A general method to study equilibrium partitioning of macromolecules

The distribution of macromolecules between a confined microscopic solution and a macroscopic bulk solution plays an important role in understanding separation processes such as Size Exclusion Chromatography (SEC). In this study, we have developed an efficient computational algorithm for obtaining the equilibrium partition coefficient (pore-to-bulk concentration ratio) and the concentration profile inside the confining geometry. The algorithm involves two steps. First, certain characteristic structure properties of the studied macromolecule are obtained by sampling its configuration space, and second those data are used for the computation of partition coefficient and concentration profile for any confinement size. Our algorithm is versatile to the model and type of the macromolecule studied, and is capable of handling three types of confining geometries (slit, rectangular channel and rectangular box). Results for linear random walk chain, linear self avoiding walk chain, and nonlinear random walk chain of various architectures (star, pom-pom, comb, and centipede etc.) will be presented. From these results, a characteristic molecular dimension can be deduced, which is relevant to SEC separation. The use of this dimension rather than Rg (radius of gyration) or Rh (hydrodynamic radius) gives a better universality in the plot of the partition coefficient as a function of the chain dimension relative to the pore size.