Dimethylsulfoxide structures

The curvature may be an artifact of a selection of nucleophiles of mixed structural types chosen to display a wide range in pAo. Buncel et al. ° varied pK by changing the solvent composition over a limited range rather than by changing the structure. They studied the reaction between X-C6H4-CT and p-nitrophenyl acetate in 40-90 mol% dimethylsulfoxide—water mixtures with just three X substituents... 

Figure 17.8 Catal3ftic zinc center of horse liver alcohol dehydrogenase revealed from an X-ray crystallographic structure (PDB file 20HX) [Al-Karadaghi et al., 1994]. The bound NADH cofactor, a molecule of the inhibitor dimethylsulfoxide (DMSO), and the amino acid residues that coordinate the Zn are shown as sticks shaded according to the elements, and the Zn center is shown as a gray sphere, while the protein is shown in thin gray lines.

In order to gain information on the environments of certain atoms in dissolved species, in melts or in solids (crystalline or noncrystalline), which are not accessible to diffraction studies for one reason or another, X-ray absorption spectrometry (XAS) can be applied, with the analysis of the X-ray absorption near-edge structure (XANES) and/or the extended X-ray absorption fine structure (EXAFS). Surveys of these methods are available 39,40 a representative study of the solvation of some mercury species, ElgX2, in water and dimethylsulfoxide (DMSO) by EXAFS and XANES, combined with quantum-chemical calculations, has been published.41... 

The octet rule is one of the cornerstones of chemical bonding theory. While the vast majority of molecules conform, apparent exceptions occur for molecules incorporating second-row (and heavier) main-group elements. Apparent refers to the fact that molecules such as dimethylsulfoxide and dimethylsulfone may either be represented in terms of structures with ten and twelve valence electrons, respectively, surrounding sulfur, or as zwitterions with the normal complement of eight valence electrons (see also discussions in Chapters 5 and 16). 

One difference in behavior between the hydrophilic alkali halides and hydrophobic solutes like the larger tetraalkylammonium halides in water is expressed by the enthalpy. The enthalpies of solution of the larger tetraalkylammonium halides in water are more exothermic than those of the corresponding alkali halides but in other solvents, e.g., several amides, propylene carbonate (PC), and dimethylsulfoxide (DMSO), the reverse is true. Generally, this phenomenon is attributed to an enhanced hydrogen bonding in the highly structured solvent water in the vicinity of the tetraalkylammonium ions (hydrophobic hydration) (i). This idea is substantiated by the absence of the effect in solvents like N,N-dimethylformamide (DMF), PC, and DMSO (2), where specific structural effects are not present in the pure solvents. 

The heats of solution and dilution of electrolytes in nonaqueous-aqueous solvent mixtures have been limited mostly to alcohol-water systems and a few measurements in dimethylsulfoxide-water and dioxane-water mixtures (J, 2). The structural maximum in aqueous-organic solvents at high water content has been well established by a variety of techniques (3,4), but few systems have been explored over the whole composition range. 

Thus, with methyl and also ethyl cyanoacetate, the anilinomethylene chromonedione (220) (this form has been found94 to predominate in dimethylsulfoxide solution rather than the tautomeric structure of the formal SchifFs base from 3-formyl-4-hydroxycoumarin and aniline) yields, initially, with potassium hydroxide in DMF, following hydrolysis, the pyronocoumarin 221. 

C6C15) with dimethylsulfoxide (DMSO), which leads to the synthesis of [Tl2 Au(C6F5)2 2 p-DMSO 3] or [Tl2 Au(C6Cl5)2 2 h-DMSO 2]ra, respectively.62 The crystal structure of the complex with fluorine shows a monodimensional polymer formed by repetition of [Au--Tl(p-0 = SMe2)3Tl] units, with gold-thallium interactions of 3.2225(6)-3.5182(8), while the pentachlorophenyl derivative contains two bridging DMSO molecules and an additional [Au(C6Cl5)2] anion. In addition, a thallium-thallium interaction of 3.7562(6) A appears in the latter (Fig. 21). 

The different conformational behavior of the azobenzoyl- and the azobenzenesul-fonyl-L-lysine polymers was explained on the basis that the monomeric units VI may interact with HFP differently than units V do (Scheme 4). The strongly proto-nating solvent HFP (pKa = 9.30) 36 is known to form electrostatic complexes with various organic compounds, including amines and dimethylsulfoxide 1371 on the other hand, sulfonamides are significantly protonated in acid media 38 so it may be presumed that protonation and formation of electrostatic complexes can occur for azobenzenesulfonyl-L-lysine residues, as well. In HFP therefore, polypeptides of structure V can adopt the ordered a-helix structure, while polypeptides of structure VI should be forced by the electrostatic interactions arising from complexation with HFP to adopt a disordered conformation. 

Crude 4-amino-2-methyl-10H-thieno[2,3-b][l,5]benzodiazepine, hydrochloride (4.3 g) was refluxed in a mixture of N-methylpiperazine (15 mL), dimethylsulfoxide (20 mL) and toluene (20 mL) under a nitrogen atmosphere for 20 hours. The mixture was cooled to ca. 50°C, water (20 mL) added, and the product allowed to crystallise at 5°C over night. The product was filtered and crystallised from acetonitrile (30 mL) to give the title compound (1.65 g) m.p. 195°C. The structure of the compound was confirmed spectroscopically. 

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