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At least 19 recordsLinked to original sources

PubMed1992

Solvent effects on chemical processes. I: Solubility of aromatic and heterocyclic compounds in binary aqueous-organic solvents.

The standard free energy change (delta G0) for equilibrium dissolution in binary solvent mixtures is written as a sum of effects arising from solvent-solvent interactions (the general medium effect), solvent-solute interactions (the solvation effect), and solute-solute interactions (the intersolute effect). The general medium effect is given by gA gamma, where g is a curvature correction factor to the surface tension (gamma) and A is the molecular cavity surface area. A new feature is the definition of gamma to be that value appropriate to the equilibrium mean solvation shell composition. The solvation effect is modeled by stoichiometric stepwise competitive equilibria between the two solvent components for the solute. The intersolute effect includes the crystal energy and solution phase interactions. In this work, water was solvent component 1, and various miscible organic cosolvents served as solvent component 2. Relating all data to the fully aqueous solution gives an explicit expression for delta M delta G0, the solvent effect on the free energy change, as a function of the mole fractions x1 and x2. This function is a binding isotherm. Nonlinear regression leads (for a two-step solvation scheme) to estimates of the solvation exchange constants K1 and K2 and the parameter gA. This relationship was applied to 44 systems comprising combinations of 31 solutes and eight organic cosolvents. Curve fits were good to excellent, and most of the parameter estimates had physically reasonable magnitudes.

Chemical Phenomena↗

PubMed1992

Analysis of solvent structure in proteins using neutron D2O-H2O solvent maps: pattern of primary and secondary hydration of trypsin.

A method of determining the water structure in protein crystals is described using neutron solvent difference maps. These maps are obtained by comparing the changes in diffracted intensities between two data sets, one in which H2O is the major solvent constituent, and a second in which D2O is the solvent medium. To a good first approximation, the protein atom contributions to the scattering intensities in both data sets are equal and cancel, but since H2O and D2O have very different neutron-scattering properties, their differences are accentuated to reveal an accurate representation of the solvent structure. The method also employs a series of density modification steps that impose known physical constraints on the density distribution function in the unit cell by making real space modifications directly to the density maps. Important attributes of the method are that (1) it is less subjective in the assignment of water positions than X-ray analysis; (2) there is threefold improvement in the signal-to-noise ratio for the solvent density; and (3) the iterative density modification produces a low-biased representation of the solvent density. Tests showed that water molecules with as low as 10% occupancy could be confidently assigned. About 300 water sites were assigned for trypsin from the refined solvent density; 140 of these sites were defined in the maps as discrete peaks, while the remaining were found within less-ordered channels of density. There is a very good correspondence between the sites in the primary hydration layer and waters found in the X-ray structure. Most water sites are clustered into H-bonding networks, many of which are found along intermolecular contact zones. The bound water is equally distributed between contacting apolar and polar atoms at the protein interface. A common occurrence at hydrophobic surfaces is that apolar atoms are circumvented by one or more waters that are part of a larger water network. When the effects on surface accessibility by neighboring molecules in the crystal lattice are taken into consideration, only about 29% of the surface does not interface ordered water. About 25% of the ordered water is found in the second hydration sphere. In many instances these waters bridge larger clusters of primary layer waters. It is apparent that, in certain regions of the crystal, the organization of ordered water reflects the characteristics of the crystal environment more than those of trypsin's surface alone.

Deuterium↗

PubMed1992

The clinical significance of sleep apnoea in workers exposed to organic solvents: implications for the diagnosis of organic solvent encephalopathy.

Among 51 patients referred for investigation of possible organic solvent encephalopathy 20 (39%) had pathological sleep apnoea [apnoea index (AI) greater than 5], compared with 5 of 16 house painters exposed to solvents (31%) who were screened for the disorder, and 1 of 18 (6%) age-matched controls. Twelve of the patients with AI greater than 5 were retested after 2 or more weeks without exposure to solvents, and showed a significant drop in AI. Likewise, significantly lower AI was seen in patients who were no longer exposed to solvents, compared with recently exposed patients. The implications of these findings for diagnostic evaluation of solvent encephalopathy and sleep apnoea are discussed.

Adult↗

PubMed1978

The role of solvents in the stabilization of helical structure: the low pH ribo A8 and A10 double helices in mixed solvents.

The order-disorder transitions of the double helices formed by the ribo-oligoadenylic acids rA8 and rA10 at pH 4.2 have been investigated in a series of organic/aqueous mixed solvents. Melting temperature data, Tm, derived from the uv melting curves were used to define the stability of the double helices in the different mixed solvent systems. It was found that the extent of helix destabilization depended in a predictable fashion on both the quantity and the nature of the added organic solvents. For the C1 through C4 aliphatic alcohols, the longer, less branched alcohols proved to be more effective destabilizers of the helical structure. Significantly, the amides proved to be more powerful destabilizers than the alcohols. Analysis of the melting curves provided the Van't Hoff enthalpy change for each transition. The data are interpreted in terms of the role of solvent in the stabilization of ribonucleic acid structure.

Hydrogen-Ion Concentration↗

PubMed1991

Derivatives of CAP having no solvent-accessible cysteine residues, or having a unique solvent-accessible cysteine residue at amino acid 2 of the helix-turn-helix motif.

The Escherichia coli catabolite gene activator protein (CAP) is a helix-turn-helix motif sequence-specific DNA binding protein. CAP contains a unique solvent-accessible cysteine residue at amino acid 10 of the helix-turn-helix motif. In published work, we have constructed a prototype semi-synthetic site-specific DNA cleavage agent from CAP by use of cysteine-specific chemical modification to incorporate a nucleolytic chelator-metal complex at amino acid 10 of the helix-turn-helix motif [Ebright, R., Ebright, Y., Pendergrast, P.S. and Gunasekera, A., Proc. Natl. Acad. Sci. USA 87, 2882-2886 (1990)]. Construction of second-generation semi-synthetic site-specific DNA cleavage agents from CAP requires the construction of derivatives of CAP having unique solvent-accessible cysteine residues at sites within CAP other than amino acid 10 of the helix-turn-helix motif. In the present work, we have constructed and characterized two derivatives of CAP having no solvent-accessible cysteine residues: [Ser178]CAP and [Leu178]CAP. In addition, in the present work, we have constructed and characterized one derivative of CAP having a unique solvent-accessible cysteine residue at amino acid 2 of the helix-turn-helix motif: [Cys170;Ser178]CAP.

Base Sequence↗

PubMed1976

Steric inhibition of conjugation in lowest excited singlet state of 9-anthramide by hydrogen bond donor solvents: role of solvent in chemical structure.

9-Anthramide has electronic absorption and fluorescence spectra that, in water, are similar to those of anthracene. This result is attributed to steric hindrance of the 9-carboxamido group with the peri-hydrogen atoms in the 1- and 8-positions of the anthracene ring. However, in aprotic solvents, although the absorption spectrum of 9-anthramide is anthracene-like, its fluorescence spectrum is red shifted and structureless. This finding is attributed to excited-state rotation of the 9-carboxamido group into coplanarity with the anthracene ring and indicates that, in water, the hydrogen-bonded solvent cage affects the steric inhibition of conjugation in excited 9-anthramide. These findings suggest that studies of structure and reactivity of drugs in nonaqueous or solid matrixes are probably of only limited value, since in the strongly interacting aqueous media the aqueous solvent cage plays a substantial role in determining molecular structure and reactivity.

Amides↗

PubMed1975

Subcutaneous secretin in dogs: influence of solvent and volume of solvent.

Pancreatic secretion in response to subcutaneously injected secretin was studied in three dogs with chronic pancreatic and gastric fistulas. Three solvents: saline, gelatin, carboxymethylcellulose (CMC); two volumes of solvent: 2 and 6 ml; and two doses of secretin: 75 and 150 clinical units were tested. Gelatin was more effective than CMC in prolonging the action of secretin; the duration of the plateau of secretion was about twice as long with gelatin as with saline. Increasing the volume of solvent from 2 to 6 ml also increased the total duration of secretion and total HCO-3 output. It is concluded that gelatin is a suitable vehicle for prolonging the action of secretin.

Analysis of Variance↗

PubMed1978

Studies related to nitrosamide formation: nitrosation in solvent: water and solvent systems, nitrosomethylurea formation in the rat stomach and analysis of a fish product for ureas.

Nitrous acid (HNO2) was partly extracted from water by organic solvents, especially polar ones. Carbaryl was nitrosated in solvent: water mixtures most rapidly when nonpolar solvents, e.g. methylene chloride, carbon tetrachloride and hexane, were used. Under given conditions, carbaryl was nitrosated in methylene chloride:water mixtures 20 times faster than in water alone, mainly because of its insolubility in water. For ethylurea, hexylurea, ethyl N-ethyl-carbamate and aminopyrine, nitrosation by sodium sulfate-dried methylene chloride extracts of nitrous acid ('dried HNO2') was at least 88% complete after reaction for 5 seconds at 6 degrees C. Nitrosation of N-butylacetamide by the same extract proceeded more slowly, with a second-order rate constant 31,000 times greater than for nitrosation in water at pH 2. Butylacetamide was nitrosated in methylene chloride by equivalent concentrations of 'dried HNO2', dinitrogen trioxide and dinitrogen tetroxide at similar rates. Nitrosomethylurea (NMU) formation was measured radioactively in the stomach contents of rats fed [3H]methylurea (MU) and sodium nitrite. When both compounds were given in the food (100 mg MU and 4 g NaNO2/kg) and the rats were killed 3 hours later, the NMU yield was 0.46% of the MU. When the food also contained 11.5 g sodium ascorbate/kg, NMU production was completely inhibited. With 2-4 g/1 sodium nitrite in the drinking water and MU in the food, no NMU was detected. Ureas were determined in dried, salted bonito fish from Japan, by a method involving ion-exchange and paper chromatography. The fish sample contained 80 mg urea/kg, but no MU. When the fish was nitrosated at pH 1 and then denitrosated at pH 0, 25 mg MU/kg was detected. Mu identity was confirmed by mass spectrometry.

Animals↗

PubMed1975

Evaluation and selection of optimal solvents and solvent combinations in thin-layer chromatography. Application of the method to basic drugs.

A series of simple mathematical techniques for the evaluation of solvents and solvent combinations in thin-layer chromatography have been investigated. A strategy for the rapid selection of the optimum combination is proposed. It uses classification procedures based on calculation of the similarity between systems. The classification is carried out using a simple graph-theoretical procedure (Kruskal's algorithm) or numerical taxonomy. The selection of optimal sets from the clusters which appear in the classification is based on the information content as derived from Shannon's equation. The method has been applied to an RF data set for basic drugs. It is concluded that these methods indeed allow the selection of optimal systems or combination of systems.

Chromatography, Thin Layer↗

PubMed1975

Solubility of nonelectrolytes in polar solvents III: Alkyl p-aminobenzoates in polar and mixed solvents.

The relative solubilities of n-alkyl p-aminobenzoates in water, proplyene blycol-water mixtures, proplyene glycol, and several other pharmaceutically important solvents can be predicted on the basis of a theoretical equation. This equation relates the activity coefficient of the hydrophobic portion of the molecule to the product of its surface area and its interfacial tension [free energy per unit area of a hydrocarbon (tetradecane) against the polar or semipolar solvent of interest]. The assumptions, conclusions, and applicability of the theorectical relationship are compared to those of the Scatchard-Hildebrand approach.

Alcohols↗

PubMed1976

Solubility of nonelectrolytes in polar solvents IV: nonpolar drugs in mixed solvents.

The molecular and group surface area approach to solubility is shown to be applicable to mixed aqueous solvent systems. An equation is derived which is consistent with the exponential increase in the aqueous solubility of nonpolar drugs that frequently accompanies the addition of a cosolvent. This equation predicts that: (a) the ability of a drug to be solubilized by a cosolvent is proportional to its hydrophobic surface area per molecule, and (b) the ability of a cosolvent to solubilize any drug is inversely proportional to its interfacial tension against a reference liquid hydrocarbon. These predictions are experimentally verified with solubility studies of several alkyl p-aminobenzoates in propylene glycol-water mixtures and of hexyl p-aminobenzoate in mixtures of water the ethanol, methanol, ethylene glycol, propylene glycol, glycerin, and formamide.

Aminobenzoates↗

PubMed1976