The Use of a Magnetic Field and Magnetic Force as a Means to Improve the Quality of Protein Crystals - Molecules: Nucleation, Aggregation and Crystallization - page 212

determining more accurate structure. Similar improvement in the quality
of the orthorhombic form of crystals of hen egg-white lysozyme in a
15 T homogeneous magnetic field was confirmed independently by
Yin et al . (2004). However, it is not entirely clear why the orthorhombic
form was used for study since among the structural information
deposited in Protein Data Bank a different crystal form, the tetragonal
one, gives better resolution. It may be that the quality enhancement was
achieved for a low-resolution crystal form, that the choice was a mere
accident, or that the results obtained for the tetragonal form were not
reported for some reason.
Yin et al . (2003b) showed more uniform morphology in a horizontal
as well as a vertical magnetic field by growing tetragonal crystals of hen
egg-white lysozyme with a crystallizing agent of NiCl 2 containing a para-
magnetic cation. With this crystallizing agent, the crystal symmetry
becomes orthorhombic through a slight distortion between the a - and
b -axes. Also, a Ni 2+ cation was shown to be incorporated into the active
cleft by X-ray crystallography (Yin et al . (2003a)). Yin et al . (2003a)
found a difference in morphology when NiCl 2 and the magnetic field were
used in combination. This is an example of the possibility that magnetic
field application may be used to change or regulate crystalline morphol-
ogy, although in this case it was used in combination with a less common
crystallizing agent.
Lübbert et al . (2004) used X-ray rocking-curve measurements and
statistical data analysis to evaluate tetragonal hen egg-white lysozyme
crystals grown in the absence and presence of a homogeneous magnetic
field of 2.4 T. Their results showed that the magnetic field (as well as gel)
does not affect the growth of the main volume of the crystals but can
reduce or partially prevent the formation or growth of separated and mis-
oriented small outlier crystallites. This is consistent with the potential of
a magnetic field to more easily orient sedimenting crystallites with
respect to the main crystal, as we discussed above. However, in the study
by Lübbert et al . the tetragonal crystals were already of high quality
without any magnetic field, and the improvement was, at least with the
2.4 T field used, small.