Thermodynamic and Raman spectrometry studies showing the role of the silica tube in the chemical vapor transport of titanium diboride

Chemical vapor transports of titanium diboride using different halides as transport agents (t.a.) produce crystals with various size, morphology and stoechiometry. The explanation of this fact is to be research in the interaction of the system TiB₂/t.a. with the silica of the tube in which the transport occurs. Thermodynamic calculations and Raman spectroscopy “in situ” both show the presence of silicon compounds in the gas phase. Their relative quantities following the nature of the t.a. allow us to explain the differences observed in the growth.     

Stereospecificity of ketoreductase domains of the 6-deoxyerythronolide B synthase

6-Deoxyerythronolide B synthase (DEBS) is a modular polyketide synthase (PKS) responsible for the biosynthesis of 6-dEB (1), the parent aglycone of the broad spectrum macrolide antibiotic erythromycin. Individual DEBS modules, which contain the catalytic domains necessary for each step of polyketide chain elongation and chemical modification, can be deconstructed into constituent domains. To better understand the intrinsic stereospecificity of the ketoreductase (KR) domains, an in vitro reconstituted system has been developed involving combinations of ketosynthase (KS)-acyl transferase (AT) didomains with acyl-carrier protein (ACP) and KR domains from different DEBS modules. Incubations with (2S,3R)-2-methyl-3-hydroxypentanoic acid N-acetylcysteamine thioester (2) and methylmalonyl-CoA plus NADPH result in formation of a reduced, ACP-bound triketide that is converted to the corresponding triketide lactone 4 by either base- or enzyme-catalyzed hydrolysis/cyclization. A sensitive and robust GC-MS technique has been developed to assign the stereochemistry of the resulting triketide lactones, on the basis of direct comparison with synthetic standards of each of the four possible diasteromers 4a-4d. Using the [KS][AT] didomains from either DEBS module 3 or module 6 in combination with KR domains from modules 2 or 6 gave in all cases exclusively (2R,3S,4R,5R)-3,5-dihydroxy-2,4-dimethyl-n-heptanoic acid-delta-lactone (4a). The same product was also generated by a chimeric module in which [KS3][AT3] was fused to [KR5][ACP5] and the DEBS thioesterase [TE] domain. Reductive quenching of the ACP-bound 2-methyl-3-ketoacyl triketide intermediate with sodium borohydride confirmed that in each case the triketide intermediate carried only an unepimerized d-2-methyl group. The results confirm the predicted stereospecificity of the individual KR domains, while revealing an unexpected configurational stability of the ACP-bound 2-methyl-3-ketoacyl thioester intermediate. The methodology should be applicable to the study of any combination of heterologous [KS][AT] and [KR] domains.     

Stereospecificity of ketoreductase domains 1 and 2 of the tylactone modular polyketide synthase

Tylactone synthase (TYLS) is a modular polyketide synthase that catalyzes the formation of tylactone (1), the parent aglycone precursor of the macrolide antibiotic tylosin. TYLS modules 1 and 2 are responsible for the generation of antidiketide and triketide intermediates, respectively, each bound to an acyl carrier protein (ACP) domain. Each module harbors a ketoreductase (KR) domain. The stereospecificity of TYLS KR1 and TYLS KR2 has been determined by incubating each of the recombinant ketoreductase domains with reconstituted ketosynthase-acyltransferase [KS][AT] and ACP domains from the 6-deoxyerythronolide B synthase (DEBS) in the presence of the N-acetylcysteamine thioester of syn-(2S,3R)-2-methyl-3-hydroxypentanoate (6), methylmalonyl-CoA, and NADPH resulting in the exclusive formation of the ACP-bound (2R,3R,4S,5R)-2,4-methyl-3,5-dihydroxyhepanoyl triketide, as established by GC-MS analysis of the TMS ether of the derived triketide lactone 7. Both TYLS KR1 and KR2 therefore catalyze the stereospecific reduction of the 2-methyl-3-ketoacyl-ACP substrate from the re-face, with specificity for the reduction of the (2R)-methyl (D) diastereomer. The dehydration that is catalyzed by the dehydratase (DH) domains of TYLS module 2 to give the unsaturated (2E,4S,5R)-2,4-dimethyl-5-hydroxyhept-2-enoyl-ACP2 is therefore a syn elimination of water.     

An ellipsoidal mirror for detection of laser-induced fluorescence in capillary electrophoresis system: applications for labelled antibody analysis

An LIF detector was integrated into a CE system which uses a ball lens to focus the laser beam on the CE capillary. The detector employs an ellipsoid that is glued on the capillary window, to permit the collection of the fluorescence in the capillary. This 'trapped' fluorescence stays in the capillary because the angle of the silica/air interface is greater than the critical angle. The performance of this new detector setup is found to be identical to the collinear setup using the same ball lens. An application to the analysis of FITC-labeled IgG was optimized using a 14 cm effective length capillary. The LOD of an FITC-labeled IgG2 at an excitation wavelength of 488 nm was 150 pg/mL, which was 10 times better than the LOD recorded with slab gel silver staining. Using a tetramethylrhodamine (TAMRA)-labeled IgG2 and a 532 nm excitation wavelength the LOD is 50 pg/mL. The electropherograms of four different commercial FITC conjugates of IgG were studied. The presence of aggregates was observed in two samples while close kinetics of reduction was observed between free aggregates and high aggregates concentration samples. The integrated LIF detector provides an extremely powerful and convenient tool for antibody analysis and should be useful for therapeutic MAb control in pharmaceutical facilities.     

Plasma-deposited metal-containing polymer films

The plasma treatment of vapors containing organometallic compounds (of Pd, Ni, Co, Sn, Au) and various alkenes has been used to prepare thin films, the composition and electrical resistivity of which could be varied over a wide range.     

Sequential Formation of Heteroternary Cucurbit[10]uril (CB[10]) Complexes

The globular and monocationic guest molecule trimethyl-azaphosphatrane ( AZAP , a protonated Verkade superbase) was shown to form a host:guest 1 : 1 complex with the cucurbit[10]uril (CB[10]) macrocycle in water. Molecular dynamics calculations showed that CB[10] adopts an 8-shape with AZAP occupying the majority of the internal space, CB[10] contracting around AZAP and leaving a significant part of the cavity unoccupied. This residual space was used to co-include planar and monocationic co-guest ( CG ) molecules, affording heteroternary CB[10]⋅ AZAP ⋅ CG complexes potentially opening new perspectives in supramolecular chemistry.     

Spectroscopy and lasing of new mixed Nd-doped (Sc,Y)VO4 crystals

We report the first successful growth of Nd-doped Sc _x Y_1?x VO₄ crystals (0<x≤1) by the Czochralski technique. For a successful synthesis, the upper limit of the Sc concentration in the YVO₄ melt was found to be below 10 at%. The spectroscopic characterization of an Nd-doped laser crystal, i.e., Nd:Sc_0.011Y_1.003V_0.986O_3.986, is discussed. Diode pumping of a c-cut crystal yielded an output power of 1.1 W at a wavelength of 1.06 μm with a slope efficiency of 43%: The slope efficiency of an a-cut crystal was 37%.     

A new high-resolution femtosecond pulse shaper

A novel liquid crystal display (LCD) with 640 stripes was successfully implemented and investigated for femtosecond pulse shaping. As compared to previously used devices, the large active area allows for operation in high-power laser systems. The increased number of pixels greatly enlarges the manifold of accessible pulse modulations, making the device an ideal tool for coherent control experiments and optical information processing. Received: 1 February 2001 / Published online: 21 March 2001