Confinement as a determinant of macromolecular structure and reactivity. II. Effects of weakly attractive interactions between confined macrosolutes and confining structures.

The effect of weak, nonspecific interaction between molecules confined within restricted elements of volume ("pores") and the boundary surfaces of the pore, upon the reactivity and physical state of the confined molecules, is explored by means of simple models. A confined molecule is represented by a rectangular parallelopiped having one of six orientations aligned with the cartesian coordinate axes, and the confining volume element is represented by a pair of parallel surfaces (planar pore), a tube of square cross section (square pore), or a cubical box (cubical pore). Weak interactions are modeled by square-well potentials having a defined range and well depth. Partition coefficients for distribution of molecules between the bulk and confined phase are calculated using an extension of the statistical-thermodynamic theory of Giddings et al. (1968). It is calculated that surface attraction with a potential of only a few kcal/mol monomer may result in large increases in the extent of self- or heteroassociation of confined molecules (as much as several orders of magnitude in favorable cases) linked to adsorption of the oligomeric species onto boundary surfaces. Calculations are also presented suggesting that surface attraction can lead to deformation of the native structure of adsorbed macromolecules. It is suggested that these findings are relevant to an understanding of the structure of eukaryotic cytoplasm.