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 208

0.2-0.6 T. These fields were provided with an electromagnet. These facts
unambiguously showed that diamagnetic orientation is quite a general
phenomenon.

Sedimenting crystals

We have mentioned above our studies on the emergence of the crystals
within the solution, not at the surface of the vessel material. Another line
of thought was provided from atomic force microscope (AFM) observa-
tions by the groups of McPherson and DeYoreo (McPherson, 1999). They
noticed that when they were observing the surface of various protein and
virus crystals with an AFM instrument, one of the surface probing micro-
scopes, there are occasions where suddenly a microcrystalline mass
appears on the surface and grows as a part of the existing, larger crystal.
This kind of phenomenon was found to be rare for the usual inorganic and
semiconductor crystals, whereas for protein and virus crystals it was not
uncommon. For this reason, the investigators called this phenomenon
“three-dimensional nucleation” and considered it as a more or less general
mechanism of crystal growth for this group of substances. By comparison,
other crystal growth mechanisms usually considered are: screw disloca-
tion, two-dimensional nucleation, and kinetic roughening. Three-dimen-
sional nucleation differs from these more common mechanisms in that an
already existing crystal, though much smaller than a bulk crystal, seems
to merge into the crystal being observed. The authors do not necessarily
decide where the merging microcrystals originate from, but it is likely that
they nucleated independently within the supersaturated solution and were
transferred in the solution either by sedimentation or by convection flow.
It must be recalled that protein and virus crystals need in general a much
higher degree of supersaturation for spontaneous nucleation than smaller
molecules, and for this reason there can be differences in nucleation
behavior. Three-dimensional nucleation was independently observed via
AFM by Astier et al . (2001) on porcine -amylase crystals.

The above mechanism, merging microcrystals that nucleate sepa-
rately and participate in growth, will certainly increases the chances of

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