The bottle neck of a three-dimensional structural analysis of protein molecules is the crystallization of large (>0.1μm) and suitable high quality crystals. To overcome this difficulty, the growth process of protein crystals, such as the mass transfer of the molecules and the growth kinetics of the crystals, must be quantitatively analyzed. Two-beam interferometry is a powerful technique for an in situ observation of the concentration distribution around a growing crystal. By using interferometer of Michelson type, we have been successfully observed the concentration distribution around a hen egg-white lysozyme crystal. Here, we outline the principle of two-beam interferometry, and explain how the growth mode and the diffusion coefficient of lysozyme molecules can be obtained. We also show a rapid solubility measurement of protein crystals by using the same interferometric technique, which can determine the solubility of lysozyme crystals at one point in a phase diagram within 1-2 hrs. This technique is about ten times quicker than other ordinary methods so far used for a solubility measurement.
