Biomimetic zinc funnel complexes based on calix[6]N3ArO ligands: an acid-base switch for guest binding

The coordination chemistry of Zn in an N(3)ArOH environment has been explored. The ligands are based on calix[6]arenes that present two imidazole arms and an amino phenol moiety at the narrow rim. Three different types of complexes have been characterized. One is dicationic with Zn(2+) coordinated to the three nitrogen atoms and to the oxygen of the phenol group of the calix[6]ligand. This complex is very sensitive to exogenous coordinating molecules and exists as a 5-coordinate species due to the endo-complexation of a guest. The second species is a monocationic complex for which the phenol group has been deprotonated. The resulting N(3)ArOZn complex can also bind a guest ligand albeit with a lower affinity than the dicationic complex. The third species is neutral. It can be obtained upon reaction with a base to yield a hydroxo complex or with an anion such as a chloride that coordinates the metal center from the outside of the calixarene cavity. The simultaneous binding of two anionic donors decreases the Zn Lewis acidity, allowing an impressive conformational reorganization of the system. One imidazole arm is released by the metal center. The other one undergoes self-inclusion into the pi-basic calixarene cavity because the low affinity of the metal center for neutral ligand does not allow the endo-coordination of an exogenous guest. Hence, the calix[6]N(3)ArOH-based Zn complexes act as an acid-base switch for guest binding. Several aspects of this system appear reminiscent of Zn-peptidases of the astacin and serralisin families.     

A new protecting group for the exocyclic amino groups of nucleosides

A new protecting group has been developed for the exocyclic amino groups of nucleosides that occur in DNA. 3-Phenyl-[{N-(2-trimethylsilyl-ethoxycarbonyl)-2-amino}]-propanoic acid used as the protective agent.     

A post-Hartree-Fock model of intermolecular interactions

Intermolecular interactions are of great importance in chemistry but are difficult to model accurately with computational methods. In particular, Hartree-Fock and standard density-functional approximations do not include the physics necessary to properly describe dispersion. These methods are sometimes corrected to account for dispersion by adding a pairwise C6R6 term, with C6 dispersion coefficients dependent on the atoms involved. We present a post-Hartree-Fock model in which C6 coefficients are generated by the instantaneous dipole moment of the exchange hole. This model relies on occupied orbitals only, and involves only one, universal, empirical parameter to limit the dispersion energy at small interatomic separations. The model is extensively tested on isotropic C6 coefficients of 178 intermolecular pairs. It is also applied to the calculation of the geometries and binding energies of 20 intermolecular complexes involving dispersion, dipole-induced dipole, dipole-dipole, and hydrogen-bonding interactions, with remarkably good results.     

Sequential ortho effects: characterization of novel [M - 35]+ fragment ions in the mass spectra of 2-alkyl-4, 6-dinitrophenols

High-resolution mass spectrometry (HRMS), hybrid tandem mass spectrometry (MS/MS) (EBqQ), and photoelectron-photoion coincidence (PEPICO) experiments were conducted to examine a possible ortho-ortho effect resulting in a novel [M - 35]⁺ fragment ion in 2-alkyl-4, 6-dinitrophenols. For compounds having ethyl or larger alkyl substituents, [M35]⁺ was observed only when [M - 18]⁺ ions were present, with the ortho nitro group being involved in the reaction to [M- 35]⁺. For [M - 18]⁺ and [M - 35]⁺, HRMS results were consistent with losses of H₂O and H₂O + OH, respectively, whereas MS/MS results indicated a sequential reaction due to metastable dissociations. The appearance energy determined by PEPICO for [M - 35]⁺ was found to be greater than the appearance energy for [M - 18]⁺, thus supporting a sequential reaction. 69–75).     

Syntheses and configurations of some heterocyclic amidoximes. X-Ray crystal structure of 3-phenyl-5,6-dihydro-2(1H)-pyrazinone-O-methyloxime

O-Methyl-α-ketophenylacetohydroximoyl chloride ( 1 ) was prepared by the reaction of O-methyl-α-methoxyphenylacetohydroximoyl chloride ( 5 ) with N-bromosuccinimide and concentrated hydrobromic acid. Reaction of 1 with ethylenediamine gave 3-phenyl-5,6-dihydro-2(1H)-pyrazinone-O-methyloxime ( 6 ). 3-Phenyl-5,6-cyclohexano-5,6-dihydro-2(1H)-pyrazininone-O-methyloxime ( 7 ) was prepared by reaction of 1 with trans-1,2-diaminocyclohexane. The X-ray structure of 6 has been determined. The crystals are orthorhombic, space group Pbca with a = 10.264(3), b = 18.262(4), c = 23.530(4)Å, V = 4411(2)ų, and Z = 16. The structure, which was refined to R = 0.038 using 1652 observed reflections, shows the amidoxime moiety to be the Z configuration. Reaction of benzohydroximoyl chloride with aziridine gave (Z)-aziridinylbenzaldoxime ( 16a ). Ultraviolet irradiation of a benzene solution of 16a gave a mixture of the Z and E isomers 16a and 16b . The E isomer 16b underwent thermal isomerization to 16a at 100°. Reaction of 16a with dimethyl sulfate in sodium hydroxide solution gave (Z)-O-methylaziridinylbenzaldoxime ( 17a ). Photoisomerization of a hexane solution of 17a gave a mixture of the Z and E isomers 17a and 17b which were separated by preparative glc. The isomers 17a and 17b are resistant to thermal Z = E isomerization. The mechanisms of thermal isomerization of benzamidoximes are discussed.     

ABSORPTION SPECTRAL SHIFTS OF CAROTENOIDS RELATED TO MEDIUM POLARIZABILITY

Abstract–Solvent induced absorption spectral shifts of the electronic transition from ground 1 A_g state to the excited 1B_u state in carotenoids have been studied. It is shown that the shift depends only on dispersion interactions in non-polar solvents. In polar media there is just a small extra contribution to the red-shift, due to other forms of interactions. The spectral shifts are well described by the theory, which expresses the shift relative to the gas phase value, as a function of solvent polarizability. The main conclusion is that the dominating mechanism behind the large red-shifted absorbance of carotenoids in the proteinacous environment, in vivo, is the mutual polarizability interactions between the carotenoids and the surrounding medium. The solution-phase values of the dipole moments of the lA_g to 1B_u transitions and the differences of isotropic polarizability between 1B_u and lA_g states of carotenoids in non-polar solvents are calculated and found to be around 13 D and 360 ų respectively. From the great overlap of absorption spectra between carotenoids in quinoline and carotenoids in vivo in purple bacterial antenna complexes, it can be expected that the carotenoids are surrounded by several aromatic amino acids in vivo. Comparisons have been done between the exicted states in carotenoids and in linear conjugated polyenes.     

Reduction of nitric oxide,nitrous acid and nitrogen dioxide at platinum electrodes in acidic solutions: Review and new voltammetric results

The literature concerning the chemical and electrochemical reactions of nitric oxide, nitrous acid and nitrogen dioxide in aqueous solutions is reviewed briefly, with emphasis on electrochemical reductions at platinum electrodes in acidic solutions. The voltammetric behavior of NO and NO₂ at a Pt electrode in perchloric acid is virtually identical to that for HNO₂ and this is explained on the basis of a common electroactive precursor concluded to be NO⁺. Three cathodic waves are obtained for acidic solutions of NO, HNO₂ and NO₂. The first two waves correspond to reduction of NO⁺ to NO and N₂O₃ to NO, respectively. The presence of N₂O₃ results from decomposition of the parent compounds. The presence of Br^- or Cl^- in acidic solutions of the title compounds promotes the voltammetric reductions at lower H⁺ concentrations. This probably results from formation of electroactive nitrosyl halides.     

Advantages to Conversion of Lattice Heat Capacity to Cv in the Resolution of Excess Properties The Ln2S3's as an example

This paper represents a fitting (modeling) of the temperature dependence of the Komada-Westrum characteristic temperature for those γ-, δ- and ε-phase lanthanide sesquisulfides for which the total heat capacities, including internal degrees of freedom (e.g., Schottky and magnetic contributions), were connected to the residue of only lattice vibrations yielding lattice heat-capacity contributions. These characteristic temperatures (θ_KW) at 298.15 K are seen to behave smoothly (nearly linearly) as a function of (cationic) atomic number within the region of stability of each phase as does the density. The trends between the phases also show some consistency but not predictability of one from the other. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of mixed metal decaosmium carbido clusters: The X-ray structures of the monoanions [Os10C(CO)24Cu(NCMe)]− and [Os10C(CO)24AuPPh3]−

The high nuclearity mixed metal cluster monoanions [Os₁₀C(CO)₂₄Cu(NCMe)]^? (I) and [Os₁₀C(CO)₂₄AuPPh₃]^? (II) have been obtained by reaction of the carbido-dianion [Os₁₀C(CO)₂₄]^2? (III) with one equivalent of [Cu(NCMe)₄] [BF₄] and Ph₃PAuCl, respectively, in CH₂Cl₂. X-ray analysis of the [PPh₃Me]⁺ salts of I and II show that the Cu and Au ligands have added to capping tetrahedra of the dianion III in μ₃- and μ₂-bridging positions, respectively.