Optical sensor materials for the detection of amines in organic solvents

A new optical polymer-based sensor was developed, which is able to recognize amines in organic solvents with high sensitivity. Thin polymer membranes were prepared and investigated, which contain a chromogenic functional dye (reactand) that shows a significant colour change during a reversible chemical reaction with the analyte. For that purpose the azo dye 4-trifluoroacetyl-4′-[N-(methacryloxyethyl)-N-(ethyl)amino]-azobenzene (CR-465) was synthesized, which contains a trifluoroacetyl moiety (receptor for interaction with amines) and in addition, a polymerizable methacrylate group. The methacrylate group links the dye covalently to the polymer matrix and the receptor recognizes the analyte via covalent binding. For immobilisation of the dye cross-linked methacrylate polymers with different composition were used. The highly cross-linked polymer network was stable against most organic solvents and exhibited enhanced stability against mechanical strain compared to plasticized PVC. The sensitivity of the reaction between the analyte and the dye was tailored by the choice of the solvent in which the analysis of the sensor layer was performed, with equilibrium constants for 1-butylamine ranging from 80 to 2000 M⁻¹ in chloroform and DMSO, respectively. In toluene as the solvent, sensor layers typically exhibited equilibrium constants of 100 M⁻¹ for 1-butylamine, 1300 M⁻¹ for 1,4-diaminobutane and 20,000 M⁻¹ for tris-(2-aminoethyl)amine. We have also investigated the cross-linked sensor layers with respect to molecular imprinting and did not find any enhancement in selectivity through imprinting in the presence of different analyte molecules.     

Synthesis of functionalized rab GTPases by a combination of solution- or solid-phase lipopeptide synthesis with expressed protein ligation

Prenylated proteins with non-native functionalities are generally very difficult to obtain by recombinant or enzymatic means. The semisynthesis of preparative amounts of prenylated Rab guanosine triphosphatases (GTPases) from recombinant proteins and synthetic prenylated peptides depends largely on the availability of functionalised prenylated peptides corresponding to the proteins' native structure or modifications thereof. Here, we describe and compare solution-phase and solid-phase strategies for the generation of peptides corresponding to the prenylated C terminus of Rab7 GTPase. The solid-phase with utilisation of a hydrazide linker emerges as the more favourable approach. It allows a fast and practical synthesis of pure peptides and gives a high degree of flexibility in their modification. To facilitate the analysis of semisynthetic proteins, the synthesised peptides were equipped with a fluorescent group. Using the described approach, we introduced fluorophores at several different positions of the Rab7 C terminus. The position of the incorporated fluorescent groups in the peptides did not influence the protein-ligation reaction, as the generated peptides could be ligated onto thioester-tagged Rab7. However, it was found that the positioning of the fluorescent group had an influence on the functionality of the Rab7 proteins; analysis of the interaction of the semisynthetic Rab7 proteins with REP (Rab escort protein) and GDI (guanosine diphosphate dissociation inhibitor) molecules revealed that modification of the peptide side chains or of the C-terminal isoprenoid did not significantly interfere with complex formation. However, functionalisation of the C terminus was found to have an adverse effect on complex formation and stability, possibly reflecting low structural flexibility of the Rab GDI/REP molecules in the vicinity of the lipid-binding site.     

Laser desorption mass spectrometry on thin liquid jets

A recently developed soft desorption method for mass spectrometry is presented, which is called Laser Induced Liquid Beam Ionization/Desorption (LILBID). Analyte ions are desorbed from a thin jet of analyte solution directly into vacuum by means of an IR laser pulse, which has been tuned to a vibrational resonance of the solvent. A comparative experiment with ammonium chloride and aniline hydrochloride shows that ion formation via proton transfer takes place in the solution. Thermally unstable compounds, as well as supra- and biomolecular complexes, can be detected intact and mass analyzed in a reflectron time-of-flight (Re-TOF) mass spectrometer. During the desorption process, noncovalent interactions and some solvation characteristics are preserved. Three examples for the capacity of LILBID are given in this short overview: (a) ion-solvent interactions with the formation of a clathrate structure Cs⁺(H₂O)₂₀, (b) host-guest interactions with the K⁺ selectivity of valinomycin, and (c) noncovalent interactions with the dimerization of gramicidin. Received: 29 July 1997 / Revised: 4 September 1997 / Accepted: 12 September 1997     

The determination of traces of thiocyanate and copper(II) ions by cathodic stripping voltammetry

Cathodic stripping methods are described for the determination of traces of thiocyanate ions down to 2 × 10^-8 mol l^-1 and Cu(II) ions down to 1 × 10^-8 mol l^-1. The method involves electrolytic accumulation of copper(I) thiocyanate on the surface of a hanging mercury drop electrode followed by stripping of the deposit during the cathodic scan. For the determination of thiocyanate, a copper amalgam electrode can be used. Examples of application of the method for the determination of traces of thiocyanate in common salts, in saliva and urine as well as for the determination of copper(II) ions in tap water are described.     

Voltammetry at the mercury film electrodes: Comparison of theoretical and experimental results obtained for indium in thiocyanate medium

The voltammetric behaviour of the In(III)-In(Hg) system was studied at the silver-, graphite-, and glassy carbon-based mercury film electrodes under cyclic and stripping conditions in thiocyanate media. The reversible curves obtained at high thiocyanate ion concentration showed a good agreement with theoretical predictions, particularly for the cathodic process. The anodic curves obtained at thin silver-based mercury film electrodes deviated from theoretical predictions due to the interactions between indium, dissolved in mercury, and the silver substrate of the film electrode. At low thiocyanate ion concentrations, where the current was controlled partly by the rate of the preceding chemical step and partly by diffusion, the variations in the film thickness affected the position of the curve as predicted theoretically for the reversible diffusion controlled case.     

Un substrat pour le dosage chimique de la rénine

A fluorogenic renin substrate, N-CBO-L -prolyl-L -phenylalanyl-L -histidyl-L -leucyl-L -leucyl-L -valyl-L -tyrosyl-L -seryl-β-naphthylamide, has been synthesized. Upon incubation at pH 5,6 with renin and an excess of the auxiliary enzyme aminopeptidase M, it gives rise to β-naphthylamine at a rate related to the quantity of renin.     

NMR spectra of a microcrystalline protein at 30 kHz MAS

Proteins are not always available in amounts desirable for solid-state magic-angle spinning (MAS) nuclear-magnetic resonance (NMR) spectroscopy. To maximize the signal-to-noise ratio achievable with small samples, the filling factor must be optimized by using small-diameter MAS rotors. These rotors have the added benefit of allowing higher radio frequency field amplitudes during polarization transfer steps and during decoupling periods as well as allowing higher spinning frequencies. We demonstrate the advantages of relatively fast MAS (30 kHz using a 2.5 mm rotor) compared to MAS at 12 kHz for the 10.4 kDa model protein Crh with 93 residues and show that the signal-to-noise ratio in two-dimensional correlation spectra can be significantly improved by taking advantage of optimized pulse sequences available with rapid MAS.     

Adsorption—Nucleation and growth mechanism of CuSCN monolayer formation on a copper amalgam electrode in thiocyanate solutions

Using voltammetric, galvanostatic and potentiostatic techniques, the electrodeposition of cuprous thiocyanate on a copper amalgam electrode has been studied in acidic solutions containing various thiocyanate ion concentrations. Under the conditions used, the formation of two successive monolayers of CuSCN could be studied before the onset of bulk deposition. It was found that the formation of the first monolayer proceeds according to a complex adsorption-nucleation and growth mechanism. In the first stage, cuprous thiocyanate is adsorbed on the amalgam electrode; in the second, the disordered layer of adsorbate rearranges to form two-dimensional crystalline centres; and in the third stage, these centres grow, completing the formation of the entire monolayer. The second monolayer is formed in a much simpler way, according to a mechanism of two-dimensional, instantaneous nucleation and growth. At high thiocyanate ion concentration, the formation of the CuSCN layer is obscurred by the extensive formation of solution soluble species.