Formation of polyaniline/Pt nanoparticle composite films and their electrocatalytic properties

Polyaniline (PANI) thin films modified with platinum nanoparticles have been prepared by several methods, characterised and assessed in terms of electrocatalytic properties. These composite materials have been prepared by the in situ reduction of a platinum salt (K₂PtCl₄) by PANI, in a variety of solvents, resulting in the formation of platinum nanoparticles and clusters of different sizes. The further deposition of platinum clusters at spin cast thin films of PANI/Pt composites from a neutral aqueous solution of K₂PtCl₄ has also been demonstrated. Thin-film electrodes prepared from these materials have been investigated for their electrocatalytic activity by studying hydrazine oxidation and dichromate reduction. The properties of the composite materials have been determined using UV–visible spectroscopy, atomic force microscopy and transmission electron microscopy. The nature of the material formed is strongly dependent on the solvent used to dissolve PANI, the method of preparation of the PANI/Pt solution and the composition of the spin cast thin film before subsequent deposition of platinum from the aqueous solution of K₂PtCl₄.Dedicated to Professor Dr. Alan Bond on the occasion of his 60th birthday.     

Rapid sample-mixing technique for transient NMR and photo-CIDNP spectroscopy: applications to real-time protein folding

We describe the development and application of a novel rapid sample-mixing technique for real-time NMR (nuclear magnetic resonance) spectroscopy. The apparatus consists of an insert inside a conventional NMR tube coupled to a rapid injection syringe outside the NMR magnet. Efficient and homogeneous mixing of solutions in the NMR tube is achieved with a dead time of tens of milliseconds, without modification of the NMR probe or additional hardware inside the magnet. Provision is made for the inclusion of an optical fiber to allow in situ laser irradiation of samples, for example to generate photo-CIDNP (chemically induced dynamic nuclear polarization). An NMR water suppression method has been implemented to allow experiments in H(2)O as well as in deuterated solvents. The performance of the device has been tested and optimized by a variety of methods, including sensitive detection of residual pH gradients and the use of NMR imaging to monitor the extent of mixing in real time. The potential utility of this device, in conjunction with the sensitivity and selectivity of photo-CIDNP, is demonstrated by experiments on the protein hen lysozyme. These measurements involve the direct detection of spectra during real-time refolding, and the use of CIDNP pulse labeling to study a partially unfolded state of the protein under equilibrium conditions. Magnetization transfer from this disordered state to the well-characterized native state provides evidence for the remarkable persistence of nativelike elements of structure under conditions in which the protein is partially denatured and aggregation prone.     

Peptide sequencing with the 2-pyridinecarboxaldehyde schiff base peptide degradation

Dedicated to Professor John C. Sheehan on the occasion of his sixty-fifth birthday. Peptides treated with 2-pyridinecarboxaldehyde are converted to Schiff bases, pyridoimidazoles, α-ketoacyl amino acids and peptide amides, and diketopiperazines. Nitrogen abstraction and condensation with 2-pyridinecarboxaldehyde gives heterocyclic products. The products are separable by gas chromatography and when analyzed by mass spectrometry afford information for sequence assignment. Thus, a nonvolatile peptide can be converted to a mixture of products in one step that by mass spectrometric analysis affords the sequence. Peptides of up to eight and nine amino acids have been analyzed using this degradation as the basis of a one-step procedure.     

Schiff base derivatives of peptide esters: Relative abundance of N-terminal,C-terminal and ‘internal’ fragments as a function of the blocking group

Electron-impact (EI) mass spectrometry of peptide derivatives is usually interpreted in terms of fragmentation where the charge resides on the N-terminal fragments and to a lesser degree on the less common, charged C-terminal fragments. Substituted and unsubstituted benzylidene, cinnamylidene, α- and β-naphthylidene derivatives of a reference tripeptide, valileala, gave both N- and C-terminal fragments as well as molecular ions. The order of increasing ion current (normalized) in C-terminal fragments was: acetylacetonyl, 4-dimethylaminonaphthylidene, p-dimethyl-aminobenzylidene, 3-pyridylmethylidene, p-diethylaminocinnamylidene, benzylidene, 2-hydroxy-naphthylidene, 4-pyridylmethylidene, p-nitrobenzylidene, p-methoxybenzylidene, p-cyanobenzylidene, cinnamylidene, p-dimethylaminocinnamylidene, β-indolylmethylidene, β-naphthylidene, 2-pyridylmethylidene and α-naphthylidene. The order for this value among the N-terminal fragments is significantly different, however (Day, Falter, Lehman and Hamilton, J. Org. Chem. in press). In addition to N- and C-terminal fragments, many spectra contain internal fragments, arising from loss of fragments from both ends, which provide sequence information. These fragments are found in the mass spectra of Schiff bases formed from various aromatic aldehydes with peptide esters. The interpretation of the latter pattern is facilitate in some cases by deuterium labeling at the α-carbon of the N-terminal amino acid residue of peptides. Such a pattern provides sequence information supplemental to that available involving N- and C-terminal fragmentations. In derivatives of hexaglycine, tetraphenylalanine and tryptophylmethionylaspartyl (β-OEt) phenylalanine amide, for example, substantial sequence information was contained in the internal fragments; in some cases the sequence could be deduced only if the internal fragments were utilized. The 4-dimethylamino-naphthylidene derivatives have proven to be the most useful to date in terms of volatility, tendency to maximize cleavage into N-terminal fragments, intensity of molecular ions and generation of useful mass spectra of certain peptide esters refractory to mass spectrometry in the form of any other derivative investigated.     

A study of some octahydroimidazo[1,5-a] pyridines. A new example of ring-chain tautomerism

A series of 3-substituted octahydroimidazo[1,5-a]pyridines has been prepared. An nmr study at several temperatures showed that 3-phenyloctahydroimidazo[1,5-a] pyridine (XIII) and 3-t-butyloctahydroimidazo[1,5-a] pyridine (XIV) exhibited ring-chain tautomerism. The n-propyl (X), isopropyl (XI) and benzyl (XII) derivatives did not show this form of isomerism. It has also been shown that Crabb and Newton's statement (3) that Freed and Day (1) had failed to obtain the parent compound, octahydroimidazo[1,5-a]pyridine (1), is incorrect.     

Parallel microwave-assisted library of imidazothiazol-3-ones and imidazothiazin-4-ones

A methodology for the generation of a microwave-assisted parallel library and its conversion into a second library is described. A 24-membered library of substituted 4(5)-sulfanyl-1H-imidazoles was generated and subsequently converted into a second library of bicyclic imidazo[5,1-b]thiazol-3-ones and imidazo[5,1-b]thiazin-4-ones. The first library was generated using a three-component reaction and transformed into a daughter library with a polymer-supported coupling agent. The procedure involved the use of an array of expandable reaction vessels, which can accommodate pressure buildup due to microwave heating without loss of volatile solvents or reagents. Library generation time for each library was 16 min.     

Cluster ion formation in laser mass spectrometry of substituted pyridines

Emission of cluster ions occurs during laser irradiation of substituted pyridines even at threshold laser power densities. The clusters generated include dimers and trimers, and appear in both positive-ion and negative-ion laser mass spectra. Fragments of cluster ions are observed and can be rationlized as losses of neutral molecules from (nM ± H)^±. Dissociation of clusters occurs primarily from substituents on the pyridine ring. Laser mass spectrometry of pyridoxine hydrochloride and pyridoxamine-dihydrochloride resulted in the emission of clusters analogous to those observed for nicotinic acid. In contrast to these results, secondary-ion and field-desorption mass spectra of salts contain the ions C_nA_n?1⁺ and C_nA_n+1^?, that were not detected in the laser mass spectra.