Stereoisomere Aromastoffe LIX. 3-Mercaptohexyl- und 3-Methylthiohexylalkanoate — Struktur und Eigenschaften der Enantiomeren

Zusammenfassung Die Synthese der optisch reinen Stereoisomeren der als Aromastoffe der gelben Passionsfrucht bekannten Acetate, Butanoate und Hexanoate des 3-Mercaptohexanols und des 3-Methylthiohexanols wird vorgestellt. Die sensorische Bewertung der Stereoisomeren wird aufgezeigt und die Möglichkeiten zur analytischen Stereodifferenzierung werden beschrieben.

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Stereoisomeric flavour compounds LIX. 3-Mercaptohexyl- and 3-methylthiohexylalkanoates — structure and properties of the enantiomers

Summary The synthesis of the optically pure stereoisomers of the acetates, butanoates and hexanoates of 3-mercaptohexanol and 3-methylthiohexanol, which are known to be flavour compounds of yellow passionfruits, is presented. The sensory characteristics of the stereoisomers are given and the chirospecific differentiation using capillary gas chromatography and Octakis (2,3-di-O-acetyl-6-Otert.-butyldimethylsilyl)--cyclodextrin as the chiral stationary phase is discussed.

     

Structural stability of oligomeric proteins: A mean-field theoretical approach

We present a mean-field approach for calculating thermodynamic properties(free energies) of protein–solvent systems. We apply this method to thetumor suppressor protein p53, where we study the stability of itstetramerization domain when subjected to site-directed mutagenesis. Acomparison with experimental results is included.     

Electronic Interactions of Cyclopropyl Groups: Synthesis and Photoelectron Spectra of Model Compounds Derived from Triasterane,Nortricyclane and Norbornane

The syntheses of the new hydrocarbons trimethylenetriasterane 12 , trispirocyclopropanetriasterane 3 , trispirocyclopropanenortricyclane 4 , and of the trispirocyclopropanenorbornanes 5, 6 , and 7 are reported. 12 contains six vinylcyclopropane units rigidly held in an antiperiplanar (ap) conformation in a D_3h symmetric framework. 3 and 4 are the first model compounds with bicyclopropyl units rigidly fixed in an s-trans (ap) conformation (Θ = 180°). Their PE spectra reveal that the interaction parameter β_ww = −1.73 eV for this arrangement is the same as that for Θ = 0°.     

ESEM investigations for assessment of damage kinetics of short glass fibre reinforced thermoplastics - Results of in situ tensile tests coupled with acoustic emission analysis

The damage mechanisms of short glass fibre reinforced polypropylene (PP) and polybutene-1 (PB-1) materials were investigated. For this purpose, in situ tensile tests were conducted in the environmental scanning electron microscope (ESEM) while simultaneously recording the acoustic emission (AE). To be able to observe damage mechanisms directly during loading, notched specimens were used. This method allows the direct correlation of the recorded load - elongation data with observed damage mechanisms, as well as correlations with acoustic emission data. Hence, it is possible to describe the damage kinetics of short glass fibre composite.It was found that different bonding conditions of the two investigated materials result in different damage mechanisms as well as in different AE behaviour. For fibre reinforced PP with excellent bonding conditions of the fibres in the polymeric matrix, fibre fracture, slipping of fibres in the delamination area, debonding and pull-out with matrix yielding was observed. The determined AE parameter amplitude A_p and energy E_AE for the PB-1 material are lower because of the weak bonding of the fibres to the PB-1-matrix. Hence, energy dissipative damage mechanisms like pull-out with matrix yielding can occur only in a limited part of such materials.     

Integrated scanning Kelvin probe-scanning electrochemical microscope system: development and first applications

The integration of a scanning Kelvin probe (SKP) and a scanning electrochemical microscope (SECM) into a single SKP-SECM setup, the concept of the proposed system, its technical realization, and first applications are presented and discussed in detail. A preloaded piezo actuator placed in a grounded stainless steel case was used as the driving mechanism for oscillation of a Pt disk electrode as conventionally used in SECM when the system was operated in the SKP mode. Thus, the same tip is recording the contact potential difference (CPD) during SKP scanning and is used as a working electrode for SECM imaging in the redox-competition mode (RC-SECM). The detection of the local CPD is established by amplification of the displacement current at an ultralow noise operational amplifier and its compensation by application of a variable backing potential (V(b)) in the external circuit. The control of the tip-to-sample distance is performed by applying an additional alternating voltage with a much lower frequency than the oscillation frequency of the Kelvin probe. The main advantage of the SKP-SECM system is that it allows constant distance measurements of the CPD in air under ambient conditions and in the redox-competition mode of the SECM in the electrolyte of choice over the same sample area without replacement of the sample or exchange of the working electrode. The performance of the system was evaluated using a test sample made by sputtering thin Pt and W films on an oxidized silicon wafer. The obtained values of the CPD correlate well with known data, and the electrochemical activity for oxygen reduction is as expected higher over Pt than W.