Association kinetics and equilibrium in solutions of cross-associating chains that contain inactive spacers

Abstract

Thermodynamic and kinetic characteristics of chemical reactions in soft matter are affected by volume exclusion caused by an inert component (crowder). In previous molecular dynamics simulations we examined the crowding effect on the association equilibrium in solutions of cross-associating macromolecules that contain, as part of the chains, inactive monomers acting as crowders. Here, we extend our studies to association kinetics. For a number of mixtures where one or both chain species contain inert spacers, a simple mean field approximation describes quite well the composition dependence of the association equilibrium in a wide concentration range. Two main factors determine this dependence: the local distribution of the stickers (the relative excess of one kind of stickers in vicinity of the other) and the local steric effects. The kinetic characteristics of dissociation/association show somewhat similar pattern: the bonded pairs dissociate and recombine slower in solutions with higher content of inactive monomer units. However, the dissociation and recombination dynamics also depends on the reactants’ diffusion and its interplay with effects of chain connectivity and chemical structure. Particularly, when one of the associating components does not carry inert spacers, the recombination accelerates because it makes easier for the sticker from the other chain species to find a new partner for bond formation near its previous partner. These local effects and variations in the subdiffusive behavior of chain molecules are important to take into account for advancing the theory of reversible association in macromolecular systems.