Conformational order

As an example of the third case, we may have conformationally ordered chains, parallel among themselves, with short-range order in the lateral packing. Fiber spectrum features are well-defined layer lines, with diffuse reflections only. 

Differently from the cases of ETFE, PTFE, 1,4-trans-polybutadiene, and PE, in the mesomorphic form of i-PP it is believed that there is a nearly complete conformational order, in the chains packed with parallel axes, (intrachain long range 3-D order) and only short-range lateral order in the positioning of the chain axes [60, 61]. 

Norton, I. T., Goodall, D. M., Morris, E. R., and Rees, D. A. Kinetic evidence for intramolecular conformational ordering of the extracellular polysaccharide (xanthan) from Xanthomonas campestris,. Chem. Soc. Chem. Comm., 545,1980. 

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

Hilvert s group used the same hapten [26] with a different spacer to generate an antibody catalyst which has very different thermodynamic parameters. It has a high entropy of activation but an enthalpy lower than that of the wild-type enzyme (Table 1, Antibody 1F7, Appendix entry 13.2a) (Hilvert et al., 1988 Hilvert and Nared, 1988). Wilson has determined an X-ray crystal structure for the Fab fragment of this antibody in a binary complex with its TSA (Haynes et al., 1994) which shows that amino acid residues in the active site of the antibody catalyst faithfully complement the components of the conformationally ordered transition state analogue (Fig. 11) while a trapped water molecule is probably responsible for the adverse entropy of activation. Thus it appears that antibodies have emulated enzymes in finding contrasting solutions to the same catalytic problem. 

An Overview of Spectroscopy (and Other Experimental) Techniques Used to Determine Stationary-Phase Conformational Order... 

A complete review of spectroscopic methods applied to the analysis of alkyl-modified surfaces with a comprehensive list of spectroscopic indicators of alkyl chain conformational order is provided elsewhere [9] this review will focus on the application of spectroscopic and other relevant experimental techniques for the characterization of shape-selective chromatographic materials. On the whole, it has been observed experimentally that any increase in alkyl stationary-phase conformational order promotes an increase in selectivity for shape-constrained solutes in RPLC separations [9], As a complement to the wealth of spectroscopic and chromatographic data, the use of molecular simulation techniques to visualize and characterize alkyl-modified surfaces may also provide new insights into molecular-level features that control shape selectivity. A review of progress in the field of chromatographic material simulations will also be discussed. 

One of the initial spectroscopic methods applied to stationary-phase characterization was Fourier transform infrared spectroscopy (FTIR). This originated from several important studies of phase conformational order in crystalline n-alkanes conducted in the late 1960s and early 1980s by Snyder, Maroncelli, and coworkers [111-114], In this work, assignments of C—H bond wagging modes were associated with chain... 

An FTIR comparison between C18 and C30 sorbents prepared by both surface-and solution-polymerized synthetic routes over a range of temperatures was recently described [119]. Little difference in conformational order was observed for C30 sorbents prepared by both surface and solution-polymerized synthetic routes, as indicated by kink/gfg, double- and end-gauche conformations. Differences in conformational... 

FIGURE 5.26 (See color insert following page 280.) A representation of the slot model illustrating potential constrained-shape solute (BaP) interactions with the conformational ordered cavities of a polymeric Cjg stationary-phase simulation model. Also included on the chromatographic surface is an identical-scale molecular structure of 1,2 3,4 5,6 7,8-tetrabenzonaphthalene (TBN). 

Srinivasan, G., Neumann-Singh, S., and Muller, K., Conformational order of n-alkyl modified silica gels as evaluated by Fourier transform infrared spectroscopy, J. Chromatogr. A, 1074, 31, 2005. 

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