Adsorption and desorption kinetics of water in lysozyme crystal investigated by confocal Raman spectroscopy

Adsorption and desorption are critical to crystal engineering for the protein crystal applications. In this paper, we present a new method to study the adsorption and desorption kinetics of waters in lysozyme crystals. H ₂O and D₂O were used as ideal indicators for the purpose. The lysozyme crystals were prepared in a complete D₂O environment to ensure that all the water molecules in crystal were D₂O. The H ₂O in the gas phase directly exchanges with the D₂O in the crystal by exposing the crystal to an ambiance with saturated water vapor. Using in situ confocal Raman microscopy, H₂O adsorption and D ₂O desorption in the lysozyme crystals were found to follow first-order kinetics. The rate constants of H₂O adsorption and D ₂O desorption were obtained to be equal to each other. The kinetic rates were found to linearly depend on the surface area-to-volume ratio of the crystals.