• excluded volume;
  • steric repulsion;
  • weak binding;
  • equivalent hard particle model


The term “macromolecular crowding” denotes the combined effects of high volume fractions of nominally unrelated macromolecules upon the equilibrium and transport properties of all macrosolutes, dilute as well as concentrated, in the crowded medium. We present a formal partitioning of the total crowding effect into contributions from steric exclusion (excluded volume) and weak, nonspecific attractive interactions between a concentrated “crowding agent” and reactant and product species present at trace concentration. A numerical example of the combined effect of both steric and chemical interactions between crowder and tracer upon the reversible dimerization of tracer is presented, based upon reasonable estimates of the magnitude of both repulsive and attractive interactions between tracer and crowder species. © 2012Wiley Periodicals, Inc. Biopolymers 99: 239–244, 2013.