Selective Enzymatic Oxidation of Silanes to Silanols

Compared to the biological world's rich chemistry for functionalizing carbon, enzymatic transformations of the heavier homologue silicon are rare. We report that a wild‐type cytochrome P450 monooxygenase (P450_BM3 from Bacillus megaterium, CYP102A1) has promiscuous activity for oxidation of hydrosilanes to give silanols. Directed evolution was applied to enhance this non‐native activity and create a highly efficient catalyst for selective silane oxidation under mild conditions with oxygen as the terminal oxidant. The evolved enzyme leaves C?H bonds present in the silane substrates untouched, and this biotransformation does not lead to disiloxane formation, a common problem in silanol syntheses. Computational studies reveal that catalysis proceeds through hydrogen atom abstraction followed by radical rebound, as observed in the native C?H hydroxylation mechanism of the P450 enzyme. This enzymatic silane oxidation extends nature's impressive catalytic repertoire.     

CE and asymmetrical flow‐field flow fractionation studies of polymer interactions with surfaces and solutes reveal conformation changes of polymers

Amphiphilic diblock copolymers consisting of a hydrophobic core containing a polymerized ionic liquid and an outer shell composed of poly(N‐isoprolylacrylamide) were investigated by capillary electrophoresis and asymmetrical flow‐field flow fractionation. The polymerized ionic liquid comprised poly(2‐(1‐butylimidazolium‐3‐yl)ethyl methacrylate tetrafluoroborate) with a constant block length (n = 24), while the length of the poly(N‐isoprolylacrylamide) block varied (n = 14; 26; 59; 88). Possible adsorption of the block copolymer on the fused silica capillary, due to alterations in the polymeric conformation upon a change in the temperature (25 and 45 °C), was initially studied. For comparison, the effect of temperature on the copolymer conformation/hydrodynamic size was determined with the aid of asymmetrical flow‐field flow fractionation and light scattering. To get more information about the hydrophilic/hydrophobic properties of the synthesized block copolymers, they were used as a pseudostationary phase in electrokinetic chromatography for the separation of some model compounds, that is, benzoates and steroids. Of particular interest was to find out whether a change in the length or concentration of the poly(N‐isoprolylacrylamide) block would affect the separation of the model compounds. Overall, our results show that capillary electrophoresis and asymmetrical flow‐field flow fractionation are suitable methods for characterizing conformational changes of such diblock copolymers.     

Asynchronous sampling for ultrafast experiments with low momentum compaction at the ANKA ring

A high‐repetition‐rate pump–probe experiment is presented, based on the asynchronous sampling approach. The low‐α mode at the synchrotron ANKA can be used for a time resolution down to the picosecond limit for the time‐domain sampling of the coherent THz emission as well as for hard X‐ray pump–probe experiments, which probe structural dynamics in the condensed phase. It is shown that a synchronization of better than 1 ps is achieved, and examples of phonon dynamics of semiconductors are presented.     

Macrocyclization of polymers via ring‐closing metathesis and azide/alkyne‐“click”‐reactions: An approach to cyclic polyisobutylenes

The end‐to‐end cyclization of telechelic polyisobutylenes (PIB's) toward cyclic polyisobutylenes is reported, using either ring‐closing metathesis (RCM) or the azide/alkyne‐“click”‐reaction. The first approach uses bisallyl‐telchelic PIB's (M_n = 1650, 3680, 9770 g mol^?1) and Grubbs 1st‐, 2nd‐, and 3rd‐generation catalyst leading to cyclic PIB's in 60–80% yield, with narrow polydispersities (M_w/M_n = 1.25). Azide/alkyne‐“click”‐reactions of bisalkyne‐telechelic PIB's (M_n = 3840 and 9820 g mol^?1) with excess of 1,11‐diazido‐undecane leads to the formation of mixtures of linear/cyclic PIB's under formation of oligomeric cycles. Subsequent reaction of the residual azide‐moieties in the linear PIB's with excess of alkyne‐telechelic PEO enables the chromatographic removal of the resulting linear PEO‐PIB‐block copolymers by column chromatography. Thus pure cyclic PIB's can be obtained using this double‐“click”‐method, devoid of linear contaminants. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 671–680, 2010     

Convergence of nonconforming multigrid methods without full elliptic regularity

We consider nonconforming multigrid methods for symmetric positive definite second and fourth order elliptic boundary value problems which do not have full elliptic regularity. We prove that there is a bound () for the contraction number of the -cycle algorithm which is independent of mesh level, provided that the number of smoothing steps is sufficiently large. We also show that the symmetric variable -cycle algorithm is an optimal preconditioner.

     

Structuring cholesteric polysiloxanes by photoinduced rotational diffusion

Cholesteric polysiloxanes containing an azo dye were irradiated with linearly or circularly polarized laser light or with the unpolarized light from a HgXe lamp. Photoinduced rotational diffusion was observed leading to new textures with a completely different orientational distribution of the chromophoric and the mesogenic side chains. A broad variety of new structures was detected depending on the polarization state of the light, the helical arrangement of the azo dye and the optional application of an electrical d.c. field. The new structures were characterized by spectroscopy with polarized light. Co-operative reorientation of chromophoric and mesogenic side chains was proven. The formation of a Bragg grating upon short term irradiation with linearly polarized light was observed. Continued irradiation leads in most cases to a strongly birefringent texture with a high dichroic ratio of the azo dye resembling a planar nematic texture. With circularly polarized light or with unpolarized light, a uniaxial homeotropic texture appeared predominantly. The formation of the new textures occurred in the glassy state of the samples leading to new structures with good optical quality and an exceptionally good long-term stability.