Biology Reference
In-Depth Information
can only be detected directly in TEM diffraction mode if the crystals have
a rather isotropic extension of about 1
m (Fig. 3). For images it may be
sufficient to have areas which include a few thousand unit cells in micro-
graphs recorded at a magnification of about 50 000 times.
µ
Order
The ultimate resolution that can be obtained from 2D crystals will depend
not only on the size of the crystals but also on the order. Deviation from
perfect crystallinity will show up as blurred diffraction spots. In images,
problems of continuous limited in-plane disorder can be reduced by
unbending (see below). An approach to any kind of disorder is to treat one
or a few unit-cells as separate entities and apply single-particle processing
(Koeck
et al
., 2007).
Flatness
The requirement to work with large crystalline areas puts a critical
demand on the flatness of the layer, which to some extent may depend on
the support film. Deviation from perfect planarity will cause most severe
distortions for data collected at high tilt angles. Successful attempts have
been reported for improving flatness and other mechanical and conduct-
ing properties of the specimen support (Gyobu
et al
., 2004; Rhinow,
Kühlbrandt, 2007).
Stacking, multilayers
Ideally, the 2D crystals should consist of a single layer of the protein. For
membrane proteins the arrangement may often be a double layer. This is
for instance the case when the crystals grow as large tubes which flatten
on the support film (Figs. 1 and 2). The two layers may be in perfect reg-
ister with a relative in-plane rotation. In such cases the two layers may be
separated during processing since two sets of diffraction spots appear.
However, two layers may also be crystallographically arranged relative to
Search WWH ::
Custom Search