Optimization of the Synthesis of 2,6–Dinitro-4-trifluoromethylphenyl Ether Substituted Cyclodextrin Bonded Chiral Stationary Phases

The comparisons of five different chiral stationary phases (CSPs) based on 2,6-dinitro-4-trifluoromethylphenyl (DNP-TFM) ether substituted β-cyclodextrin are presented. The five CSPs differ from each other in the linkage/spacer chemistry, or on the position of the substituents on β-cyclodextrin, or in the sequence of the synthetic procedure. The results show that there are two optimum combinations: (1) DNP-TFM randomly substituted on the β-cyclodextrin as the chiral selector along with a carbamate linkage chain bonding it to the silica support; and (2) β-cyclodextrin derivatized by DNP-TFM substituents only on the C-2 and C-3 positions of the cyclodextrin with an ether linkage chain anchoring it to the silica gel. These two combinations show complementary separations for some enantiomers. The spacer chain effect is much more pronounced for the CSP based on the β-cyclodextrin derivatives with DNP-TFM substituents only on C-2 and C-3 positions than its randomly substituted counterpart. The sequence of derivatizing the cyclodextrin and attaching it to silica gel also affects its selectivity and efficiency. The β-cyclodextrin should be derivatized before it is linked to the silica gel.     

Chromatographic evaluation of poly (trans-1,2-cyclohexanediyl-bis acrylamide) as a chiral stationary phase for HPLC

Chiral stationary phases (CSPs) based on polymeric (R,R)- or (S,S)-1,2-diaminocyclohexane (DACH) derivatives are synthesized. When bonded to 5 microm porous spherical silica gel, the poly (trans-1,2-cyclohexanediyl-bis acrylamide) based poly-cyclic amine polymer (P-CAP) stationary phases is proved to be effective chiral stationary phases that could be used in the normal-phase mode, polar organic mode and with halogenated solvents mobile phases, if desired. Since these are entirely synthetic CSPs, the elution order of all enantiomers can be reversed between the (R,R) P-CAP and (S,S) P-CAP columns. Because of the high loading of chiral selectors, the columns exhibit very high sample capacities. Thus, P-CAP columns are useful for preparative and semi-preparative enantiomeric separations. The application of these CSPs and optimization of their separations are discussed.     

The function of Shp2 tyrosine phosphatase in the dispersal of acetylcholine receptor clusters

Background

Tone languages such as Thai and Mandarin Chinese use differences in fundamental frequency (F₀, pitch) to distinguish lexical meaning. Previous behavioral studies have shown that native speakers of a non-tone language have difficulty discriminating among tone contrasts and are sensitive to different F₀ dimensions than speakers of a tone language. The aim of the present ERP study was to investigate the effect of language background and training on the non-attentive processing of lexical tones. EEG was recorded from 12 adult native speakers of Mandarin Chinese, 12 native speakers of American English, and 11 Thai speakers while they were watching a movie and were presented with multiple tokens of low-falling, mid-level and high-rising Thai lexical tones. High-rising or low-falling tokens were presented as deviants among mid-level standard tokens, and vice versa. EEG data and data from a behavioral discrimination task were collected before and after a two-day perceptual categorization training task.

Results

Behavioral discrimination improved after training in both the Chinese and the English groups. Low-falling tone deviants versus standards elicited a mismatch negativity (MMN) in all language groups. Before, but not after training, the English speakers showed a larger MMN compared to the Chinese, even though English speakers performed worst in the behavioral tasks. The MMN was followed by a late negativity, which became smaller with improved discrimination. The High-rising deviants versus standards elicited a late negativity, which was left-lateralized only in the English and Chinese groups.

Conclusion

Results showed that native speakers of English, Chinese and Thai recruited largely similar mechanisms when non-attentively processing Thai lexical tones. However, native Thai speakers differed from the Chinese and English speakers with respect to the processing of late F₀ contour differences (high-rising versus mid-level tones). In addition, native speakers of a non-tone language (English) were initially more sensitive to F₀ onset differences (low-falling versus mid-level contrast), which was suppressed as a result of training. This result converges with results from previous behavioral studies and supports the view that attentive as well as non-attentive processing of F₀ contrasts is affected by language background, but is malleable even in adult learners.     

Secondary ion yields produced by keV atomic and polyatomic ion impacts on a self-assembled monolayer surface

A suite of keV polyatomic or 'cluster' projectiles was used to bombard unoxidized and oxidized self-assembled monolayer surfaces. Negative secondary ion yields, collected at the limit of single ion impacts, were measured and compared for both molecular and fragment ions. In contrast to targets that are orders of magnitude thicker than the penetration range of the primary ions, secondary ion yields from polyatomic projectile impacts on self-assembled monolayers show little to no enhancement when compared with monatomic projectiles at the same velocity. This unusual trend is most likely due to the structural arrangement and bonding characteristics of the monolayer molecules with the Au(111). Copyright 1999 John Wiley & Sons, Ltd.     

Development of dinitrophenylated cyclodextrin derivatives for enhanced enantiomeric separations by high-performance liquid chromatography

The synthesis and evaluation of new dinitrophenyl (DNP) substituted beta-cyclodextrin (beta-CD) chiral stationary phases (CSPs) for the enantioseparation of various classes of chiral analytes by HPLC are presented. The dinitrophenyl substituted beta-CD derivatives are synthesized and covalently bonded to functionalized 5 microm spherical porous silica gel. These are the first reported derivatized cyclodextrin which contains pi-electron deficient substituents (i.e., pi-acidic moieties). The column performance in terms of their ability to separate enantiomers is evaluated. A variety of different dinitro-substituted aryl groups are investigated and compared. The pH of the mobile phase buffers, the buffer composition, the number and position of the dinitro groups on the phenyl ring substituent, the degree of substitution, and the bonding strategy all greatly affect the performance of the CSPs. A large variety of racemic compounds have been separated successfully on these CSPs. The bonded dinitrophenyl-derivatized cyclodextrins are stable in all three mobile phase modes, namely, the reversed-phase, polar organic, and normal phase modes. No degradation in column performance was observed in any mode of operation even after more than 1000 injections. The analytical applicability of these types of CSPs for enantiomeric separations is discussed in detail.     

On the mechanism of the formation of benzocyclobutene by flash vacuum pyrolysis of ortho-methylbenzyl chloride

Flash vacuum pyrolysis of $\text{o}$-methylbenzyl chloride-$\text{∝}$,$\text{∝}$-$\text{d}$₂ gives benzocyclobutene-$\text{∝}$,$\text{∝}$-$\text{d}$₂. This finding indicates that the reaction involves net δ-elimination of hydrochloric acid.     

Room temperature observation of p-xylylenes by 1H NMR and evidence for diradical intermediates in their oligomerization

p-Quinodimethanes (p-QDMs) are reactive molecules that have been invoked as transient intermediates in a number of reactions. Dilute solutions of benzene-based p-QDMs, p-xylylene (1), alpha-methyl-p-xylylene (10), and 2,5-dimethyl-p-xylylene (11) can be prepared by fluoride-induced elimination of trimethylsilyl acetate from the appropriate precursor. It has been found that these solutions are stable enough to allow these reactive p-QDMs to be observed by 1H NMR spectroscopy at room temperature. For the first time, the 13C NMR spectrum of p-QDM 1 was observed. After several hours at room temperature, these p-QDMs form dimers, trimers, and insoluble oligomers. Formation of trimers provides evidence that p-QDMs 1, 10, and 11 dimerize by a stepwise mechanism involving dimeric diradicals as intermediates.