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plane. In publications dealing with such crystals, the
xy
-plane refers to the
layer whereas the
z
-direction is the perpendicular direction, where
x
,
y
and
z
are the directions of the “lab” coordinate system. The
z
-direction is par-
allel to the direction of the electron beam. Like for 3D crystals the small-
est repeating unit of the 2D counterpart is the unit cell defined by the
a
,
b
and
c
unit cell vectors (Fig. 2). In other words,
a
,
b
and
c
define the crys-
tal coordinate system. For untilted specimens, the
z
- and
c
-directions are
parallel so that the
xy
- and
ab
-planes coincide. The length of
c
corre-
sponds to the thickness of the 2D crystal.
(A)
Fig. 2.
Properties of a melibiose permease 2D crystal. (A) The collapsed tube shows two
lattices with a relative in-plane rotational difference close to 90
(calculated diffraction
pattern in the inset). The depicted in-plane unit cell vectors
a
and
b
show the directions rel-
ative to the crystal. Scale bars: 1/75 Å
−1
in the inset and 100 nm. (B) The projection map
shows two unit cells and the packing of four protein molecules in one unit cell. These are
in the p222
1
two-sided plane group related by a two-fold rotation perpendicular to the
plane of the membrane, a two-fold rotation parallel to the short
b
-axis and a two-fold screw
axis along the
a
-axis direction, respectively.
°
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