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) x 10(-15) m(2)/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 magnitude higher than that estimated from a concentration of a bulk solution, demonstrating that interaction from the crystal surface drastically condensed the molecules at the interface. The average residence time and average diffusion length of the molecules at the interface were 0.47 s and 0.11 mu m, respectively.