Cation-binding to biomolecules

Ab initio SCF computations indicate that Mg²⁺ should bind essentially to the oxygen atoms of uracil, the remaining part of the base being rather repulsive towards such an interaction. The Coulombic component predominates in the interaction, the essential feature of which may thus be deduced from the study of the molecular electrostatic potential of uracil. These ab initio results contradict an earlier CNDO prediction that the binding of uracil and Mg²⁺ should occur preferentially at the C₅=C₆ double bond of the base. It is shown that the CNDO result is an artifact due to an exaggeration by this method of the charge transfer between the ligand and the cation. The small amount of available experimental data seem in favor of the ab initio results.     

The influence of vibronic interactions on the chiroptical spectra of dissymmetric pseudo tetragonal metal complexes

The influence of vibronic interactions on the chiroptical spectra associated with a threesome of nearly degenerate electronic excited states in a dissymmetric molecular system is examined on a formal theoretical model. The model considers two vibrational modes to be effective in promoting pseudo Jahn-Teller (PJT) type interactions between the three closely spaced electronic excited states. Formal expressions are developed for the rotatory strengths of individual vibronic levels derived from the coupled electronic states. Two mode (vibrational)-three state (electronic) vibronic Hamiltonians are constructed (basis set size, 63–108, depending upon interaction parameters used) and diagonalized for a large number of different parameter sets representative of various vibronic coupling strengths, electronic energy level spacings, oscillator (vibrational mode) frequencies, and electronic rotatory strengths. Diagonalization of these vibronic Hamiltonians yields vibronic wave functions and energies which are then used to calculate gif" alt="ldquo" align="MIDDLE" BORDER="0">rotatory strength spectragif" alt="rdquo" align="MIDDLE" BORDER="0"> for the model system. The calculated results demonstrate the profound influence which vibronic interactions of the PJT type may have on the sign patterns and intensity distributions within the rotatory strength spectrum associated with a set of nearly degenerate electronic states. The implication of these results for the interpretation of circular dichroism spectra of chiral transition metal complexes with pseudo tetragonal symmetry are discussed.     

syn,Anti isomerization of 5,5′-Bis-5H-dibenzo[a,d]cyclohepten-5-ylidene

syn-5,5gif" alt="prime" align="BASELINE" BORDER="0">-Bis-5H-dibenzo[a,d]cyclohepten-5-ylidene (2syn), an overcrowded bistricyclic aromatic ene with central seven-membered rings, undergoes a thermal isomerization to the anti stereoisomer (2anti) at elevated temperatures, with gif" alt="Delta" align="BASELINE" BORDER="0">G _inv ^{gif" alt="ngt" align="MIDDLE" BORDER="0">} (174°C)=36.4 kcal/mole. The high-energy barrier for syn gif" alt="rarr" align="BASELINE" BORDER="0"> anti isomerization of2 is compared with the corresponding inversion barrier of 5H-dibenzo[a,d]cycloheptene derivatives and interpreted in terms of a lower degree of overcrowding in2 relative to bistricyclic enes (1) with central five- and six-membered rings. The overcrowding features of2syn,2anti, and theiranti-10,11,10gif" alt="prime" align="BASELINE" BORDER="0">,11gif" alt="prime" align="BASELINE" BORDER="0">-tetrahydro derivative (3) is analyzed, and a mechanism for the syn gif" alt="rarr" align="BASELINE" BORDER="0"> anti isomerization of 2 is proposed.     

Quantum mechanical studies of environmental effects on biomolecules

LCAO SCF calculations with a (7s, 3p/3s) atomic basis are performed on urea and hydrated urea in order to determine the preferred sites of water fixation using the supermolecule approach. It appears that up to five water molecules can be directly bound to urea in the first hydration shell.     

A comparison of eight commercial reverse-phase (ODS) columns for the separation of hydroxylated derivatives of 7,12-dimethylbenz(a)anthracene

This paper presents an evaluation of a number of commercially available octadecylsilane columns. The separation of four 7,12-dimethylbenz(a)anthracene (DMBA) derivatives namelycis- andtrans-5,6-dihydroxy DMBA,trans-8,9-dihydro-8,9-dihydroxy DMBA, and 7,12-dihydroxymethyl benz(a)anthracene (BA) was compared under identical conditions comprising of a concave gradient (No. 4) from 20 to 70% acetonitrile/water for 75 minutes at flow rate of 1ml/min. A direct correlation between percent carbon loading and retention behavior was observed for bonded phases prepared from silica particles with similar surface areas. Other column performance parameters, such as capacity factors, separation ratios and resolution between peaks for the solutes examined were also calculated and are discussed.     

Recent progress in tartaric acid‐modified Raney nickel system for enantio‐differentiating hydrogenation

Tartaric acidgif" alt="dash" align="MIDDLE" BORDER="0">modified Raney nickel (TAgif" alt="dash" align="MIDDLE" BORDER="0">MRNi) is an enantiogif" alt="dash" align="MIDDLE" BORDER="0">differentiating catalyst for hydrogenation of ketones. The stereochemical models explaining the enantiogif" alt="dash" align="MIDDLE" BORDER="0">differentiation of gif" alt="beta" align="MIDDLE" BORDER="0">gif" alt="dash" align="MIDDLE" BORDER="0">ketoester and 2gif" alt="dash" align="MIDDLE" BORDER="0">alkanone were unified as an extended stereochemical model by the experimental supports. Based on this new model, a working hypothesis to improve the enantiomeric excess (ee) of the gif" alt="beta" align="MIDDLE" BORDER="0">gif" alt="dash" align="MIDDLE" BORDER="0">ketoester/TAgif" alt="dash" align="MIDDLE" BORDER="0">MRNi system was developed, and the ee was improved to 96%. By further fine tuning of this system, almost perfect enantiogif" alt="dash" align="MIDDLE" BORDER="0">differentiation resulting in over 98% ee was achieved.     

Chemical interactions between 2-mercaptobenzazoles and π-acceptors

Summary 2-Mercaptobenzazoles (1a–c) interact with several gif" alt="pgr" align="BASELINE" BORDER="0">-acceptors such as tetracyanoethylene (TCNE) 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL) dicyanomethyleneindane-1,3-dione (CNIND), 2,3-dicyano-1,4-naphthoquinone (DCNQ), 9-dicyanomethylene-2,4,7-trinitrofluorene (DTF), and 2,3-dichloro-1,4-naphthoquinone (DCHNQ)via the formation of charge-transfer (CT) complexes to yield various heterocyclic compounds.

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Chemische Wechselwirkungen zwischen 2-Mercaptobenzazolen und gif" alt="pgr" align="BASELINE" BORDER="0">-Akzeptoren

Zusammenfassung Die 2-Mercaptobenzazole1a–c reagieren mit verschiedenen gif" alt="pgr" align="BASELINE" BORDER="0">-Akzeptoren wie Tetracyanoethylen (TCNE), 2,3-Dichlor-5,6-dicyano-1,4-benzochinon (DDQ), 2,3,5,6-Tetrachlor-1,4-benzochinon (CHL), Dicyanomethylenindan-1,3-dion (CNIND), 2,3-Dicyano-1,4-naphthochinon (DCNQ), 9-Dicyanomethylen-2,4,7-trinitrofluoren (DTF) und 2,3-Dichlor-1,4-naphthochinon (DCHNQ) unter Ausbildung von charge transfer — Komplexen (CT) zu heterocyclischen Verbindungen.

     

Synthese von α-Methyl-homocysteinthiolacton

Summary A facile high yield large scale methylation procedure affording2c is reported utilizing the N-[bis(methylthio)-methylen-protected derivative4a as an intermediate. The optical resolution of racemic2c is described leading to (S)-2b. In addition the thiolactone2c undergoes oxidative ring opening by bromine to the corresponding sulphonic acid5.

     

Basis set dependence of localized orbitals

The effects of Gaussian basis set contraction and addition of polarization functions on H₂O localized orbitals have been studied at the experimental geometry. It is shown that the electric moments and moment features of localized orbitals are not influenced very much by basis set quality variations, as going from medium size to enlarged basis sets. The difference between bond pair and lone pair charge densities was found to be larger on approaching the Hartree-Fock limit. A minimal basis set, however, does not suitably characterize the localized charge distributions.     

Hexafluoroacetone as Protecting and Activating Reagent. Glycosylated Malic, Citramalic, and Thiomalic Acid Derivatives, New Glycosylated Building Blocks for Drug Design [1]

Summary. Glycosylated gif" alt="agr" align="BASELINE" BORDER="0">-hydroxy and gif" alt="agr" align="BASELINE" BORDER="0">-mercapto acids have been synthesized starting from malic/citramalic/thiomalic acid and Ac₄-gif" alt="beta" align="MIDDLE" BORDER="0">-D-Glc-NH₂/Bzl₄-gif" alt="beta" align="MIDDLE" BORDER="0">-D-Glc-NH₂ using hexafluoroacetone as protecting and activating reagent.Dedicated to Prof. Dr. Horst Wilde on the occasion of his 65^th birthday