Metastable ion study of organosilicon compounds part II—hexamethyldisiloxane

Unimolecular reactions of the metastable silicenium ion (CH₃)₃SiOSi(CH₃)₂ ⁺ generated by dissociative ionization of bexamethyldisiloxane were investigated by mass-analysed ion kinetic energy (PUKE) Spectrometry. The characteristic fragmentations observed were losses of CH₄ and (CH₃)₂Si?O molecules. Complete scrambling of all methyl groups prior to these reactions was found by investigating the MIKE spectra of deuterium labelled analogues (CD₃)₃SiOSi(CH₃)₂⁺ and (CH₃)₃SiOSi(CD₃)₃⁺. The loss of methane was accompanied by a large kinetic energy release (T_0.5 = 482 meV). The MIKE spectra of silicenium ions were compared with those of their carbon analogues. The most predominant reaction of metastable (CH₃)₃COC(CH₃)₃⁺ ion was the loss of CH₂?C(C H₃)₂ leading to protonated acetone. Significant differences between the ion fragmention characteristics of silicon and carbon compounds were found.     

Physical Methods for the Microanalysis of Solids—General Significance,Recent Developments,and Limitations

The characterization of solid systems by physical methods (“physical analysis”) is one of the most important future-orientated areas of analytical chemistry. Important developments are the increase of the informational content of analytical signals by application of mathematical methods, the investigation of extremely small amounts (microanalysis), as well as concentrations (trace analysis). New developments in analytical methodology and strategy in micro and surface analysis permit, for example, the direct identification of compounds in individual phases in the solid state, the quantitative elemental analysis of aerosol particles in the submicrometer region (identification of asbestos fibers), and highly sensitive distribution analysis of trace elements in semiconductors. Important technological-scientific problems can be explained in this way.     

Discovery of the New Metastable HONF. Radical

The new radical HONF has been detected in the gas phase by neutralization–reionization mass spectrometry (NRMS). The radical has been identified and directly characterized as a gaseous isolated species, having a lifetime of at least 1 μs and a linear cis–trans structure of H‐O‐N‐F connectivity. Detection of this molecule, which is highly unstable towards the dissociation into HF and NO and kinetically sufficiently stable to be observed, represents an advance in the search for high‐energy species.     

Metastable ion study of organosilicon compounds. Part III—trimethoxyphenylsilane

The unimolecular decomposition of trimethoxyphenylsilane (1) was investigated by mass-analysed ion kinetic energy (MIKE) spectrometry, deuterium-labelling studies and from high resolution data. The characteristic fragmentations of metastable molecular ion of 1 were losses of C₆H₆ and C₇H₇^· with rearrangements. Almost complete H/D scrambling occurred in the molecular ion prior to these decompositions. The other important fragmentation routes corresponded to expulsions of CH₃O^· and C₆H₅^·. These fragmentations were followed by consecutive elimination of an H₂CO molecule, as commonly observed in the mass spectra of alkoxysilanes. In these fragmentation processes, H/D scrambling increased as the internal energy of the molecular ion was lowered. The fragmentations of 1 were compared with those of its carbon analogue, α,α,α-trimethoxytoluene.     

Metastable ion study of organosilicon compounds. Part I—Diethoxydimethylsilane

Electron impact induced fragmentations of diethoxydimethylsilane (1) and its deuterium-labelled analogues (2 and 3) were investigated by mass-analysed ion kinetic energy spectrometry. The principal fragmentation processes of compound 1 are dominated by silicenium ions which undergo consecutive losses of ethylene and aldehyde molecules. The fragmentations of 1 led to the formation of protonated silanoic acid [CH₃Si(OH)₂]⁺ and protonated dimethyl-silanone [(CH₃)₂Si = OH]⁺, and their unimolecular reacdvities were clarified. The fragmentation characteristics of compound 1 are compared with those of the carbon analogue, acetone diethyl acetal (4).     

Metastable ion study of organosilicon compounds. Part V—tetramethoxysilane and trimethoxymethylsilane

The fragmentations of tetramethoxysilane ((CH₃O)₄Si (1)) and trimethoxymethylsilane ((CH₃O)₃SiCH₃ (3)) induced by electron impact were investigated by mass-analysed ion kinetic energy (MIKE) spectrometry and a deuterium-labelling study. These molecular ions begin to fragment by the loss of CH₃ or CH₃O. These fragmentations are followed by the loss of an aldehyde molecule (H₂CO), as commonly observed in the mass spectra of alkoxysilanes. Almost complete scrambling of the methoxy hydrogens takes place in the metastable molecular ion, [1]⁺˙, prior to the decomposition. On the other hand, a moderate extent of scrambling of the hydrogens takes place in [3]⁺˙. The fragmentations of [1]⁺˙ and [3]⁺˙ were compared with those of the corresponding carbon analogues, tetramethoxymethane ((CH₃O)₄C (2)) and 1,1,1-trimethoxyethane ((CH₃O)₃CCH₃ (4)), respectively.     

Metastable ion studies: XIII—the measurement of appearance energies of metastable peaks

A simple, accurate and reproducible method is described for measuring the appearance energy of a metastable peak. The method involves the comparison of the metastable peak intensity with that of a ‘standard’ metastable peak, over a small range of ionizing electron energies above their onsets. Six fragmentation processes whose reaction energetics are well established were selected to provide suitable calibrant metastable peaks. Results are also presented for fragmentations having large kinetic shifts and for some primary and secondary ion decomposition of current interest.     

Rearrangements and fragmentations of phenyl styryl sulfides: 1—Metastable ion spectra

The mass spectra of two isomeric phenyl styryl sulphides show abundant peaks for fragment ions formed after interesting and unusual rearrangements, e.g. loss of CHS^˙ and C₇H₇^˙. Information about these rearrangements was obtained by comparison of the metastable peak intensities and peak shapes with those of four isomers. It appears that isomerization reactions between the phenyl styryl sulfides and the isomers are possible and that most of the metastable ion fragmentations proceed via reacting configurations that are common to two or more of the compounds investigated. The results indicate that an earlier proposed mechanism for the fragmentation of phenyl vinyl sulfides is incomplete and only partly correct for the compounds studied. Metastable ion spectra were obtained using high voltage scans and constant B/E scans: a short comparison of the results obtained with the two methods is given.     

Metastable ion studies. VII—loss of water from the molecular ion of cyclopentanol

Metastable peaks for the loss of water from the molecular ion of cyclopentanol and six ²H labelled analogues have been studied. The results, together with some thermochemical date and a detailed examination of metastable peak shapes, have allowed the identification of the two mechanisms for water loss, both of which take place from the α-cleaved molecular ion. Daughter ion structures are proposed.     

Metastable ion studies. III—composite metastable peaks in fragmentations of some small hydrocarbon cations

Composite metastable peaks are generated in the unimolecular fragmentations (i) [C₃H₅]⁺ → [C₃H₃]⁺ + H₂ (flat-top upon flat-top) and (ii) [C₄H₉]⁺ → [C₃H₅]⁺ + CH₄ (flat-top and gaussian). The measurement of appearance potentials and kinetic energy releases lead us to conclude, in agreement with earlier proposals, that in (i) the components can arise from the generation of the isomeric cyclopropenium and propargyl daughter cations. In (ii) the components are proposed to arise from the fragmentation of tert- and sec-butyl cations yielding allyl as the common daughter ion. The composite peak observed in the fragmentation (iii) [C₃H₄]⁺· → [C₃H₃]⁺ + H· is shown to be present only if the decomposing molecular ion is large enough to also produce [C₆H₈]²⁺ ions. The second component in (iii) then arises from the reaction [C₆H₈]²⁺ → [C₆H₆]²⁺ + H₂.     

Peptidomimetics for Receptor Ligands—Discovery,Development, and Medical Perspectives

Peptides, as neurotransmitters, neuromodulators, and hormones, influence a multitude of physiological processes by signal transduction mediated through receptors. In addition, during the last 20 years their role in the appearance or maintenance of various diseases could be unequivocally proven. Agents that can imitate or block the biological functions of bioactive peptides (agonists or antagonists, respectively) can be considered as aids for the investigation of peptidergic systems and also as therapeutic agents. The suitability of bioactive peptides as therapeutic agents was examined after preliminary pharmacological experiments. It was thereby shown that based on their pharmacological properties, for example degradation by peptidases or poor bioavailability, they could be employed as drugs in only a few cases. To solve this problem peptidomimetics, compounds that act as substitutes for peptides in their interaction with receptors, have been synthesized. In comparison with native peptides they show higher metabolic stability, better bioavailability, and longer duration of action. Peptidomimetics with antagonistic properties were also developed within the range of these investigations. As a result, new types of treatment and therapy for a series of diseases are possible. Although peptidomimetics have been developed largely by empirical methods (e.g. modification of native peptides, optimization of lead structures), methods for rational design based on investigations into the structure of peptide? peptide receptor complexes and studies of conformation energies, among others, are gradually being established.     

Metastable ion Characteristics. XXXXIII—separation and comparison of collisional activation spectra and metastable ion spectra

Additional ion acceleration in a separate collision chamber in the field free drift region makes it possible to add extra kinetic energy to those ions undergoing decomposition in the chamber. In this way product ions formed in the collision chamber (the [collisional activation] spectrum) can be distinguished from ions formed from decompositions in the low pressure portion of the field free drift region (the [metastable ion] spectrum). This technique is used to show that often a relatively small proportion of the ions in the [pure] collisional activation spectrum arises from processes which produce the metastable ion spectrum.     

Insulin—From its Discovery to the Industrial Synthesis of Modern Insulin Analogues

After the discovery of insulin as a drug for diabetes, the pharmaceutical companies were faced with the challenge to meet the demand for insulin with the highest possible degree of purity in the required quantities from animal sources. The observation of an immune reaction of patients to insulin from animal pancreatic extracts made the availability of human insulin of highest priority. Only the enzyme‐catalyzed semisynthesis at the C‐terminus of the insulin B‐chain led to a commercial process, but it depended on porcine insulin and was aggravated by supply concerns. The advent of rDNA technology allowed the commercial preparation of human insulin by biosynthesis in virtually unlimited quantities. An increased chemical diversity was only envisaged through chemical synthesis, which was simplified by advances in solid‐phase peptide synthesis and chemical ligation. Single‐chain insulin precursors are now being synthesized that should enable fast screening of insulin analogues for improved biophysical, biological, and thus promising new therapeutic properties, as well as for the industrial manufacture of insulin analogues not accessible by biosynthesis.