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Figure 9.
Figure 9. Comparing model A with model B from initial
molecular replacement. (a) Model A. Presumably, the carbo-
nyl group of the proline residue in the X position should
make a hydrogen bonding contact with the N atom of the
glycine residue in the neighboring chain. Model A displays
``normal'', N(Gly) to O(Pro)X hydrogen bonds in terms of
length and orientation (broken single line). The distance to
the C
a
of the third chain is longer and the orientation is not
as appropriate for a hydrogen bond (broken double line). (b)
Model B. Interchain hydrogen bonding geometry is per-
turbed. In model B, the oxygen atom appears to be some-
what pointed toward the C
a
(broken double line), rather
than toward the N(Gly), and this length is reasonable for a
hydrogen bond while the distance to the N(Gly) becomes
longer (broken single line). (c) The high-symmetry sequence
and quasi-infinite helical nature of Pro-Pro-Gly implies that
an end-to-end rotation of the peptide chain would give ana-
logous models with only the N and C
a
positions transposed.
Parts (a) and (b) of the Figure were generated with MOL-
SCRIPT (Kraulis, 1991).
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