CROSSOVER OF LYSOZYME AGGREGATES BETWEEN SUPER- AND UNDER- SATURATED STATES IN AQUEOUS SOLUTION. N.Niimura, M.Ataka*, I.Tanaka, Y.Minezaki and Y.Karasawa, Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-11, Japan, *National Institute of Bioscience and Human-Technology, 1-1, Higashi, Tsukuba, 305, Japan

The structure of lysozyme aggregates at the wide concentration range of lysozyme and NaCl in the supersaturated state has been studied by small angle x-ray scattering (SAXS). Based on the interest in the nucleation stage of protein crystallization, we have already reported the aggregate formation in undersaturated aqueous lysozyme solutions (N.Niimura, Y.Minezaki, M.Ataka & T.Katsura: J. Crystal Growth 137(1994) 671, Y.Minezaki, N.Niimura, M.Ataka & T.Katsura, Biophys. Chem. 58(1996)355) and two types (Type I & II) of aggregates in supersaturated states by small angle neutron scattering measurement. (N.Niimura, Y.Minezaki, M.Ataka and T.Katsura: J. Cryst. Growth 154 (1995) 136). The SAXS measurements were performed by using a synchrotron-radiation x-ray scattering instrument. The radius of gyration Rg of Type II aggregates was obtained by carrying out Guinier plot in the Q range of 0.03<Q< 0.08. (V-V0) vs. natural logarithm of supersaturation ratio, where V and V0 are the aggregates volume in the supersaturated and saturated states, respectively, are plotted. The volume increment from the one on the solubility curve at different NaCl concentration is described by the one unique straight line. On the other hand, the data in the undersaturated states are off the line. This means that the nature of the aggregates in the supersaturated states could be unified in reference to the supersaturation degree and the crossover between super and under saturated states was clearly demonstrated by the treatment.