ΔG°) of the cross-linked collagenous peptides increased, and the thermal stabilization effect corresponded to an elongation by two units of the Gly-Pro-Hyp triad. The van't Hoff enthalpy (ΔH°) and the entropy (ΔS°) values of the cross-linked peptides increased with chain elongation, although the increments were smaller than those of the linear peptides. When the same chain lengths were compared, the ΔH° was increased and the ΔS° was nearly the same or increased by the cross-linking. These results suggest that the cross-linking of the collagenous peptides with the Lys-Lys-dimer template(s) for stabilization contributes to the enthalpic effect, rather than the entropic effect.     

Structural stability of β-lactoglobulin in the presence of kosmotropic salts A kinetic and thermodynamic study

The thiol group of β-lactoglobulin reacted very sluggishly with dithio-bis-nitro-benzoic acid as compared to that of glutathione at pH 6.85. The pK_app value of the thiol group of the protein was 9.35. In the presence of 3 M urea, the thiol group reacted completely with dithio-bis-nitrobenzoic acid at pH 6.85. Heating (from 50° to 80°) increased the exposure of the thiol by dissociating the dimer unit. From the pseudo-first order rate constants of heat-exposure of thiol, thermodynamic activation parameters, ΔG⁺₊, ΔH⁺₊, and ΔS⁺₊, for the heat-dissociation of β-lactoglobulin dimer were estimated to be 23 290 cal/mol, 31 160cal/mol, and 22.9 e.u. (at 70°), respectively. Addition of kosmotropic salts, chloride, tartrate, sulfate, phosphate, and citrate (0.2 m ) decreased the heat-induced exposure of the thiol group (at 70°), probably by decreasing the dissociation of the dimer at pH 6.85. The relative change in free energy of activation for the dissociation of the dimer, Δ(ΔG⁺_+dimer), in the presence of the salts was positive, suggesting that these additives increase the stability of the dimer against heat. These salts also increased the conformational stability of β-lactoglobulin as revealed by an increase in - Δ(ΔG⁰_conf) values in their presence. Both Δ(ΔG⁺_+dimer) and - Δ(ΔG⁰_conf) values followed the order, chloride < tartrate < sulfate < phosphate < citrate. These salts seem to manifest their structure-stabilizing effect by increasing both inter- and intramolecular hydrophobic interactions via changes in structure of water.     

Codissolution of calcium hydrogenphosphate and sodium hydrogencitrate in water. Spontaneous supersaturation of calcium citrate increasing calcium bioavailability

The sparingly soluble calcium hydrogenphosphate dihydrate, co-dissolving in water during dissolution of freely soluble sodium hydrogencitrate sesquihydrate as caused by proton transfer from hydrogencitrate to hydrogenphosphate, was found to form homogenous solutions supersaturated by a factor up to 8 in calcium citrate tetrahydrate. A critical hydrogencitrate concentration for formation of homogeneous solutions was found to depend linearly on dissolved calcium hydrogenphosphate: [HCitr^2?] = 14[CaHPO₄] - 0.05 at 25 °C. The lag phase for precipitation of calcium citrate tetrahydrate, as identified from FT-IR spectra, from these spontaneously formed supersaturated solutions was several hours, and the time to reach solubility equilibrium was several days. Initial calcium ion activity was found to be almost independent of the degree of supersaturation as determined electrochemically. The supersaturated solutions had a pH around 4.7, and calcium binding to hydrogencitrate as the dominant citrate species during precipitation was found to be exothermic with a determined association constant of 357 L mol^?1 at 25 °C for unit ionic strength, and ΔH° = ?22 ± 2 kJ mol^?1, ΔS° = ?26 ± 8 J K^?1 mol^?1. Calcium binding to hydrogencitrate and, more importantly, to citrate is suggested to decrease the rate of precipitation by lowering the driving force of precipitation, and becoming important for the robust spontaneous supersaturation with perspectives for design of functional foods with increased calcium bioavailability.     

1H-n.m.r. study of the folding of ribonuclease 12-(β-(3-pyridyl)-l-Ala) S-peptide (1–14)

The ¹H-n.m.r. spectra (360 MHz) of 12-(β-(3-pyridyl)-l -Ala) ribonuclease S-peptide (1–14), a tetradecapeptide incorporating (β-3-pyridyl-l -Ala) instead of His at position 12, have been assigned. The shift vs. temperature dependence has been analyzed at three different pD's in terms of a two-state helix (3–13) ± coil equilibrium, and the corresponding values for the thermodynamic quantities ΔH° and ΔS° determined. Helix populations at 0°C have been measured as a function of pD, showing their dependence on two apparent pK_a's at ? 3.3 and 5.5, with a maximum at pD ? 4.2. All the obtained results show that the new peptide has very similar folding properties to those shown by S-peptide and particularly to those of C-peptide. The 3–13 helix formed is stabilized by two interactions: a salt-bridge Glu 2^-. Arg 10+ and a partial stacking between the aromatic rings of residues Phe 8 and His 12. Calculations involving ring current shifts and potential energies validate the possible existence of this latter interaction, which must present a local geometry defined by χ₁X⁸ 180°, χ₂X⁸ 100°, χ₁¹² – 60 and χ₂¹² 80.     

Conformational investigation of α, β-dehydropeptides. VIII.*N-Acetyl-α, β-dehydroamino acid N'-methylamides: conformation and electron density perturbation from infrared and theoretical studies

The Fourier transform infrared spectra are analyzed in the regions of v_s(N-H), amide I, amide II and v_s(C^α=C^β) bands for a series of Ac-ΔXaa-NHMe, where ΔXaa =ΔAla, (Z)-ΔAbu, (Z)-ΔLeu, (Z)-ΔPhe and AVal, to determine the predominant solution conformation of these α, β-dehydropeptide-related molecules and the electron distribution perturbation in their amide bonds. The measurements were performed in dichloromethane (DCM). To confirm and rationalize the assignments, the spectra of the respective series of saturated Ac-Xaa-NHMe, recorded in DCM, and the spectra of these two series of unsaturated and saturated compounds, recorded in acetonitrile, were examined. To help interpret the spectroscopic results, the equilibrium geometrical parameters for some selected amides were used. These were optimized with ab initio methods in the 6-31G** basis set. Each of the dehydroamides studied adopted a C₅ structure, which in Ac-ΔAla-NHMe is fully extended and accompanied by the strong C₅ hydrogen bond. Interaction with the C^α=C^β bond lessened the amidic resonance within each of the flanking amide groups. The N-terminal C=O bond was noticeably shorter, both amide bonds were longer than the corresponding bonds in the saturated entities and the N-terminal amide system was distorted. Ac-ΔAla-NHMe constituted an exception. Its C-terminal amide bond was shorter than the standard one and both amide systems were prototypically planar.     

Peptide aggregates: a novel model system to study self-assembly of peptides

Ordered aggregates of Val-Leu-Pro-Phe, tetrapeptide 1 , have been found in aqueous solutions. Evidence for the formation of aggregates for the above peptide was obtained by conductometric, pH metric, UV and fluorescence spectroscopic techniques. Values of critical micelle concentration (CMC) for the above peptide obtained by these methods are in good agreement with each other. The formation of organized aggregates of the peptide is favoured upon increasing the temperature (viz. the process of aggregation is endothermic). The aggregation number has been determined at different temperatures. Values of ΔG°_mΔH°_mΔS°_m and ΔC°_p have also been estimated. Binding studies with the 8-anilinonaphthyl sulfonic acid (ANS) and pyrene indicate that the interior of the aggregate is nonpolar. There are two processes with regard to the change of thermodynamical parameters like ΔG°_mΔH°_mΔS°_mΔC°_p aggregation number (N). In the first process (from 5°C to 40°C) the driving force for aggregation seems to be the positive entropy because of water release due to intermolecular association of ionic moieties. The second process (from 40°C and above) is due to intramolecular ionic interaction. The chemical shifts of the amide protons of the peptide have been presented in the light of inter- and intramolecular hydrogen-bond formation, and forces implicated in aggregation for both the first and second processes. © Munksgaard 1995.     

Conformations of dehydrophenylalanine containing peptides Nuclear Overhauser effect study of two acyclic tetrapeptides

Two isomeric, acyclic tetrapeptides containing a Z-dehydrophenylalanine residue (Δ^z-Phe) at position 2 or 3, Boc-Leu-Ala-Δ^z-Phe-Leu-OMe (1) and Boc-Leu-Δ^z-Phe-Ala-Leu-OMe (2), have been synthesized and their solution conformations investigated by 270MHz ¹H n.m.r. spectroscopy. In peptide 1 the Leu(4) NH group appears to be partially shielded from solvent, while in peptide 2 both Ala(3) and Leu(4) NH groups show limited solvent accessibility. Extensive difference nuclear Overhauser effect (n.O.e.) studies establish the occurrence of several diagnostic inter-residue n.O.e.s (Cα_jH ? N_i+1H and N_iH ? N_i+1H) between backbone protons. The simultaneous observation of “mutually exclusive” n.O.e.s suggests the presence of multiple solution conformations for both peptides. In peptide 1 the n.O.e. data are consistent with a dynamic equilibrium between an -Ala-Δ^z-Phe- Type II β-turn structure and a second species with Δ^z-Phe adopting a partially extended conformation with Ψ values of ± 100° to ± 150°. In peptide 2 the results are compatible with an equilibrium between a highly folded consecutive β-turn structure for the -Leu-Δ^z-Phe-Ala- segment and an almost completely extended conformation.     

Conformational investigation of α,β-dehydropeptides VII*. Conformation of Ac-Pro-ΔAla-NHCH3 and Ac-Pro-(E)-ΔAbu-NHCH3: comparison with (Z)-substituted α,β-dehydropeptides†

The crystal structure and solution conformation of Ac-Pro-ΔAla-NHCH₃ and the solution conformation of Ac-Pro-(E)-ΔAbu-NHCH₃ were investigated by X-ray diffraction method and NMR, FTIR and CD spectroscopies. Ac-Pro-ΔAla-NHCH, adopts an extended-coil conformation in the crystalline state, with all-trans peptide bonds and the ΔAla residue being in a C₅ form, φ₁=– 71.4(4), ψ₁=– 16.8(4), φ₂=– 178.4(3) and ψ₂= 172.4(3)°. In inert solvents the peptide also assumes the C₅ conformation, but a γ-turn on the Pro residue cannot be ruled out. In these solvents Ac-Pro-(E)- ΔAbu-NHCH₃ accommodates a βII-turn, but a minor conformer with a nearly planar disposition of the CO—NH and C=C bonds (φ₂～0°) is also present. Previous spectroscopic studies of the (Z)-substituted dehydropeptides Ac-Pro-(Z)- ΔAbu-NHCH, and Ac-Pro-ΔVal-NHCH₃ reveal that both peptides prefer a βII-turn in solution. Comparison of conformations in the family of four Ac-Pro-ΔXaa-NHCH₃ peptides let us formulate the following order of their tendency to adopt a β-turn in solution: (Z)- ΔAbu > (E)- δAbu > ΔVal; ΔAla does not. None of the folded structures formed by the four compounds is stable in strongly solvating media. © Munksgaard 1996.     

A solubility equation for non-electrolytes in water

A solubility equation is developed based on the steps: solid → super-cooled liquid → octanol → water. Estimating the standard Gibbs energy change for each step gives the following result: logC^s_2.w = 0.80 ? 7.3 × 10^?4[Δ^f(T_m ? 298)] ? 7.3 × 10^?4[V₂(10.3 ? δ₂)²] ? logPC where C^s_2,w is the molar aqueous solubility of the solute, ΔS^f its entropy of fusion, T_m its melting point, V₂ its molar volume, δ₂ its solubility parameter and PC its octanol-water partition coefficient. Comparison of this equation with a regression equation based on experimental data shows the derived result above to be nearly as good as the regression equation. The equation appears to provide good aqueous solubility estimates for solids and liquids.     

Solubilities of testosterone propionate in non-polar solvents at 100°

THE solubilities of the formate to valerate esters of testosterone in non-polar solvents at 25° were determined by James & Roberts (1968) who also compared them with ideal mole fraction solubilities (X₂), calculated from the equation, (Hildebrand & Scott 1962). ΔH^F is the heat of fusion of the solute and T_M the melting point. Changes in solubility as the homologous series is ascended were predicted by equation (1), but the individual experimental results did not agree with the calculated values. ΔH^F was calculated from the heat of fusion at the melting point, ΔH^F_M, by correcting for the differences in heat capacity of the solid and the supercooled liquid between T_M and T. The correction was estimated with a differential scanning calorimeter by extrapolating the liquid enthalpy line back to 25° and measuring the area between the extrapolation and the enthalpy line of the solid. The method was considered questionable, however, because it assumed that the enthalpy line of the supercooled liquid decreased linearly over the whole range of temperature. This theory is tested below by comparing the measured and calculated solubilities of testosterone propionate at a temperature just below its melting point, where the heat capacity correction is small and ΔH^F_M can be used for ΔH^F. The solvents examined by James & Roberts (1968) had smaller molar volumes than the testosterone esters, and it was suggested that the difference in molar volume between solute and solvent could prevent the random distribution assumed by regular solution theory. Prediction of solubility would thus improve as the molar volume of the solvent approached that of the solute. The test is applied below by determining the solubility of testosterone propionate in a range of solvents.     

The identification, isolation, and preservation of delta-9-tetrahydrocannabinol (delta-9-THC)

(—)-trans-Δ⁹-Tetrahydrocannabinol (Δ⁹-THC) was isolated from marihuana plant extract, by adsorptive column and glc. The adsorptive column chromatography method consisted of chromatographing marihuana extract on a column packed with a mixture of silica gel (gas chromatography grade (100/120 mesh), silver nitrate and calcium sulphate (CaSO₄·H₂O) (3:1:0·5) with benzene as the eluting solvent. The glc method consisted of chromatographing the extract on a 3 ft silanized glass column (3/8 inch o.d.) packed with 1·5 ft of 2% QF-1 and 1·5 ft of 2% OV-17 on chromosorb W, AW 30–60 mesh, prep grade. A purity of 99% for the isolated Δ⁹-THC was confirmed by infrared spectroscopy, nuclear magnetic resonance, mass spectroscopy. The effects of storage conditions on Δ⁹-THC stability, monitored by glc, indicated the best method for preserving Δ⁹-THC was at 0°, protected from light, stored under nitrogen.     

Design of peptides withα,β-dehydro-residues: synthesis,and crystal and molecular structure of a310-helical tetrapeptide Boc-l-Val-ΔPhe-ΔPhe-l-Ile-OCH3

Abstract: The peptide Boc-l -Val-ΔPhe-ΔPhe-l -Ile-OCH₃ was synthesized using the azlactone method in the solution phase, and its crystal and molecular structures were determined by X-ray diffraction. Single crystals were grown by slow evaporation from solution in methanol at 25°C. The crystals belong to an orthorhombic space group P2₁2₁2₁ with a = 12.882(7) Å, b = 15.430(5) Å, c = 18.330(5) Å and Z = 4. The structure was determined by direct methods and refined by a least-squares procedure to an R-value of 0.073. The peptide adopts a right-handed 3₁₀-helical conformation with backbone torsion angles: φ₁ = 56.0(6)°, ψ₁ = –38.0(6)°, φ₂ = –53.8(6)°, ψ₂ = 23.6(6)°, φ₃ = –82.9(6)°, ψ₃ = –10.6(7)°, φ₄ = 124.9(5)°. All the peptide bonds are trans. The conformation is stabilized by intramolecular 4→1 hydrogen bonds involving Boc carbonyl oxygen and NH of ΔPhe³ and CO of Val¹ and NH of Ile⁴. It is noteworthy that the two other chemically very similar peptides: Boc-Val-ΔPhe-ΔPhe-Ala-OCH₃ (i) and Boc-Val-ΔPhe-ΔPhe-Val-OCH₃ (ii) with differences only at the fourth position have been found to adopt folded conformations with two overlapping β-turns of types II and III′, respectively, whereas the present peptide adopts two overlapping β-turns of type III. Thus the introduction of Ile at fourth position in a sequence Val-ΔPhe-ΔPhe-X results in the formation of a 3₁₀-helix. The crystal structure is stabilized by intermolecular hydrogen bonds involving NH of Val¹ and carbonyl oxygen of a symmetry related (–x, y – 1/2, 1/2 + z) ΔPhe² and NH of ΔPhe² with carbonyl oxygen of a symmetry related (x, y1/2, 1/2 + z) Ile⁴. This gives rise to long columns of helical molecules linked head to tail running along [010] direction.     

Crystal structure and conformation of a highly constrained linear tetrapeptide Boc-Leu-dehydro Phe-Ala-Leu-OCH3

The conformation of a tetrapeptide containing a dehydro amino acid, Δ^zPhe, in its sequence has been determined in the crystalline state using X-ray crystallographic techniques. The tetrapeptide, Boc-Leu-Δ^zPhe-Ala-Leu-OCH₃, crystallizes in the orthorhombic space group P2₁2₁2₁ with four molecules in a unit cell of dimensions a = 11.655(1) Å, b = 15.698(6) Å and c = 18.651(3) Å V = 3414.9 Å and Dcalc =1.12 g/cm ^?3. The asymmetric unit contains one tetrapeptide molecule, C₃₀H₄₆N₄O₇, a total of 41 nonhydrogen atoms. The structure was determined using the direct methods program SHELXS86 and refined to an R-factor of 0.049 for 3347 reflections (13.0(I). The linear tetrapeptide in the crystal exhibits a double bend of the Type III-I, with Leu¹ (<φ=?54.1°, Ψ=?34.5°) and Δ^zPhe² (φ=?59.9°, Ψ=?17.1°) as the corner residues of Type III turn and Δ^zPhe² (φ=?59.9°, Ψ=?17.1°) and Ala³ (φ=?80.4°, Ψ= 0.5°) residues occupying the corners of Type I turn, with Δ^zPhe as the common residue in the double bend. The turn structures are further stabilized by two intramolecular 4→1 type hydrogen bonds.