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 207

focus of the microscope is gradually raised to the inside of the solution,
nothing is observed, except for some floating on top at the solution-air
interface. There are two possibilities — either the crystals nucleated and
started to grow at the vessel bottom surface, or they nucleated within the
solution and then sedimented to the bottom later after reaching a certain size
(usually, the density of crystals is greater than the density of surrounding
solution). Distinguishing between the two cases is not easy, since seizing
nucleation on site is difficult unless we know a priori where and when the
nucleation starts. Therefore, we considered using the magnetic orientation
phenomenon to solve this problem, which may otherwise be difficult to elu-
cidate. The crystals at the bottom of the vessel do not respond to a magnetic
field of 1 T or so and show orientation, since they adhere to the glass ves-
sel. We carried out the crystal growth experiments in a magnet, applying the
magnetic field all the time during the crystal growth. When the crystals
were observed after they grew, almost all of them were at the vessel bottom,
and nearly perfect orientation took place. By contrast, no clear orientation
was observed if the magnetic field was not applied. On the other hand,
when the magnetic field of 1.6 T (provided by an electromagnet) was
applied some time (a few to ten hours) after supersaturating the solution,
but before the crystals reached their final size, a considerable proportion of
the crystals (particularly the larger ones that were considered to start grow-
ing earlier than the smaller ones) showed random orientation, in strong
contrast with the situation where the field was present from the very begin-
ning. The difference was attributed to the fact that only floating crystals
within the solution which can freely rotate can respond to the magnetic
field and show orientation, and thus this was taken as evidence that the
crystals start growing within the solution. Also, the same observation indi-
cates that when the crystals sediment, they are already oriented in one
direction in the presence of a magnetic field of the order of 1 T. If the field
strength is larger, then the magnetic orientation will be more perfect.
We next proceeded to show that other crystalline forms of hen egg-
white lysozyme, as well as crystals of other proteins (without metal ions)
can exhibit magnetic orientation (Sakurazawa et al ., 1999). The field
strength necessary for the orientation to take place was found to be