Entropy conformational

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

Disulfides. The introduction of disulfide bonds can have various effects on protein stability. In T4 lyso2yme, for example, the incorporation of some disulfides increases thermal stability others reduce stability (47—49). Stabili2ation is thought to result from reduction of the conformational entropy of the unfolded state, whereas in most cases the cause of destabili2ation is the introduction of dihedral angle stress. In natural proteins, placement of a disulfide bond at most positions within the polypeptide chain would result in unacceptable constraint of the a-carbon chain. 

A4) Bond angle bending makes a nonnegligible contribution to conformational entropy and can affect computed equilibrium populations [11]. 

P Koehl, M Delarue. Application of a self-consistent mean field theory to predict protein side-chains conformation and estimate their conformational entropy. J Mol Biol 239 249-275, 1994. 

The porous materials that offer the narrowest possible pore size distribution are those that have cylindrical pores of uniform diameter penetrating the entire medium without branching. Branching gives polymer molecules in the junctions extra conformational entropy. An agglomerate of tiny pieces of these porous materials, interlaced with larger voids (much larger than the pore size), should also be chosen. 

At low concentrations, adsorption is a single-chain phenomenon. The adsorption takes place when the enthalpy gain by the monomer-surface contact with respect to the monomer-solvent contact surpasses the loss of the conformational entropy. In a good solvent the adsorption is not likely unless there is a specific interaction between monomers and the surface. At high concentrations, however, interactions between monomers dominate the free energy of the solution. The adsorption takes place when the enthalpy gain by the mono-... 

Ad layering (Fig. 16a). On the other hand, additional conformational entropy related to the sequenced hydrocarbon-perfluorinated chain prevents the crystallization of the aromatic parts of the molecules in the smectic layers as occurs for hydrocarbon analogues of the same tail length [4]. The stability of Cd phase in polyphilics is more delicate in nature and will be discussed below. 

If peptide residues are converted to peptoid residues, the conformational heterogeneity of the polymer backbone is likely to increase due to cis/trans isomerization at amide bonds. This will lead to an enhanced loss of conformational entropy upon peptoid/protein association, which could adversely affect binding thermodynamics. A potential solution is the judicious placement of bulky peptoid side chains that constrain backbone dihedral angles. 

These two examples show that regular patterns can evolve but, by definition, dissipative structures disappear once the thermodynamic equilibrium has been reached. When one wants to use dissipative structures for patterning of materials, the dissipative structure has to be fixed. Then, even though the thermodynamic instability that led to and supported the pattern has ceased, the structure would remain. Here, polymers play an important role. Since many polymers are amorphous, there is the possibility to freeze temporal patterns. Furthermore, polymer solutions are nonlinear with respect to viscosity and thus strong effects are expected to be seen in evaporating polymer solutions. Since a macromolecule is a nanoscale object, conformational entropy will also play a role in nanoscale ordered structures of polymers. 

We note that the resulting entropic force is particular to macromolecular systems, where the conformational entropy generates a force stabilizing the most favorable conformation. 

Other than an effect on backbone solvation, side chains could potentially modulate PPII helix-forming propensities in a number of ways. These include contributions due to side chain conformational entropy and, as discussed previously, side chain-to-backbone hydrogen bonds. Given the extended nature of the PPII conformation, one might expect the side chains to possess significant conformational entropy compared to more compact conformations. The side chain conformational entropy, Y.S ppn (T = 298°K), available to each of the residues simulated in the Ac-Ala-Xaa-Ala-NMe peptides above was estimated using methods outlined in Creamer (2000). In essence, conformational entropy Scan be derived from the distribution of side chain conformations using Boltzmann s equation... 

Cao, M., and L. Schafer. 1993. Viewpoiny 6—Characteristic Aspects of GG Sequences and the Importance of Constitutional Properties for Conformational Entropies. J. Mol. Struct. (Theochem) 284, 235-242. 

This expression accounts for the configurational entropy of an ideal binary mixture with identical molecular sizes, but not for that of a polymer solution, since polymer chains are large and flexible. For that case, more contributions arise from the chain conformational entropy, first considered by Meyer [19] and then derived by Huggins [20] and Flory [21]. In analogy with a nonreversing random walk on a lattice, the conformational contribution of polymer chains to the partition function is given by... 

Flexible, coily-structured polymers in solution possess a high conformational entropy resulting from the many rotational possibilities of the single bonds in the polymer chain. 

Adsorption of the polymer molecule causes a reduction of its conformational entropy (Norde 2003b). Flence, adsorption takes place only if the loss in conformational entropy is compensated by sufficient favorable interactions between polymer segments and the interface. Because the polymer molecule attaches with many segments at the interface, it adsorbs tenaciously with a very high affinity, even if the interaction of the individual segments with the interface is rather weak. The high affinity manifests itself by the adsorption being irreversible with respect to variations of the polymer concentration in solution. 

A change in the environment of a protein molecule, e.g. adsorption from aqueous solution onto a sorbent surface, may lead to a partial breakdown of its ordered structure, resulting in an increase of conformational entropy. This is a fundamental difference between protein adsorption and the adsorption of flexible polymers, for which attachment to a surface implies a loss of conformational entropy. 

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