Magnetic hybrid modified electrodes,based on magnetite nanoparticle containing polyaniline and poly(3,4-ethylenedioxythiophene)

In this paper, we report on the direct electrodeposition of magnetic hybrids based on magnetite nanoparticle containing poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline (PANI) in the presence of magnetite and the special conducting electrolyte, potassium tetraoxalate. The optimal electropolymerization processes (monitored by scanning electron microscopy) were performed potentiostatically, and the incorporation of the iron oxide into the polymeric film was demonstrated by Diffuse Reflectance UV-Visible Spectroscopy (DR-UV–vis) and transmission electron microscopic measurements. Electrochemical quartz crystal nanobalance proved that both the neat PEDOT and the PEDOT/magnetite hybrid show anion exchange behaviour. Cyclic voltammetric features of the polymers and their hybrids exhibited an enhanced redox capacity of the composites. The difference in the effect of the scanning rate on this capacity increase in the two cases could be interpreted by the assumption that the presence of magnetite manifests dominantly in the enhanced intrinsic electroactivity of PANI, while in the case of the PEDOT composite, the extra charge is more connected to the charge surplus originating from the redox activity of the nanoparticles.

     

Trifluoroacetic anhydride promoted tandem conjugate addition of boronic acids/acetal ring opening

A new stereoselective tandem reaction consisting of the metal-free conjugate addition of boronic acids followed by an intramolecular ring opening of a cyclic acetal has been disclosed. Optically pure polysubstituted tetrahydropyrans have been synthesized diastereoselectively by this new reaction. Two new C-C bonds and up to three stereocenters are formed in a single step, allowing the generation of quaternary stereocenters.     

Liquid-phase oxidation of hydrocarbons in the presence of different types of phase-transfer reagents

The autoxidation of cyclohexene, tetralin and cumene was investigated in the presence of non-ionic, anionic and cationic surfactants and it was found that all three types of phase-transfer reagents are able to influence the rate of oxidation. If their HLB values are not too low (> 3) or not too high (< 15) all the non-ionic surfactants increase the rate of oxidation, otherwise they exert a slight inhibitory effect. If present in acid form, anionic surfactants all increase the rate of oxidation, while their Na-salts slightly inhibit the oxidation. Cationic surfactants all increase the autoxidation rate, but their catalytic efficiencies depend strongly on the experimental conditions. It was shown that the phosphonium ion-type surfactants are quickly oxidized by hydroperoxide present, but their catalytic and phase-transfer capabilities are not detectably reduced. In biphasic systems, the catalytic activities of cationic surfactants are strongly reduced by the presence of water as a separate phase. Light scattering measurements demonstrated that all three types of surfactants are prone to self-association when their concentrations are increased, and this limits their influence on the rate of oxidation. The oxidation rate is strongly reduced when non-ionizing but strongly solvating solvents are simultaneously applied in the reaction mixture. The rate-diminishing effect seems to correlate with the sequence of solvating ability of the solvents. It was pointed out that the presence of hydroperoxide is essential for the PTC-catalyzed oxidation of hydrocarbons. It is thought that the cationic and the anionic surfactants interact with the more nucleophilic (the inner one) and the more electrophilic (the outer one) O-atom of hydroperoxide, respectively, whereby homolysis of the O---O bond is facilitated. The non-ionic surfactants exert their rate-influencing effects through H-bond formation. In the PTC-catalyzed oxidation of hydrocarbons, the rate changes linearly only in a narrow range of PTC, hydroperoxide and substrate concentrations, and consequently the turnover number is not suitable to characterize the oxidation.     

Toward direct determination of conformations of protein building units from multidimensional NMR experiments VI: chemical shift analysis of his to gain 3D structure and protonation state information

NMR--chemical shift structure correlations were investigated by using GIAO-RB3LYP/6-311++G(2d,2p) formalism. Geometries and chemical shifts (CSI values) of 103 different conformers of N'-formyl-L-histidinamide were determined including both neutral and charged protonation forms. Correlations between amino acid torsional angle values and chemical shifts were investigated for the first time for an aromatic and polar amino acid residue whose side chain may carry different charges. Linear correlation coefficients of a significant level were determined between chemical shifts and dihedral angles for CSI[(1)H(alpha)]/phi, CSI[(13)C(alpha)]/phi, and CSI[(13)C(alpha)]/psi. Protonation of the imidazole ring induces the upfield shift of CSI[(13)C(alpha)] for positively charged histidines and an opposite effect for the negative residue. We investigated the correspondence of theoretical and experimental (13)C(alpha), (13)C(beta), and (1)H(alpha) chemical shifts and the nine basic conformational building units characteristic for proteins. These three chemical shift values allow the identification of conformational building units at 80% accuracy. These results enable the prediction of additional regular secondary structural elements (e.g., polyProlineII, inverse gamma-turns) and loops beyond the assignment of chemical shifts to alpha-helices and beta-pleated sheets. Moreover, the location of the His residue can be further specified in a beta-sheet. It is possible to determine whether the appropriate residue is located at the middle or in a first/last beta-strand within a beta-sheet based on calculated CSI values. Thus, the attractive idea of establishing local residue specific backbone folding parameters in peptides and proteins by employing chemical shift information (e.g., (1)H(alpha) and (13)C(alpha)) obtained from selected heteronuclear correlation NMR experiments (e.g., 2D-HSQC) is reinforced.     

Ab initio analysis of the conformational changes of the prolyl-proline model peptide

For- -Pro- -Pro-NH₂ is an ab initio model of the prolyl-proline sequence unit present in numerous peptides and proteins. Cis–trans isomerization of the peptide linkage is a crucial step in accessing the active conformation of several proline containing macromolecules.

The present study focuses on the flexibility of the five-membered pyrrolidine ring, which is considered to help other conformational changes as well as cis–trans isomerization. Ring flexibility is characterized by the pseudorotational amplitude, A, and the phase angle, P. Calculations are carried out at the RHF/6-31+G(d) level of theory. The choice of method and level of theory is further supported by single point DFT calculations.

In the course of NMR structure determination of peptides or proteins, proline residues present in the sequences need special attention. Because of the lack of an amide hydrogen, sequential assignment of proline is rather complicated. Furthermore, in solution state, peptide cis–trans isomers are almost always present. Ab initio study on the For- -Pro- -Pro-NH₂ model is a useful tool to discover the structural characteristics of the prolyl-proline sequence unit.     

Peptide models XXXI. Conformational properties of hydrophobic residues shaping the core of proteins. An ab initio study of N‐formyl‐L‐valinamide and N‐formyl‐L‐phenylalaninamide

Employing introductory (3‐21G RHF) and medium‐size (6‐311++G** B3LYP) ab initio calculations, complete conformational libraries, containing as many as 27 conformers, have been determined for diamide model systems incorporating the amino acids valine (Val) and phenylalanine (Phe). Conformational and energetic properties of these libraries were analyzed. For example, significant correlation was found between relative energies from 6‐311++G** B3LYP and single‐point B3LYP/6‐311++G**//RHF/3‐21G calculations. Comparison of populations of molecular conformations of hydrophobic aromatic and nonaromatic residues, based on their ab initiorelative energies, with their natural abundance indicates that, at least for the hydrophobic core of proteins, the conformations of Val (Ile, Leu) and Phe (Tyr, Trp) are controlled by the local energetic preferences of the respective amino acids. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 732–751, 2001     

Capillary electrophoresis tandem mass spectrometry of bromine-containing charged derivatives of peptides

Novel bromine-containing positively charged labels 5-bromo-1-ethyl-thiazolium (BET+) and 5-bromo-1-ethyl-pyridinium (BEP+) ions were studied for improving the interpretation of MS/MS spectra of peptides. 2,5-Dibromo-1-ethyl-thiazolium tetrafluoroborate (DBET) reacts in the order: varepsilon->alpha-amino group>hydroxyl group of Tyr while 2,5-dibromo-1-ethyl-pyridinium tetrafluoroborate (DBEP) reacts preferably with thiol group of Cys>hydroxyl group of Tyr. In this study a simple and fast CE/MS/MS method is presented for investigating the labeling reaction with these new reagents, where the difference in migration times of labeled and unlabeled peptides also gives us information about the position of labeling. These bromine-containing reagents simplify the MS/MS spectra of peptides: the charge of the derivatives increases the intensity of the corresponding ions, thus enhancing the sensitivity of the detection and the characteristic distribution of the bromine isotope (the 79Br and 81Br ratio is nearly one) facilitating the recognition. By eliminating the non-doubled peaks, clear and easily interpretable MS/MS spectra can be produced that contain only the labeled fragments.     

Synthesis and properties of triphenylantimony(V) dithiolate derivative

Interaction of substituted benzene-1,2-dithiols I–III with triphenylantimony(V) dichloride was studied. As a result of the exchange reaction of 3,6-dichlorobenzene-1,2-dithiol with Ph₃SbCl₂ in the presence of triethylamine triphenylantimony(V) 3,6-dichlorobenzene-1,2-dithiolate IV was obtained. Electrochemical properties of the free dithiols and the complex were explored. According to the cyclic voltammetry, the oxidation of the complex is irreversible and corresponds to one electron transfer with follow-up chemical stage. The antioxidant activity of compounds I–IV was examined in the process of oleic acid autoxidation. A more effective inhibition of the oleic acid oxidation was found at adding dithiol and better protective effect was observed in the presence of Ph₃SbCl₂ compared with the complex IV.