Recently, we showed that titanocene silyls are much more
stable with Ti in the oxidation state +3. The current study demonstrates
that analogous Zr and Hf compounds can also be obtained by reaction
of a suitable metalate precursor with an oligosilanyl dianion. As
the obtained complexes formally possess a d1 electron configuration,
they were investigated using EPR spectroscopy. The corresponding spectra
indicate that the compounds can be considered to also exhibit some
cyclosilanyl radical anion character. In order to understand the strong
preference of disilylated titan(IV)ocenes for reductive elimination,
a theoretical study of the thermodynamics of these reactions was conducted,
revealing that this behavior is essentially caused by the weak Si–Ti(IV)
bond.
Many peptides, proteins, and drugs interact with biological membranes, and knowing the mode of binding is essential to understanding their biological functions. To obtain the complete orientation and immersion depth of such a compound, the membrane-mimetic system (micelle) is placed in an aqueous buffer containing the soluble and inert paramagnetic contrast agent Gd(DTPA-BMA). Paramagnetic relaxation enhancements (PREs) of a specific nucleus then depend only on its distance from the surface. The positioning of a structurally characterized compound can be obtained by least-squares fitting of experimental PREs to the micelle center position. This liquid-state NMR approach, which does not rely on isotopic labeling or chemical modification, has been applied to determine the location of the presumed transmembrane region 7 of yeast V-ATPase (TM7) and the membrane-bound antimicrobial peptide CM15 in micelles. TM7 binds in a trans-micelle orientation with the N-terminus being slightly closer to the surface than the C-terminus. CM15 is immersed unexpectedly deep into the micelle with the more hydrophilic side of the helix being closer to the surface than the hydrophobic one.
Thermal and UV degradation of four common space-grade polymer films have been studied in situ using electron spin resonance (ESR) spectroscopy. By recording subsequent spectra at a sufficient rate, the time dependence of the radical concentration could be followed, allowing more detailed study of the kinetics of the degradation process. The thermal degradation was found to consist of two main processes, one being a stabilization process and the other the actual degradation, whereas the UV experiments showed biexponential degradation kinetics. Additionally, to compare in situ and ex situ experiments, we monitored the stability of the generated radicals after the exposure. The radical concentration decreased only slightly when samples were stored in a vacuum, whereas storage in air led to a significant loss of radicals.
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