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Figure 5.
Fig. 5. Electrostatics of packing. A: The antiparallel packing arrangement
of three symmetry-related four-helix bundles in the ortho-1 crystal form is
shown. The bundles are represented as a ribbon diagram and are shown
superimposed with semitranslucent isocontours of the electrostatic poten-
tial. Red surface represents negative potential ~contoured at 23 mV! and
blue surface represents positive potential ~contoured at 4 mV!. For illus-
trative purposes, the molecules are displayed somewhat apart from their
actual packing arrangement. It is apparent that in this arrangement favor-
able electrostatic interactions take place such that positive and negative
electrostatic potentials are juxtaposed. This figure was generated with MOL-
SCRIPT ~Kraulis, 1991!, SPOCK ~Christopher, 1997!, and RASTER3D
~Merritt & Bacon, 1997!. B: Relative orientation of packed molecules in
the three crystal forms, superimposed on one molecule of the pseudodimer.
Perspective is down the principal axis of the four-helix bundle. The model
from the ortho-1 form is shown in red, the trigonal form in magenta, and
the ortho-2 form in yellow. While maintaining the commonality of the
antiparallel arrangement, packing is not identical in any two crystal forms.
The pseudodimers of the ortho-2 and ortho-1 forms differ by a small
translational shift, while the pseudodimer of the trigonal crystal form is
related to that of ortho-1 by an additional, nearly 908 rotation. This figure
was generated with MIDAS ~Ferrin et al., 1988!. C: Schematic of the
interactions of the electrostatic lobes in different packing arrangements.
The arrow represents the principal axis of the helix bundle, the blue oval
represents the positive lobe of electrostatic potential, and the red oval
represents the negative lobe of electrostatic potential. The upper sections of
the sketch represent the packing plane, the lower sections represent a
cross-section thereof ~left panel, ortho forms; right panel, trigonal form!. In
both packing arrangements, positive and negative electrostatic potential are
juxtaposed.
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