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[en] Complete text of publication follows. The reactions Lm Mn+1 - X + R · → Lm Mn + RX (where M is transition metal cation and x halides, H, alkyls etc.) are of importance in a large variety of radical induced catalytic processes. The factors affecting the kinetics of these reactions have still not been fully elucidated. It was decided to measure the kinetics of the reactions: L (Cl) RhIII - H2+ R · → (L(Cl)RhII)2+ RH hindrance. The first radical studied was the methyl radical, formed radiolytically in aqueous solutions. The rate constants of the latter reactions are 1.3x105 M-1s-1 and 5.6x104 M-1s-1 for the trans- and cis- isomers of the complex respectively. These low rate constants might be due to the RhIII-H bond strength. In order to verify this assumption it was decided to measure the isotope effects of these reactions. Surprisingly, large inverse isotope effects are observed: k((trans - L(Cl)RhIII - H)) / k((trans - L(Cl)RhIII - D)) = 0.50 ± 0.10 k((cis - L(Cl)RhIII - H)) / k((cis - L(Cl)RhIII - D)) = 0.49 ± 0.11. These isotope effects indicate that the radical first forms a metal-carbon bond followed by reductive elimination of methane.
[en] This study presents the first application of the model-free analysis (MFA) (Meiler in J Am Chem Soc 123:6098-6107, 2001; Lakomek in J Biomol NMR 34:101-115, 2006) to methyl group RDCs measured in 13 different alignment media in order to describe their supra-τc dynamics in ubiquitin. Our results indicate that methyl groups vary from rigid to very mobile with good correlation to residue type, distance to backbone and solvent exposure, and that considerable additional dynamics are effective at rates slower than the correlation time τc. In fact, the average amplitude of motion expressed in terms of order parameters S2 associated with the supra-τc window brings evidence to the existence of fluctuations contributing as much additional mobility as those already present in the faster ps-ns time scale measured from relaxation data. Comparison to previous results on ubiquitin demonstrates that the RDC-derived order parameters are dominated both by rotameric interconversions and faster libration-type motions around equilibrium positions. They match best with those derived from a combined J-coupling and residual dipolar coupling approach (Chou in J Am Chem Soc 125:8959-8966, 2003) taking backbone motion into account. In order to appreciate the dynamic scale of side chains over the entire protein, the methyl group order parameters are compared to existing dynamic ensembles of ubiquitin. Of those recently published, the broadest one, namely the EROS ensemble (Lange in Science 320:1471-1475, 2008), fits the collection of methyl group order parameters presented here best. Last, we used the MFA-derived averaged spherical harmonics to perform highly-parameterized rotameric searches of the side chains conformation and find expanded rotamer distributions with excellent fit to our data. These rotamer distributions suggest the presence of concerted motions along the side chains
[en] The loss of the methyl group from methylacetophenone (MAcPh) isomers is investigated. The formation of C8H7O+ ion is defined with loss of methyl group from the acyl of acetophenone. The appearance potential values of the ion C8H7O+, which are 10.45, 10.52 and 10.72 eV from ortho-, meta- and para-MAcPh, respectively, as well as the heats of formation of this ion lead to the formation of C8H7O+ ion having the same structure in the case of the three isomers. (author)
[en] Polydimethylsiloxane (PDMS) is used by many for nanoimprint applications due to its affordability, ease of preparation, mechanical flexibility, compatibility with imprint resists and transparency to UV light. However PDMS is notoriously flexible, tacky and permeable to air. Here fluorinated ethylene–propylene (FEP) is considered as a viable and versatile alternative material for nanoimprint stamps. FEP possesses many of the desirable nanoimprint attributes associated with PDMS but crucially also features a range of complementary characteristics, including an order of magnitude more mechanical strength allowing it to handle higher loads than PDMS, an intrinsically non-stick surface and is compatible with oxygen sensitive resists. Unlike elastomeric polymers, FEP is glassy so patterning may be realised via hot embossing. Not only is this a facile and rapid means of physical structuring but it also facilitates combinatorial patterning, providing a versatility beyond that of traditional casting materials. Due to the intrinsically slow creep of FEP both micro- and nanopatterning are successfully performed sequentially. Feature sizes from 45 nm were successfully realised via the hot-embossing method. To further demonstrate the potential of the material, a modified computer numerical control machine is used. It is capable of photo-, nanoimprint- and laser lithography in conjunction with patterned FEP foils. The tool is used to perform pattern transfer into a developmental nanoimprint resist from Micro Resist Technology, mr-NIL210 XP, and Nano SU-8 3005 negative tone photo resist from MicroChem. Ultimately three-tier lithography is performed in unison and advantageous step-and-repeat performance is achieved with fabricated FEP imprint stamps as they demould more compliantly and resist pressure and contamination better than PDMS. (paper)
[en] For the radiofluorination of benzenes and benzene derivatives, the electrophilic reaction with [18F]F2 is a very common route. Yet, aromatic nucleophilic substitution (SNAr) by n.c.a [18F]fluoride, which can be produced efficiently in high amounts, has been considered to be very desirable. However, to facilitate 18F-labelling via SNAr at an electron rich aromatic system, an appropriate leaving group must be present together with an auxiliary group in ortho or para position to the leaving group. An interesting alternative for the auxiliary group is the heteroatom of a heteroaromatic system, for which pyridine is a leading example. Dolci et al. (J Label Compd Radiopharm 42:975-985, 1999) have evaluated the scope of the nucleophilic aromatic fluorination of 2-substituted pyridine rings using the activated K [18F]F-K222 complex. As methyl and methoxy groups are known to enhance the electron density of an aromatic system by the +I and the +M effect, respectively, SNAr is unlikely to occur. Until now, the effect of these substituents has not been studied towards the 18F-radiofluorination of substituted 2-nitropyridines by use of [18F]fluoride. Therefore, we have investigated the effect of methoxy and methyl groups in 2-nitropyridines. The results showed that 3-methoxy-2-nitropyridine and 3-methyl-2-nitropyridine can efficiently be substituted by [18F]fluoride with high RCY's (70-89%) in short reaction times (1-30 min) at a reaction temperature of 140 deg C. Moreover, 3-methoxy-6-methyl-2-[18F]fluoropyridine was obtained from the corresponding nitro-precursor in a high yield of 81 ± 1% after 30 min at 140 deg C. In case of 2-nitropyridines data indicates the effect of methyl and methoxy groups on SNAr to be of minor importance. (author)
[en] The 1H nuclear magnetic resonance spectrum of hexamethylbenzene orientationally ordered in the nematic liquid crystal ZLI-1132 is analysed using covariance matrix adaptation evolution strategy. The spectrum contains over 350 000 lines with many overlapping transitions, from which four independent direct dipolar couplings are obtained. The rotations of the six methyl groups appear to be correlated due to mutual steric hindrance. Adjacent methyl groups show counter-rotating or geared motion. Hexamethylbenzene thus behaves as a molecular hexagonal gear.
[en] Methyl radicals generated by γ-irradiation in the proton form of synthetic rho zeolite exposed to CH4 have been investigated by electron paramagnetic resonance (EPR) in the temperature range 110-370 K. Depending on the methane adsorption temperature two EPR spectra of ·CH3 radicals were recorded. The isotropic quartet observed after CH4 adsorption at room temperature was assigned to ·CH3 radicals freely rotating in the middle of octagonal prism. After adsorption at 413 K, the isotropic quartet is overlapped with the second signal characterized with anisotropy of hyperfine splitting and g-value. It is postulated that anisotropic signal represents the ·CH3 radicals located in α-cages and strongly interacting with silicaalumina framework. The mechanisms of radiolytic formation of methyl radicals and the possible stabilization sites are also discussed. (author)
[en] Resolution of (E)-phenyl[2-2H1]oxirane via an amino hydrin, reduction with super-tritide, and oxidative degradation afforded chiral acetate in high radiochemical yield, albeit with some racemization (67-78% ee). Chiral methyl mevalonates prepared from phenyl acetate were used as substrate for kaurene biosynthesis with an enzyme preparation from Marah macrocarpus. Re-creation of a chiral methyl at C-17, regiospecific degradation to acetate, and enzymatic analysis revealed overall net retention of configuration. It is concluded that the CH3 → CH2 elimination forming the exocyclic methylene group of kaurene occurs with an endo orientation. 9 refs.; 4 schemes; 1 table