Segmental flexibility

If the branching density is sufficiently high to hinder segmental flexibility and impose strong excluded volume and even steric interactions, molecular dimensions become rigid. Measurements of solution and melt viscosity showed that the properties of dendritic molecules approached that of solid spheres as the... 

In an early attempt to model the dynamics of the chromatin fiber, Ehrlich and Langowski [96] assumed a chain geometry similar to the one used later by Katritch et al. [89] nucleosomes were approximated as spherical beads and the linker DNA as a segmented flexible polymer with Debye-Huckel electrostatics. The interaction between nucleosomes was a steep repulsive Lennard-Jones type potential attractive interactions were not included. 

Plasticizers extend the lower temperature range for use of materials since they discourage polymer chain associative behavior and encourage segmental flexibility, thus increasing the rotational freedom and effectively decreasing the material s typical Eg. 

Segregation of the incompatible molecular segments takes place with formation of distinct nano-compartments organized on a one-dimensional (ID), two-dimensional (2D), or three-dimensional (3D) periodic lattice, separated by interfaces. These interfaces tend to be minimal in order to reduce the interfacial energy stored in the system. For amphiphilic molecules without anisometric segments (flexible amphiphiles) the mesophase type is mainly determined by the relative volume of the two incompatible segments, as shown in Fig. 2. 

Here three constants appear Go is the equilibrium modulus of elasticity 0p is the characteristic relaxation time, and AG is the relaxation part of elastic modulus. There are six measured quantities (components of the dynamic modulus for three frequencies) for any curing time. It is essential that the relaxation characteristics are related to actual physical mechanisms the Go value reflects the existence of a three-dimensional network of permanent (chemical) bonds 0p and AG are related to the relaxation process due to the segmental flexibility of the polymer chains. According to the model, in-termolecular interactions are modelled by assuming the existence of a network of temporary bonds, which are sometimes interpreted as physical (or geometrical) long-chain entanglements. 

Tan LK, Shopes RJ, Oi VT, Morrison SL. Influence of the hinge region on complement activation, Clq binding, and segmental flexibility in chimeric human immunoglobulins. Proc Natl Acad Sci USA 1990 87(l) 162-6. 

Figure 33.5. Segmental Flexibility. The linkages between the and the regions of an IgG molecule are flexible,... 

The strength of mucoadhesion decreases with an increase in cross-linking as this leads to a decrease in the polymer s diffusion coefficient and chain segment flexibility and mobility (which in turn reduces interpenetration). 

Figure 33.8 Segmental flexibility. The linkages between the F b and the F<- regions of an IgG molecule are flexible, allowing the two antigen-binding sites to adopt a range of orientations with respect to one another. This flexibility allows effective interactions with a multivalent antigen without requiring that the epitopes on the target be a precise distance apart.

Segmental flexibility in IgE appears less than that generally found for IgG subclasses [40,97] but, it has been argued, is still significant [97], In the model discussed above there appears to be scope for flexibility between Cel and Ce2 domains and between Ce2 and CE3 domains [49], The latter flexibility corresponds to that described earlier for IgM. 

Fluorescence studies have been taken to indicate a non-perpendicular conformation of IgE to the cell surface when receptor bound [104] possibly dislocated in the manner envisaged for IgG (Fig. 9, [105]). These studies also indicate a retention of most of the segmental flexibility of IgE when receptor bound [97]. 

Many polymers are themselves brittle at room temperature. For these polymers to become more pUable, additives called plasticizers that allow segmental mobility, and consequently segmental flexibility, are added. For syn-tiietic pol)miers such as poly(vinyl chloride) (PVC) and polystyrene (Figure 13), plasticizers are added that allow tiie polymers to be flexible. 

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