Conformational asymmetry

Fig. 8 Phase diagram for PI-fc-PEO system. Only equilibrium phases are shown, which are obtained on cooling from high temperatures. ODT and OOT temperatures were identified by SAXS and rheology. Values of /AT were obtained using /AT = 65/T + 0.125. Dashed line spinodal line in mean-field prediction. Note the pronounced asymmetry of phase diagram with ordered phases shifted parallel to composition axis. Asymmetric appearance can be accounted for by conformational asymmetry of segments. Adopted from [53]...

We should also be familiar with the meaning of the term conformational asymmetry. We know that different conformations of the same compound have different symmetry and different statistical contribution (i.e., their percentage content is different). Therefore, the total effect on the polarization of light depends on the arrangement of atoms in different conformations and also on the statistical contribution of each conformation. This is called conformational asymmetry. The compound CH3-CH2-CH (CH3)C1 has conformational asymmetry because two identical atoms (c) are situated at the asymmetric centre. This compound has three staggered conformations. 

Figure B3.5.5 Near-UV CD spectra. (A) Bovine a -casein peptide under a variety of conditions (data from Alaimo et al., 1999). Peptide concentration 0.631 mg/ml in 2 mM PIPES, 4 mM KCI, pH 6.75 scan rate 40 sec/nm path length 10 mm bandwidth 1.5 nm. The loss of aromatic dichroism with increasing temperature indicates denaturation, which is, however, not complete at 70°C or in 6 M guanidine hydrochloride. The shift in maximum wavelength indicates loss of tryptophan asymmetry, but less so of tyrosine. (B) Seed coat soybean peroxidase under native and denaturing conditions (data from Kamal and Behere, 2002). Protein concentration 15 pM and path length 10 mm. The negative aromatic band centered around 280 nm and the Soret band around 410 nm both disappear at 90°C, indicating the loss of net conformational asymmetry of the aromatic and heme chromophores.

Another interesting driving force for surface enrichment in a blend is conformational asymmetry, described e.g. by different statistical segment lengths bA,... 

In the strong-segregation limit, the phase behavior is controlled mainly by /a, the fraction of component A in the diblock, and, to a lesser extent, by a conformational asymmetry parameter, which is the ratio (Hamley et al. 1994)... 

When racemic 173 (R = Me) was hydrolyzed in the presence of chymo-trypsin, the resulting optically active acid 173 (R = H) exhibited an ORD absorption spectrum characteristic of L-phenylalanine the starting ester possessed an axially oriented carbomethoxy group.338-362 Atropisomerism and conformational asymmetry of a precisely definable nature in a substrate are therefore recognized by chymotrypsin. X-ray diffraction studies confirmed that the chymotrypsin-active isomer has an axial ester moiety in the solid state, and that the ester mutarotates in solution to a CD-inactive isomer, whose ester group is in the equatorial position.363... 

In a similar manner, the coalescence temperature for the methyl groups of the tetramer was determined as 4°C. AG was calculated as 12.7 kcal/mol which was 3.7 kcal/mol smaller than that for the pentamer. The hexamer showed a total of seven signals with narrow linewidth due to two methyl groups and six methine protons even at 70°C, indicating that the rate of helix sense reversal is much slower than the rate for the pentamer. This suggests the possibility that the symmetrical oligomers over the pentamer level may be optically resolved at room temperature based entirely on conformational asymmetry. This was confirmed by the chiral HPLC technique using (+)poly(triphenylmethyl methacrylate) as a stationary phase.282... 

Zhou, N., Lodge, T.P. and Bates, RS. (2006) Influence of conformational asymmetry on the phase behavior of ternary homopolymer/block copolymer blends around the bicontinuous microemulsion channel. /. Phys. Chem. B, 110, 3979-3989. 

A different approach was used by Milner [326] in order to predict the phase diagram for asymmetric copolymer architectures (for example A2B, A3B etc. types of miktoarm stars). The free energy of the system can be calculated by summing the free energies of the polymer brushes existing on the two sides of the interphase. Milner described the effects of both chain architecture (i.e., number of arms) and elastic (conformational) asymmetry of the dissimilar chains, in the strong segregation limit, by the parameter... 

Due to the conformation asymmetry in rod-coil diblock copolymer systems, the packing is expected to be totally different from conformationally symmetric coil - coil block copolymers. Semenov and Vasilenko [71] have predicted that a N-SmA transition can be either a first-order transition (in the case of large coil fraction) or a second-order transition (in the case of small coil fraction) and a SmC phase in a rod-coil system is also expected for f <036. 

The mismatch can be characterized by a conformational asymmetry parameter defined as [35]... 

As we have seen, mean field theory predicts that the phase behavior of conformationaly symmetric diblocks can be described in terms of just two quantities, /Ac and/A. A general result of the NSCFT is that, as long as each microphase can be described by a single independent lattice parameter, then the entire microphase diagram of a system of diblock copolymers depends on three parameters, which are / Aeff, the compositional asymmetry /a, and the conformational asymmetry e. Aeff is an effective degree of polymerization, defined by... 

Although all copolymer phase diagrams are qualitatively similar, they vary quantitatively from copolymer to copolymer, and they are generally not perfectly symmetric about /a = 0.5. The small differences in phase diagrams for different molecules of the same architectures, for example, diblock PS-PI versus diblock PS-PDB, can thus be characterized in terms of different conformational asymmetries of the polymers. 

The NSCFT microphase diagram of conformationally symmetric, diblock copolymers has been presented in Figure 3 and discussed in Section II. In this section, we present a number of other diagrams for different systems, exploring the effects of conformational asymmetry and architectures. We focus on two-species copolymers, which have been the subject of most calculations. 

The shift in the order-order boundaries is easily explained by the conformational asymmetry. The argument is most transparent for layers. In these diagrams, bs < a, which implies that the A block is naturally more extended than the B block. If the diblocks have equal volumes,/a =/b, then, in the lamellar morphology, the A and B subdomains must have the same thickness. However, in this case of bn < bA, this means that the B block is stretched beyond its unperturbed Rg more than the A block. A corollary to this is that equal stretching will occur for copolymers with > /b-This means that the tendency for these polymers to be driven toward a B-centered morphology is reduced by this conformational asymmetry. Hence, the L/C phase boundary moves toward greater /a. Similar arguments apply to the other order-order phase boundaries, which all shift in the same direction. 

Matsen and Schick also did AB starblock copolymers, with three, five, and nine armed-stars [4]. Once again, there was no change in the topology of the phase diagram, but the order-order boundaries were shifted. As in all the other cases, the conformational asymmetry simply shifted the order-order phase boundaries to the left if bA < bs or to the right if a > b-... 

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