Preparation and characterization of new micelle-forming cholesterol amphiphiles

New sulfated and sulfonated cholesterol amphiphiles were synthesized and characterized by dynamic light scattering and electron microscopy. The surfactants form micelles with diameters up to 30 nm. Their critical micelle concentration values were determined by conductometry and photometric measurements. Use of the new amphiphiles enables the transfer of an asymmetric hydrogenation reaction into aqueous solution by keeping the activity and increasing the enantioselectivity. Interesting liquid-crystalline behaviour was observed. Received: 7 July 1999 /Accepted in revised form: 5 November 1999     

Trace analysis of surfactants derived from fatty alcohols II. Hydrolysis and enrichment techniques

Summary Acid catalyzed hydrolysis of fatty alcohol sulfates under reflux condition or by microwave heating is compared. Microwave hydrolysis turned out to be faster and more convenient to use. The combination of reflux hydrolysis of fatty alcohol sulfates or alkyl polyglycosides and the simultaneous steam distillation of the generated fatty alcohols gives a very powerful sample preparation and enrichment method for fatty alcohol sulfates and alkyl polyglycosides. The preconcentrated fatty alcohols are derivatized and detected via fluorescence detection. The applicability of the method is demonstrated by the analysis of fatty alcohol sulfates in cosmetics and in river water as matrices spiked with approximately 4 ppb fatty alcohol sulfates.     

Pulse sequences for measurement of one-bond (15)N-(1)H coupling constants in the protein backbone.

A set of three improved two-dimensional (2D) NMR methods for measuring one-bond (15)N-(1)H coupling constants in the protein backbone is presented. They are tailored to suit the size of the TROSY effect, i.e., the degree of interference between dipolar and chemical shift anisotropy relaxation mechanisms. The methods edit 2D spectra into two separate subspectra corresponding to the two possible spin states of the coupling partner. Cross talk between the two subspectra is a second order effect in the difference between the actual coupling constants and the one used in setting the pertinent delays of the pulse sequences. This relatively high degree of editing accuracy makes the methods useful for applications to molecules subjected to weak alignment where the one-bond coupling constants are linear combinations of a scalar J and a residual dipolar contribution containing important structural information. A demonstration of the new methods is shown for the (15)N-labeled protein chymotrypsin inhibitor 2 in a lipid bicelle mixture.     

Optimization of Three-Dimensional TROSY-Type HCCH NMR Correlation of Aromatic H–C Groups in Proteins

Improved methods for three-dimensional TROSY-Type HCCH correlation involving protons of negligible CSA are presented. The TROSY approach differs from the conventional approach of heteronuclear decoupling in evolution and detection periods by not mixing fast and slowly relaxing coherences and usually suppressing the former. Pervushin et al. (J. Am. Chem. Soc. 120, 6394–6400 (1998)) have proposed a 3D TROSY-type HCCH experiment where the TROSY approach is applied only in one of the ¹³C dimensions. A new pulse sequence applying the TROSY approach in both indirect dimensions is advantageous when the TROSY effect of the carbons is large or when a relatively high resolution is required. For lower resolutions or moderate TROSY effects we show that it is possible to combine the best of both worlds, namely to suppress heteronuclear couplings without mixing fast and slowly relaxing coherences while at the same time superimpose the two components and thus have both contribute to the detected signal. That is possible using the novel technique of Spin-State-Selective Time-Proportional Phase Incrementation (S³ TPPI). The new 3D S³ TPPI TROSY HCCH method is demonstrated on a ¹³C,¹⁵N-labeled protein sample, RAP 18–112 (N-terminal domain of α₂-macroglobulin receptor associated protein), at 750 MHz and average sensitivity enhancements of 10% are obtained for the cross peaks in comparison to methods based on conventional decoupling on one of the carbons or on TROSY on both carbons.     

Combined constant strain rate and stress relaxation test for linear viscoelastic studies

The strain history of the combined test is formed by a constant strain rate during the period 0 <t < t₁ followed by a constant strain for t > t₁. This test includes as limiting cases the relaxation test (t₁ → 0), the constant strain rate or stress growth experiment (t₁ → ∞), and the stress relaxation after cessation of constant strain rate flow (t₁ sufficiently large). The constant t₁ can be chosen arbitrarily such that the test conditions of the combined test can be adjusted according to the problem investigated and to the capability of the testing device. From this, practical advantages follow for linear viscoelastic studies of polymeric solids and liquids, e.g., the zero shear viscosity η₀ for unstable polymer melts can be determined at least approximately. In order to obtain linear viscoelastic material functions from the measured stress record, a simple procedure is given which avoids the difficulties with the “factor-of-10 rule” for short relaxation times.     

Thermal dehydrochlorination of poly(vinyl chloride)—epoxidized butadiene/styrene triblock copolymer blends

The effect of the type of epoxidized butadiene/styrene block copolymer [ESBS; linear (B/S) or radial (E(B/S)_n), containing 0–27% of epoxy groups] on the thermal dehydrochlorination of poly (vinyl chloride) (PVC)-ESBS blends (ESBS content 10%) was investigated in the temperature range 170–180 °C, under a non-oxygen atmosphere. Thermal stability of the PVC-ESBS blends was estimated on the basis of induction time, t₀, and maximum rate of hydrochloride emission, V_max from the system. It was found that, for a similar degree of epoxidation of the SBS copolymer, the induction time i.e. the time after which emission of HCl begins, is longer in the series PVC < PVC-SBS < PVC-EB/S < PVC-E(B/S)_n, and the same is true for thermal stability. However, the maximum rate of emission of HCl is lowest in the case of PVC-EB/S blends, in the range of molar ratios from 0.5 to 2.0 × 1O⁻². On the basis of the dependence V_max = f(EB/VC), it was found that there is a certain content of epoxidized butadiene (EB) units in a mixture which causes the optimum stability of poly (vinyl chloride) during heating. During thermal destruction of the PVC-ESBS blends, the HC1 evolved undergoes addition both to epoxy fragments and to double bonds. The degree of conversion of EB units in time t₀ is 20% at 170 °C and 30% at 180 °C. The glass transition temperature, T_g, of PVC in the PVC-ESBS blends shifts towards higher temperatures by about 6–8 °C, proving the existence of crosslinking processes during moulding of the blends. The epoxidized butadiene/styrene radial block copolymer, E(B/S)_n, is a better thermal stabilizer of PVC than the linear EB/S copolymer.     

Unusual spin-transition anomaly in the crossover system [Fe(2-pic)3]Cl2·EtOH

The spin transition in [Fe(2-pic)₃]Cl₂·EtOH (2-pic = 2-picolylamine) has been reinvestigated by Mössbauer spectroscopy and magnetic susceptibility measurements. With both techniques we see a “two-step” spin conversion in the crossover region with transition temperatures at 120.7 and 114.0 K. This has never been reported for any other spin crossover system. The quadrupole splittings for both spin states change abruptly in the range of the transition.     

Synthesis of telogens and optimization of tetrafluoroethene telomerization process

The most advantageous technological parameters of tetrafluoroethene telomerization using 1-chloro-2-iodohexafluoropropane (telogen) towards the telomers n2-n4 or n1-n4 were experimentally established. The telomers n1-n4 were prepared with the yield of 48 mol% under the following conditions: temperature 170 °C, the molar ratio of 1-chloro-2-iodohexafluoropropane to tetrafluoroethene equal to 1.2, the autoclave filling of 1.5 kg dm⁻³. The maximum yield of telomers n2-n4 amounted to 27 mol% when the molar ratio of telogen to tetrafluoroethene was decreased to 0.7 and the other parameters of synthesis remained the same. The optimum parameters for the synthesis of telogens: 1-chloro-2-iodohexafluoropropane and 2-iodoheptafluoropropane have been also determined.