Convergent synthesis of noncovalent metallodendrimers containing hydrophobic dendrons at the periphery.

The noncovalent synthesis of "layer-block" metallodendrimers containing hydrophobic shells of covalent dendritic wedges at the periphery is described. Starting from first- and second-generation Fréchet wedges having phosphines at their focal point, convergent dendritic growth yields third- and fourth-generation metallodendrimers in which the coordination of nitriles, pyridines, and phosphines to SCS Pd(II) pincers is used as the assembly motif. In this convergent growth, the number of terminal hydrophobic phosphine wedges increases with generation. The solubility of the dendritic structures in apolar organic solvents such as chloroform and dichloromethane increases accordingly, in contrast to previously reported metallodendrimers. All dendritic structures were characterized by (1)H and (31)P NMR spectroscopy, elemental analysis, and MALDI-TOF mass spectrometry.     

A molecular tweezer for lysine and arginine

Lysine and arginine play a key role in numerous biological recognition processes controlling, inter alia, gene regulation, glycoprotein targeting and vesicle transport. They are also found in signaling peptide sequences responsible, e.g. for bacterial cell wall biosynthesis, Alzheimer peptide aggregation and skin regeneration. Almost none of all artificial receptor structures reported to date are selective and efficient for lysine residues in peptides or proteins. An artificial molecular tweezer is introduced which displays an exceptionally high affinity for lysine (K(a) approximately 5000 in neutral phosphate buffer). It features an electron-rich torus-shaped cavity adorned with two peripheral anionic phosphonate groups. Exquisite selectivity for arginine and lysine is achieved by threading the whole amino acid side chain through the cavity and subsequent locking by formation of a phosphonate-ammonium/guanidinium salt bridge. This pseudorotaxane-like geometry is also formed in small basic signaling peptides, which can be bound with unprecedented affinity in buffered aqueous solution. NMR titrations, NOESY and VT experiments as well as ITC measurements and Monte Carlo simulations unanimously point to an enthalpy-driven process utilizing a combination of van der Waals interactions and substantial electrostatic contributions for a conformational lock. Since DMSO and acetonitrile compete with the amino acid guest inside the cavity, a simple change in the cosolvent composition renders the whole complexation process reversible.     

Ag+ labeling: a convenient new tool for the characterization of hydrogen-bonded supramolecular assemblies by MALDI-TOF mass spectrometry

Herein we describe our results on the characterization of a wide variety of different hydrogen-bonded assemblies by means of a novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique with Ag+ labeling. The labeling technique with Ag+ ions is extremely mild and provides a nondestructive way to generate charged assemblies that can be detected by mass spectrometry. Up to now more than 25 different single (1(3).2(3)), double (3(3).2(6)), and tetrarosettes (4(3).2(12)) have been successfully characterized by the use of this method. The success of the method entirely depends on the presence of a suitable binding site for the Ag+ ion. A variety of functionalities has been identified that provide strong binding sites for Ag+, either acting in a cooperative way (pi-arene and pi-alkene donor functionalities) or individually (cyano and crown ether functionalities). The method works well for assemblies with molecular weights between 2,000 and 8,000 Da, and most likely far beyond this limit.     

Electrochemical measurements on clay minerals for possible sensor applications

Natural clays were characterized by XRD, TGA and SEM/EDX, then pressed to pellets and analyzed by impedance measurements under various defined gas compositions and temperatures. From the Nyquist plot, conductivity values were calculated for the different clay samples. A strong dependence of the conductivity on the water vapour concentration at low temperatures was found for all investigated clay samples. Partial substitution of water by methanol led to a decrease in conductivity at low temperatures, confirming the essential role water plays in the conductivity mechanism at these temperatures. Increasing the temperature resulted in a decrease in conductivity, until a turn-over point was reached. Beyond this temperature an Arrhenius behaviour was observed, suggesting a change in the conductivity mechanism. Paper presented at the 5th Euroconference on Solid State Ionics, Benamádena, Spain, Sept. 13–20, 1998.     

Characterization of Hydrogen-Bonded Supramolecular Assemblies by MALDI-TOF Mass Spectrometry after Ag+ Labeling

The high affinity of Ag ⁺ ions for aromatic π donors and cyano groups is exploited in a novel MALDI-TOF mass spectrometric method for the identification of hydrogen-bonded assemblies. The interaction with the Ag⁺ ions—which, for example, can be complexed by two phenyl groups in a sandwich-type manner (see drawing on the right)—provides positively charged assemblies in a nondestructive way.     

Site of gas-phase methylation of l-phenyl-2-aminopropane

The regioselectivity of methyl cation transfer from (CH₃)₂F⁺, (CH₃)₂Cl⁺ and (CH₃)₃O⁺ to 1-phenyl-2-aminopropane was studied by Fourier transform ion cyclotron resonance in combination with collision-induced dissociation and neutralization-reionization mass spectrometry of the stable [M + CH₃]⁺ ions formed in a chemical ionization source. The (CH₃)₂F⁺ ion transfers a methyl cation to the NH₂ group and the phenyl ring with almost equal probability. Predominant CH₃⁺ transfer to the NH₂ group is observed for the (CH₃)₂Cl⁺ ion whereas the (CH₃)₃O⁺ ion reacts almost exclusively at the amino group. The preference for methylation at NH₂ is discussed in terms of a lower methyl cation affinity of the phenyl ring than of the amino group and the existence of an energy barrier for methylation of the phenyl moiety.     

Peptide bond formation in gas-phase ion/molecule reactions of amino acids: a novel proposal for the synthesis of prebiotic oligopeptides

There is a general fascination with regard to the origin of life on Earth. There is an intriguing possibility that prebiotic precursors of life occurred in the interstellar space and were then transported to the early Earth by comets, asteroids and meteorites. It is probable that some part of the prebiotic molecules may have been generated by gas-phase ion/molecule reactions. Here we show experimentally that gaseous ion/molecule reactions of the amino acids, Glu and Met, may promote the synthesis of protonated dipeptides such as (Glu-Glu)H(+) and (Glu-Met)H(+) and their chemical growth to larger protonated peptides.     

Energy-dependent reversal of secondary isotope effects on simple cleavage reactions: Tertiary amine radical cations with deuterium at remote positions

Deuterium substitution at remote sites gives rise to inverse secondary kinetic isotope effects on the α-cleavage of a number of tertiary amines in the ion source, but to normal isotope effects on reactions occurring in the field-free regions. The change from normal to inverse secondary isotope effects when the internal energy of the reacting ions increases is consistent with transition-state models that involve slight lowering of vibrational frequencies also for bonds well removed from the site of cleavage. The isotope effect on the reactions of ions of high internal energy is caused by the influence that even small changes of isotope-dependent frequencies have on the state sums (a statistical weight effect favouring loss of the labelled radical), whereas the behaviour of low-energy ions is determined by the zero-point energy changes which favour loss of the unlabelled fragment.