Various factors influencing affinity partitioning in aqueous polymer two-phase systems are studied by model calculations using a self-consistent mean-field lattice theory. The latter is an extension of the theory of Scheutjens and Fleer, adapted to take affinity ligand binding into account. The dependence of partition coefficients on ligand concentration is studied for different binding strengths, polymer lengths, and polymer-protein interactions, and the effects of the ligand being attached at different positions of the polymer chain are investigated. The mechanism of the affinity partitioning is discussed. In particular, at saturation the enhanced partitioning by the presence of the ligand substituted polymers should be regarded as an expulsion from the minority phase rather than an attraction to the majority phase. Moreover, the results are put into relation with the multiple-equilibriascheme that has frequently been used to analyse affinity partition data, and comparisons with experimental findings are made. Some important principles for application of affinity partitioning concluded from the modelling results are: i) A higher selectivity can be expected if a polymer which has in general repulsive interactions with proteins is chosen as ligand-carrying polymer, ii) a higher Delta(lnK)(max) is expected for a longer ligand-carrying polymer as compared to a shorter one. With increased polymer length higher ligand concentration is however needed to reach the plateau value. iii) the ligand bound to the end of the polymer gives a higher Delta(lnK)(max) than when bound to the middle of the chain. iv) having both polymer ends carrying ligand should actually lead to a decrease in the Delta(lnK)(max). However, at quite low ligand concentrations the doubly substituted ligand-carrier should be more effective.
