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Quantitative Study of Crystallization Kinetics of Hen Egg-White Lysozyme Using Magnetic Orientation

N. I. Wakayama
[J. Cryst. Growth, Vol. 191, pp. 199-205, 1998]

Magnetic stabilization energy of a lysozyme crystal is a function of both magnetic strength and the number of molecules in a crystal. We, therefore, used magnetic orientation to clarify the process under normal gravity where crystals grow in solution and then sediment to the bottom of the vessel, where they continue to grow. When a magnetic field of 1.6 T was applied immediately after the solution preparation (t = 0) and was continued for various duration times, photomicrographs taken after completion of the crystallization process showed that magnetically oriented crystals sediment before the field was switched off (t = t). The ratio of the number of magnetically oriented crystals to the total number of crystals could be approximated by Ratio = 1 | exp(|kt), where k is a rate constant of the crystal growth and sedimentation. When the supersaturation ratio beta was 5.3, 6.7, and 8.0, the approximated value of k was 0.016, 0.15, and 0.48/h, respectively. k significantly increases with increasing beta. This equation explains quantitatively the transient behavior of the growth and sedimentation of crystals. We also analyzed Ratio by growing crystals under various magnetic-field strengths (0-1.6 T) and found that most crystals sediment to the bottom when their size is 2-6 mm. Numerical simulation also showed that the process of sedimentation is determined by the size of crystals. In conclusion, a rate constant k that we determined in this study characterizes the process where crystals grow to 2-6 mm and then sediment to the bottom of the vessel, and that crystallization growth strongly depends on the supersaturation ratio beta.

Table 1 Rate Constant of Crystal Growth and Sedimentation (k), Half Sedimentation Time (tau1/2), and Supersaturation Ratio (beta) As a Function of the Initial Concentration of Lysozyme (c0) for NaCl Concentration of 3.0%, pH 4.6, and 18.

                                               
  c0       beta = c0/cs       k          tau1/2
 (%)                     (h-1)         (h)
                                               
  3.2         5.3        0.016        42.5
  4.0         6.7        0.15          4.5
  4.8         8.0        0.48          1.5

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