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Figure 5.
FIGURE 5. Two proposed forms of the first tetrahedral
intermediate (A and B) interchangeable via N-inversion and/or
rotation about the C-N bond. In A, the P1' nitrogen lone pair
projects away from His-57, and in B it projects toward it. In
intermediate B, R and R' refer to either hydrogen or the P1'
residue, depending on whether N-inversion has occurred. The
amide nitrogen of Ser-195, which forms part of the oxyanion
hole, is not shown for clarity; from the viewpoint shown, it
lies behind the plane of the picture. Note that steric
constraints mean that the conformation in which the nitrogen
lone pair is exactly coplanar with the C-O(Ser-195) bond is
unlikely.
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