Isotropic conformation

More recently, a systematic study of the properties of side-chain LC dendrimers has been undertaken on these PPI dendrimers and also on PAMAM systems by Serrano et al. [211]. In these dendrimers, the mesogenic group is connected to the dendritic scaffold by an imine linkage. In all cases, it was found that the enthalpic gain of the mesogenic units arranged as in a classical liquid crystalline mesophase dominates over the entropic tendency of the dendrimer core to adopt a globular isotropic conformation. The flexibility of the dendritic PAMAM and PPI cores allows the macromolecule to adopt... 

M. R. Sridhar and M. M. Yovanovich, Elastoplastic Contact Model for Isotropic Conforming Rough Surfaces and Comparison with Experiments, ASMEJ. of Heat Transfer (118/1) 3-9,1996. 

When LCEs are synthesized in the absence of external fields, so-called polydomain LC elastomers are obtained, which show macroscopically isotropic properties similar to polycrystalline materials. This resembles, e.g., bulk material of a low molar mass liquid crystal, where thermal fluctuations prevent a uniform director orientation over the whole sample. For nematic elastomers the overall isotropic behavior also indicates an overall isotropic conformation of the polymer chains, which is the consequence of the maximization of the chain entropy. 

In a gedanken experiment, a polymer is swollen with a liquid crystal and crosslinked. By swelling with the LC polymer chains lose their isotropic conformation due to the anisotropic environment. This leads to an anisotropic conformation of the network chains as well. The respective coil dimensions differ parallel and perpendicular to the director of the LC phase. Upon heating, the LC phase will lose its anisotropy and turn isotropic. The polymer network will follow this change, but due to the crosslinks, the adaption of an isotropic conformation will lead to a deformation of the elastomer (Figure 19). 

The isotropic equivalent thermal parameters are on the whole larger than in the PbTX-1 dimethyl acetal structure or the structure of the natural product. The B values for atoms on the fused ring skeleton range from 4.7 to 12.6 A (mean square amplitudes of 0.059 and 0.16 A ). Curiously, the largest values are associated with C17-C20 of the 9-membered E ring—the ring that adopts two conformations in crystalline PbTX-1. The acyclic atoms do not have appreciably higher thermal parameters, with the exception of hydroxyl 013, which has a B of 22.4 A 2. 

Our first exploration of property space was focused on acetylcholine. This molecule was chosen for its interesting structure, major biological role, and the abundant data available on its conformational properties [15]. The behavior of acetylcholine was analyzed by MD simulations in vacuum, in isotropic media (water and chloroform) [16] and in an anisotropic medium, i.e. a membrane model [17]. Hydrated n-octanol (Imol water/4mol octanol) was also used to represent a medium structurally intermediate between a membrane and the isotropic solvents [17]. 

Conversely, in a membrane model, acetylcholine showed mean log P values very similar to those exhibited in water. This was due to the compound remaining in the vicinity of the polar phospholipid heads, but the disappearance of extended forms decreased the average log P value somewhat. This suggests that an anisotropic environment can heavily modify the conformational profile of a solute, thus selecting the conformational clusters more suitable for optimal interactions. In other words, isotropic media select the conformers, whereas anisotropic media select the conformational clusters. The difference in conformational behavior in isotropic versus anisotropic environments can be explained considering that the physicochemical effects induced by an isotropic medium are homogeneously uniform around the solute so that all conformers are equally influenced by them. In contrast, the physicochemical effects induced by an anisotropic medium are not homogeneously distributed and only some conformational clusters can adapt to them. 

The dynamic profile of carnosine was investigated by comparing MD simulations in isotropic solvents (i.e. water and chloroform) with simulation of the compound bound to serum carnosinase (CNl) [22]. This enzyme is characterized by its distribution in plasma and brain, and its ability to hydrolyze also anserine and homocarnosine [23]. The conformational profile of carnosine can be defined by... 

Fig. 27. Percec s hyperbranched liquid crystalline dendrimer that forms isotropic and nematic phases based on monomeric gauche and anti conformations...

For oriented systems, the determination of molecular conformation is a complex problem because Raman spectra contain signals inherently due to both molecular conformation and orientation. To extract only the information relative to the conformation, one has to calculate a spectrum that is independent of orientation, in a similar way to the A0 structural absorbance of IR spectroscopy (Section 4). Frisk et al. [57] have shown that for a uniaxial sample aligned along the Z-axis, a spectrum independent of orientation (so-called isotropic spectrum), fso, can be calculated from the following linear combination of four polarized spectra [57]... 

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