Single-Molecule Visualization of Diffusion at the Solution−Crystal Interface

We directly observed the diffusion of individual fluorescence-labeled protein molecules at an interface between a solution and a protein crystal using a single-molecule visualization technique, by which we can observe individual molecules with fluorescence labels. We simulated the diffusion behavior of mobile solute molecules at a solution−crystal interface, using fluorescence-labeled lysozyme and tetragonal lysozyme crystals. We found that the molecules diffused along the crystal surface 4–5 orders of magnitude slower (diffusion coefficient (6.9 ± 1.2) × 10−15 m2/s) than in a bulk solution, indicating that the molecules strongly interacted with molecules that were aligned at the crystal surface. This result denotes that slow two-dimensional diffusion inside a range of interactions from the crystal surface is a general picture of surface diffusion at an interface between an aqueous solution and a hydrophilic crystal surface. In addition, the number density of the molecules at the interface was 3 orders of...