Interactions of Some Amino Acids with Aqueous Tetraethylammonium Bromide at 298.15 K: A Volumetric Approach

The apparent molar volumes, V_,2, of glycine, L-alanine, DL--amino-n-butyric acid, L-valine, and L-leucine have been determined in aqueous 0.25, 0.75, 1.0, and 1.5 mol-dm^–3 tetraethylammonium bromide (TEAB) solutions by density measurements at 298.15 K. These data have been used to calculate the infinite dilution apparent molar volumes, V_2,m, for the amino acids in aqueous tetraethylammonium bromide and the standard partial molar volumes of transfer (_tr V_2,m) of the amino acids from water to the aqueous salt solutions. The linear correlation of V_2,m for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH₃⁺, COO^–), CH₂ group, and other alkyl chains of the amino acids to V_2,m. The results of the standard partial molar volumes of transfer from water to aqueous tetraethylammonium bromide have been interpreted in terms of ion–ion, ion–polar, and hydrophobic–hydrophobic group interactions. The volume of transfer data suggest that ion–ion or ion–hydrophilic interactions are predominant in the case of glycine and alanine, and hydrophobic–hydrophobic group interactions are predominant in the case of DL--amino butyric acid, L-valine, and L-leucine.     

Angular heterocycles. Utilization of arylaminonaphthoquinones in the synthesis of benzophenothiazines

Substituted 6-ethoxyanilino-5H-benzo[a]phenothiazin-5-ones (3a-f) were synthesized by the cyclocondensation of 2-chloro-3-ethoxyanilino-1,4-naphthoquinones with zinc mercaptides of 2-aminothiophenol in pyridine. The nmr, ir and mass spectral data are presented and discussed.     

Volumetric Properties of Aqueous 2-Chloroethanol Solutions and Volumes of Transfer of Some Amino Acids and Peptides from Water to Aqueous 2-Chloroethanol Solutions

Densities and apparent molar volumes of aqueous 2-chloroethanol were determined at temperatures from 15.0 to 34.4°C using digital densimetry. The results of the volumetric measurements have been used to calculate the following thermodynamic quantites at 25°C: V ₂ ^o = 55.05 ± 0.02 cm³-mol^–1, (V ₂ ^o/T)_p = 0.01486 ± 0.00318 cm³-K^–1-mol^–1, and (² V ₂ ^o/T ²)_p = 0.02972 ± 0.00318 cm³-K^–2-mol^–1. Partial molar volumes of transfer from water to 1 mol-dm^–3 2-chloroethanol have also been determined for L-glycine, L-alanine, L-serine, L-glutamic acid, and L-aspartic acid at 35.0°C. The transfer results have been explained in terms of the nature of the interactions of the groups in the solute and solvent. Hydration numbers of L-glycine and L-alanine have also been calculated in aqueous 2-chloroethanol.     

Carbon-13 and nitrogen-15 nuclear magnetic resonance of physostigmine (eserine)

The ¹⁵N and ¹³C nmr spectra of physostigmine are discussed along with complete assignment of the signals. This alkaloid ¹⁵ N nmr spectrum is notable because it contains nitrogens in three different environments.     

Iron(III) complexes of a tetradentate tripodal ligand N,N′,N″-tris (2-benzimidazolyl-methyl)amine

Summary Fe^III complexes of a tetradentate ligand with pendant benzimidazolyl groups have been synthesized and characterized. Room temperature Mössbauer spectra depict a quadruple split doublet in the case of NO _inf3 ^p– as co-ligand, while a nearly symmetrical one line spectrum is obtained for complexes with Cl^–as co-ligand. The isomer shift values are towards the lower end of the range found for other high spin Fe^III complexes. ¹H-n.m.r. spectra of the complexes reveal relatively broad linewidths with large isotropic shifts. Paramagnetically shifted resonances are observed in the range of –10.0–+70.0 p.p.m.Author to whom all correspondence should be directed.     

2,2,2-trifluoroethanol-induced molten globule state of concanavalin a and energetics of 8-anilinonaphthalene sulfonate binding: calorimetric and spectroscopic investigation

The interaction of 2,2,2-trifluoroethanol (TFE) with concanavalin A has been investigated by using a combination of differential scanning calorimetry, isothermal titration calorimetry (ITC), circular dichroism (CD), and fluorescence spectroscopy at pH 2.5 and 5.2. All of the calorimetric transitions at both the pH values were found to be irreversible. In the presence of 4 mol kg(-1) TFE at pH 2.5, concanavalin A is observed to be in a partially folded state with significant loss of native tertiary structure. The loss of specific side chain interactions in the transition from native to the TFE-induced partially folded state is demonstrated by the loss of cooperative thermal transition and reduction of the CD bands in the aromatic region. Acrylamide quenching, 8-anilinonaphthalene sulfonate (ANS) binding, and energy transfer also suggest that in the presence of 4 mol kg(-1) TFE at pH 2.5 concanavalin A is in a molten globule state. ITC has been used for the first time to characterize the energetics of ANS binding to the molten globule state. ITC results indicate that the binding of ANS to the molten globule state and acid-induced state at pH 2.5 displays heterogeneity with two classes of non-interacting binding sites. The results provide insights into the role of hydrophobic and electrostatic interactions in the binding of ANS to concanavalin A. The results also demonstrate that ITC can be used to characterize the partially folded states of the protein both qualitatively and quantitatively.     

Kinetics and mechanism of oxidation of D-altrose by aqua-cerium(IV) in perchloric acid medium

Kinetics of the oxidation of D-altrose by Ce(IV) perchlorate has been studied in perchloric acid medium. At equivalent concentrations the overall order of the reaction is two, being unity in each reactant. The reaction is characterized by a primary positive kinetic salt effect. Perchloric acid has been found to enhance the rate, while the addition of the reaction products retards the rate. No evidence for initial complexation has been obtained. Thermodynamic parameters have been determined and a mechanism consistent with the observed results is proposed.

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D- Ce(IV) . , . . , . . , .

     

Partial Molar Volumes of Amino Acids and Peptides in Aqueous Salt Solutions at 25°C and a Correlation with Stability of Proteins in the Presence of Salts

Partial molar volumes for a homologous series of amino acids and peptides have been measured in aqueous 1M sodium acetate, sodium thiocyanate, and sodium sulfate at 25^°C. These data have been utilized in conjunction with the data in water to deduce partial molar volumes of transfer V _2,m ⁰(tr) from water to these aqueous salt solutions. The volumes of transfer for the amino acids and peptides are found to be positive. The interpretation is that this result arises from the dominant interaction of the sodium salts with the charged centers of amino acids and peptides. Thermal denaturation of the structurally homologous proteins lysozyme and -lactalbumin has been studied in the presence of these salts. Significant thermal stabilization of hen egg-white lysozyme has been observed in the presence of sodium acetate and sodium sulfate. However, the thermal stabilization observed for -lactalbumin is very small in the presence of these salts and sodium thiocyanate leads to a lowering of its thermal denaturation temperature. The rise in the surface tension of aqueous salt solutions with salt concentration has been correlated with the calorimetric and volumetric measurements. The results show that V _2,m ⁰(tr) depends less on the type of electrolyte than on the ionic strength of the solution. The V _2,m ⁰(tr) values correlate very well with the increase in the surface tension of aqueous salt solutions, indicating significant role of surface tension in interactions of amino acids, peptides, or protein with the salts.     

Solid-phase synthesis of an oxalic acid amide library

Monoamides of oxalic acid are of interest as bioisosteric replacements for phosphate groups in the design of new enzyme inhibitors. Here, we have demonstrated the use of oxalic acid as a linker to the Wang resin to synthesize individual or libraries of phosphate biosteres. The highly reactive resin-bound acid chloride reacts with arylamines to yield resin-bound N-aryloxamic acids (oxanilic acids). This methodology is especially useful for the rapid synthesis of 2-(oxalylamino)benzoic acids (OBAs), because it can be utilized for library synthesis and eliminates the intermediate purification step necessary in solution-phase reactions. The products are cleaved off the resin with trifluoroacetic acid in dichloromethane in good yields.     

Complexes of N-aroyl-N′-thiobenzohydrazide with cobalt(II), nickel(II) and zinc(II)

Summary New potential tetradentate ligands, N-benzoyl-N-thiobenzohydrazide (H₂BTBH) and N-salicyl-N-thiobenzohydrazide (H₂SBTH) have been prepared and characterized. Their complexes with Co^II, Ni^II and Zn^II have been prepared and characterized on the basis of elemental analyses, magnetic susceptibility measurements, and u.v.-vis., i.r. and ¹H-n.m.r. spectral studies. The bonding and stereochemistries of the complexes are discussed. H₂BTBH, H₂SBTH and the complexes have been screened towards a number of bacteria.