Surface-segment interaction parameter

Fig. 3.36 Segment density profiles from a Monte Carlo computer simulation of adsorption of a BAB triblock at a planar interface, where the hydrophobic B block is preferentially adsorbed (Balazs and Lewandowski 1990). Profiles are plotted for different A segment-surface interaction parameters, AS, with Xas = 0 and a chain length - 30 units.

The DNA-nucleosome interaction parameters are not known at present. In most of the theoretical work it is deemed negligible compared to the DNA-DNA and nucleosome-nucleosome interaction, except for a hard-core excluded volume interaction. Nevertheless, recent work on the mechanism of nucleosome repositioning [67] assumes that the DNA can dynamically detach from the nucleosome surface and reattach in different conformations, such that it is conceivable that distant DNA segments may also transiently bind to open regions of the DNA-binding surface of the nucleosome. 

This definition of interaction parameters is asymmetric, i.e., the interaction parameter of a segment i with a segment j is different from the interaction parameter of j with i. This is physically counter-intuitive, and has indeed caused a lot of discussion in the literature because it leads to a mathematical inconsistency in the contact probabilities of i and j [86]. Its explicit solution for general number of surface types made UNIQUAC easy to use in the times when computers where rare and slow. Therefore, UNIQUAC... 

For situations of overlapping chains, where lateral fluctuations in the segment concentration become rather small, mean-field descriptions become appropriate. The most successful of this type of theoiy is the lattice model of Scheutjens and Fleer (SF-theoiy). In chapter II.5 some aspects of this model were discussed. This theory predicts how the adsorbed amount and the concentration profile 0(z) depend on the interaction parameters and x and on the chain length N. From the statistical-thermodynamic treatment the Helmholtz energy and, hence, the surface pressure ti can also be obtcdned. When n is expressed as a function of the profile 0(z), the result may be written as ... 

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