Slow molecular dynamics close to crystal surfaces during crystallization of a protein lysozyme studied by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) was applied to the crystallization processes of egg-white lysozyme. Utilizing FCS's high spatial resolution of about the laser wavelength used, the molecular dynamics close to crystal surfaces was investigated for both tetragonal single crystals and needlelike spherulites. When the FCS measurement was done at the point closer than 1 μm to the surface of a tetragonal single crystal, the relaxation time became several times longer than that in bulk solution, but the fluorescence intensity (thus concentration) was similar to that observed in bulk solution. On the other hand, the peculiar slow dynamics (a few orders of magnitude slower than that in bulk solution) of concentrated liquid states of the lysozyme molecules was observed in needlelike spherulites. We suggested that these observations could be explained by the formation of softly connected aggregates accumulating around the needlelike crystals, which could cause the instability of the crystal growth and thus the formation of spherulites. These aggregates gradually disappeared as the crystallization further proceeded. After the disappearance of the aggregates, the spherulites started to mature.