Water Dynamics in Cytoplasm-Like Crowded Environment Correlates with the Conformational Transition of the Macromolecular Crowder

Polyethylene glycol (PEG) is a unique polymer material with enormous applicability in many industrial and scientific fields. Here, its use as macromolecular crowder to mimic the cellular environment in vitro is the focus of the present study. We show that femtosecond mid-IR pump–probe spectroscopy using three different IR probes, HDO, HN3, and azido-derivatized crowder, provides complete and stereoscopic information on water structure and dynamics in the cytoplasm-like macromolecular crowding environment. Our experimental results suggest two distinct subpopulations of water molecules: those that interact with other water molecules and those that are part of a hydration shell of crowder on its surface. Interestingly, water dynamics even in highly crowded environment remains bulk-like in spite of significant perturbation to the tetrahedral H-bonding network of water molecules. That is possible because of the formation of water aggregates (pools) even in water-deficient PEGDME-water solutions. In such a crow...