Hydration Potential of Lysozyme: Protein Dehydration Using a Single Microparticle Technique

For biological molecules in aqueous solution, the hydration pressure as a function of distance from the molecular surface represents a very short-range repulsive pressure that limits atom-atom contact, opposing the attractive van der Waals pressure. Whereas the separation distance for molecules that easily arrange into ordered arrays (e.g., lipids, DNA, collagen fibers) can be determined from x-ray diffraction, many globular proteins are not as easily structured. Using a new micropipette technique, spherical, glassified protein microbeads can be made that allow determination of protein hydration as a function of the water activity (aw) in a surrounding medium (decanol). By adjusting aw of the dehydration medium, the final protein concentration of the solid microbead is controlled, and ranges from 700 to 1150 mg/mL. By controlling aw (and thus the osmotic pressure) around lysozyme, the repulsive pressure was determined as a function of distance between each globular, ellipsoid protein. For separation distances, d, between 2.5 and 9 Å, the repulsive decay length was 1.7 Å and the pressure extrapolated to d = 0 was 2.2 × 108 N/m2, indicating that the hydration pressure for lysozyme is similar to other biological interfaces such as phospholipid bilayers.