Observing subdiffusion in artificial cytoplasm: polyethylene glycol as a crowding agent*

Biological cells are crowded viscoelastic environments and cellular cytoplasm's spatial organization plays a massive role in biological systems, yet their underlying physical-chemical properties are not fully understood. The development of an artificial cellular cytoplasm is desired to contribute to the understanding of biological intracellular environments. We studied the motion of inert tracer particles in monodisperse and polydisperse aqueous solutions with varying concentrations and chain lengths of the crowding agent polyethylene glycol (PEG) to determine the circumstances that allow for a subdiffusive artificial intracellular environment. PEG is a chain molecule available in a variety of lengths specified by its molecular mass. We find that subdiffusion is observed consistently in aqueous solutions of approximately monodisperse longer PEG (20000 g/mol) at 25 mg/mL concentration as well as with lengths as low as 200 g/mol with concentrations of 40% by volume. Generally, subdiffusivity increases with the medium's PEG concentration or length. The dependence of subdiffusivity on higher concentration and longer polymer chain lengths suggests that PEG produces a caging effect within artificial environments. These findings support the use of PEG as a tunable crowding agent for an artificial cytoplasm and have exciting implications for the creation of an artificial cell.