Silica, helical

Tetramethylsilane (TMS) [75-76-3] M 88.2, b 26.3, n 1.359, d 0.639. Distilled from cone H2SO4 (after shaking with it) or LiAlH4, through a 5ft vacuum-jacketted column packed with glass helices into an ice-cooled condenser, then percolated through silica gel to remove traces of halide. 

The inductively coupled plasma source (Fig. 20.11) comprises three concentric silica quartz tubes, each of which is open at the top. The argon stream that carries the sample, in the form of an aerosol, passes through the central tube. The excitation is provided by two or three turns of a metal induction tube through which flows a radio-frequency current (frequency 27 MHz). The second gas flow of argon of rate between 10 and 15 L min-1 maintains the plasma. It is this gas stream that is excited by the radio-frequency power. The plasma gas flows in a helical pattern which provides stability and helps to isolate thermally the outside quartz tube. 

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

The oldest method of resolving enantiomers by TLC takes advantage of the natural chiral properties of cellulose and triacetylcellulose resulting from the helical structure of the polymers (98). Amino acid derivatives have been resolved on silica gel layers impregnated with chiral acids or bases, for example. 

Fig. 4.18 Formation ofhexagonal silica platelets with poly-L-lysine helical chains in the presence of monosilicic acid and phosphate. Reprinted with permission from [170], M.M.Tomczaketa/.J. Am. Chem. Soc. 2005, 727, 12577. 2005, American Chemical Society.

L. and Mann, S. (2002) Chiral templating of silica-lipid lamellar mesophase with helical tubular architecture. Angewandte Chemie-International Edition, 41, 2988-2991. 

The influence of adsorption on the structure of a -chymotrypsin is shown in Fig. 10, where the circular dichroism (CD) spectrum of the protein in solution is compared with that of the protein adsorbed on Teflon and silica. Because of absorbance in the far UV by the aromatic styrene, it is impossible to obtain reliable CD spectra of proteins adsorbed on PS and PS- (EO)8. The CD spectrum of a protein reflects its composition of secondary structural elements (a -helices, / -sheets). The spectrum of dissolved a-chymotrypsin is indicative of a low content of or-helices and a high content of //-sheets. After adsorption at the silica surface, the CD spectrum is shifted, but the shift is much more pronounced when the protein was adsorbed at the Teflon surface. The shifts are in opposite directions for the hydrophobic and hydrophilic surfaces, respectively. The spectrum of the protein on the hydrophilic surface of silica indicates a decrease in ordered secondary structure, i.e., the polypeptide chain in the protein has an increased random structure and, hence, a larger conformational entropy. Adsorption on the hydrophobic Teflon surface induces the formation of ordered structural elements, notably an increase in the content of O -helices (cfi, the discussion in Sect. 3.1.4). 

Figure 5.33 Electron micrographs of nanoparticle helices inside tubules of DC8,nPC+ 2% DCgj9PEOH after sequential absorption of charged polymers followed by treatment with 45-nm silica spheres. Reprinted with permission from Ref. 141. Copyright 2000 by the American Chemical Society.

Figure 1.15 Helical hybrid silicas chirality transcription from the precursor molecule to the hybrid solid. (Image courtesy of Prof. Joel Morean.)... 

Figure 1.25 A silica-lipid mineralised helical ribbon (scale bar 400 nanometres). (Reproduced from ref. 52, with permission.)...

A. M. Seddon, H. M. Patel, S. L. Burkett and S. Mann, Chiral Templating of Silica-Lipid Lamellar Mesophase with Helical Tubular Architecture, Angew. Chem. Int. Ed., 2002, 41, 2988. 

The main limitation of these CSPs is their limited pressure stability, which makes them not very suitable for HPLC application. However, they have proved to be an excellent tool for the preparative separation of drugs by low-pressure HPLC. To make these CSPs accessible to HPLC, silica gel-based phases were developed. " This type of phase is available from Merck (Darmstadt, Germany) under the name Chiraspher. Polymer phases of different types have been developed by Okamoto s group. > They are prepared by the asymmetric polymerization of triphenylmethyl-methacrylate monomers. The original character of these polymers is that they do not possess any chiral centre and therefore their chirality is only due to their helicity. However, clear mechanisms have not been proposed... 

In addition, helical silica and chiral organic-inorganic hybrid silica were found to serve as chiral triggers of asymmetric autocatalysis ... 

Tamaru S, Takeuchi M, Sano M, Shinkai S (2002) Sol-gel transcription of sugar-appended porphyrin assemblies into fibrous silica unimolecular stacks versus helical bundles as templates. Angew Chem Int Ed 41 853-856... 

In addition to the classification of liquid chromatographic enantioseparation methods by technical description, these methods could further be classified according to the chemical structure of the diverse CSPs. The chiral selector moiety varies from large molecules, based on natural or synthetic polymers in which the chirality may be based on chiral subunits (monomers) or intrinsically on the total structure (e.g., helicity or chiral cavity), to low molecular weight molecules which are irreversibly and/or covalently bound to a rigid hard matrix, most often silica gel. 

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