Multisite interactions in the lattice gas model: shapes of binding isotherm of macromolecules

摘要

The effect of interactions between binding sites of a lattice that are not pairwise additive (multisite interactions) on the shape of the ligand binding isotherm is investigated theoretically. The mean-field approximation treats the multisite interaction as an effective two-site interaction, the coupling constant becoming a function of the fraction of occupied sites. In general, the binding curve will show multiple vertical jumps. The sufficient condition for the existence of a critical temperature below which a vertical jump occurs in the binding curve is the presence of a cooperative two-site interaction. The functional dependence of the effective J on the saturation, and the value of the temperature determines the number and location of the vertical jumps. The existence of more than one point of inflection is shown to be unambiguous proof of the existence of multisite coupling. The occurrence of a point of inflection is dependent on temperature and it disappears above a critical temperature. The relationship of these results with experimental results reported in the literature on the binding of alkyl sulphates and sulphonates to the protein bovine serum albumin and alkyl ammonium chlorides on biotite surface which show the phenomenon of temperature-dependent vertical jumps in the binding isotherm is pointed out.