Acid-base equilibria in organic solvents : Part 2. Cyclic voltammetry in the study of hydrogen-bond formation

The cyclic voltammetric technique is used to study hydrogen-bond formation in some polar organic solvents (S) of electroanalytical interest (1,2-dimethoxy, tetrahydrofuran, dimethylformamide, dimethyl sulphoxide and pyridine). The cathodic shift of the proton reduction caused by stepwise addition of the solvent investigated to a solution of anhydrous perchloric acid in acetonitrile is utilized. The theoretical treatment applied produced evidence that HS⁺ and HS⁺₂ are the only acidic species involved, so that the relevant formation constants can be evaluated. The data obtained mostly compare well with those available in the literature. The features that condition the tendency to hydrogen-bonding and the effect of hydrogen bonding on solvent basicity are discussed.     

Simplex optimization procedure for evaluating equivalence points in sigmoidal and segmented titration curves

The use of the simplex procedure as a non-linear least-squares curve-fitting method is proposed for evaluation of equivalence points in sigmoidal and segmented titration curves. The application of this procedure to theoretical curves affected by different amounts of random noise indicates its effectiveness and accuracy for locating the correct end-point in titrations characterized by very low reaction constants. The relevant results are compared with those obtained by other regression methods as well as by the first-derivative and Gran approaches.     

Peptide backbone folding induced by the C(alpha)-tetrasubstituted cyclic alpha-amino acids 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) and 1-aminocyclopentane-1-carboxylic acid (Ac5c). A joint computational and experimental study

The conformational study of a new group of synthetic peptides containing 4-amino-1,2-dithiolane-4-carboxylic acid (Adt), a cysteine-related achiral residue, has been carried out through a joint application of computational and experimental methodologies. Molecular Dynamics simulations clearly suggest the tendency of this molecule to adopt a gamma-turn conformation in vacuum and help in analyzing the complex and crucial conformational behaviour of the dithiolane ring which appears to preferentially adopt a C(S)-like structure. Electronic structure calculations carried out in solution using the Density Functional Theory also indicate the preservation of the gamma-like folding in apolar solvents and the helix-like one in more polar solvents. A comparison with the achiral 1-aminocycloalkane-1-carboxylic acid (Ac5c) has been carried out using the same computational tools. NMR and IR data on dipeptide derivatives containing the Adt or Ac5c residue show that in chloroform solution all the models prefer a gamma-turn structure, centered at the cyclic residue, stabilized by an intramolecular H-bond, whereas in a more polar solvent, i.e. dimethyl sulfoxide, this folding is not maintained. The experimental conformational studies, extended to N-Boc protected tripeptides, clearly indicate the remarkable tendency of both the five-membered C(alpha)-tetrasubstituted cyclic amino acids Adt and Ac5c to induce the gamma-turn structure also in models able to adopt the beta-bend conformation.     

An electroactive nickel containing polymeric film obtained by electrochemical reduction of an aryl-nickel derivative

The electrochemical reduction of [Ni(PPh₃)₂(p-C₆H₄Br)Br] in acetonitrile results in a polymeric coating of a nickel-containing poly-p-phenylene, formulated as [-Ni(p-C₆H₄)_n-] (n=6?7) on the basis of elemental analysis, IR, UV, MS and ESCA measurements. This coating is found to undergo a two-electron reversible reduction and a one-electron reversible oxidation with a large potential window (3.6 V), without the polymeric film being destroyed. Moreover, the polymer obtained is shown to be electroactive towards dissolved reactants via permeation or mediated electron transfer, depending on the molecular size of the electroactive species employed.     

Influence of organic modifier and temperature on the enantiomeric separation of γ-lactams by HPLC

Summary A high-performance liquid chromatographic method is reported for the resolution of the enantiomers of a series of fused -lactams (2,7-diaza-3-oxo[3.3.0]octan-6-ones) with probable anti-HIV and anticancer activity. Resolution was achieved on a Chiralcel^® OD column, cellulose tris-(3,5-dimethylphenyl carbamate) adsorbed on macroporous silica gel; mixtures ofn-hexane and isopropyl alcohol in different proportions were used as the mobile phase. The analysis was studied at different temperatures.     

Cathodic behaviour of nickel(II) in acetonitrile in the presence of carbon monoxide and substituted phosphines

Summary The reduction of nickel perchlorate in the presence of carbon monoxide and substituted phosphines or diphosphines has been studied in acetonitrile by cyclic voltammetry. The results show that only mono- and bis-carbonylphosphinenickel(O) complexes are formed, while no evidence for the formation of carbonyl-nickel(I) and -nickel(II) species was obtained. Although the oxidation processes are not reversible, a good correlation between the anodic peak potentials relative to nickel(O) complexes and the -donor--acceptor abilities of the phosphines employed was observed.     

Intermolecular complexes of HXeOH with water: stabilization and destabilization effects

Theoretical and matrix-isolation studies of intermolecular complexes of HXeOH with water molecules are presented. The structures and possible decomposition routes of the HXeOH-(H(2)O)(n)(n = 0, 1, 2, 3) complexes are analyzed theoretically. It is concluded that the decay of these metastable species may proceed through the bent transition states (TSs), leading to the global minima on the respective potential energy surfaces, Xe + (H(2)O)(n+1). The respective barrier heights are 39.6, 26.6, 11.2, and 0.4 kcal/mol for n = 0, 1, 2, and 3. HXeOH in larger water clusters is computationally unstable with respect to the bending coordinate, representing the destabilization effect. Another decomposition channel of HXeOH-(H(2)O)(n), via a linear TS, leads to a direct break of the H-Xe bond of HXeOH. In this case, the attached water molecules stabilize HXeOH by strengthening the H-Xe bond. Due to the stabilization, a large blue shift of the H-Xe stretching mode upon complexation of HXeOH with water molecules is featured in calculations. On the basis of this computational result, the IR absorption bands at 1681 and 1742 cm(-1) observed after UV photolysis and annealing of multimeric H(2)O/Xe matrixes are assigned to the HXeOH-H(2)O and HXeOH-(H(2)O)(2) complexes. These bands are blue-shifted by 103 and 164 cm(-1) from the known monomeric HXeOH absorption.     

Microbial products. II. Granaticinic acid,a new antibiotic from a thermophilic streptomycete

Streptomyces sp. XT-11989 produces a mixture of two antibiotics with litmus-like indicator properties. One of them was shown to be identical with granaticin, the other was identified as [1S,3S,4S,7R,9R,10S,13R]-4,7,9,10-tetrahydro-5,12-dioxo-4, 6,10,11,13-pentahydroxy-1,9-dimethyl-(1H,3H)-7,10-ethanonaphtho[2,3-c: 6,7-c]dipyran-3-acetic acid and termed granaticinic acid. Microbial production and nuclear magnetic resonance data of these antibiotics are discussed and the antibacterial properties of the antibiotics are compared.

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Mikrobielle produkte. II. Granaticinsäure, ein neues Antibiotikum von einem thermophilen Streptomyceten

Zusammenfassung Der Streptomycetenstamm XT-11989 produziert eine Mischung von zwei Antibiotika mit Lakmus-ähnlichen Indikatoreigenschaften. Eines dieser Antibiotika erwies sich als Granaticin, das andere wurde als [1S,3S,4S,7R,9R,10S,13R]-4,7,9,10-tetrahydro-5,12-dioxo-4,6,10,11,13-pentahydroxy-1,9-dimethyl-(1H,3H)-7,10-ethanonaphto[2,3-c: 6,7-c]dipyran-3-essigsäure identifiziert und Granaticinsäure benannt. Mikrobiologische Produktion und Kernresonanzdaten dieser antibiotika werden besprochen und ihre antibakteriellen Eigenschaften verglichen.