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were poorly satisfied in the starting structures, the potential-energy term corresponding to the imposed
NOE data (E NOE) was applied gradually by increasing the scale factor in a nonlinear fashion such that it
was 0.0 after 0.2 ps, 0.1 after 1.2 ps, and 1.0 after the full 3.0 ps. Following 15 ps of equilibration, 7 ps
of incremental production dynamics was completed. During this stage the NOE scale factor was raised
from 1.0 to 4.5. By slowly raising the force constants for the NOE restraints as the target distances
became better satisfied, no dramatic increase in temperature was observed. Finally the NOE scale factor
was set to 5.0, 10 ps of production dynamics were completed, and an average structure was extracted
from the last 5 ps of the coordinate trajectory.
Analysis of the two average structures obtained from the two unique starting points demonstrated
convergence to a similar conformational species. In each, the sum of the NOE restraint energy was less
than 4.7 kcal/mol and the RMS deviation from the target distances was below 0.25 Å. Similar results
were obtained for each simulation when they were repeated without the electrostatic term being included
in the total potential-energy function. This important data lends credence to the hypothesis that the final
structures are derived from the NOE restraints and not by poorly represented electrostatic interactions.
The average dynamic structures are characterized as having all trans peptide bonds and hydrophobic side
chain groups oriented outward into solution, perhaps ready to interact with the receptor. There is a
possible 1–3 hydrogen-bonded γ turn bridging Phe 5, although it is not explicitly defined by the NOE
data set. If present, then the preferred overall geometry would be U shaped and, if absent, an S shaped
geometry is possible based on coincident conformational analyses. According to the dihedral angle
values for Pro 7 and Phe 8, a type-II β turn extending from Ser 6 to Arg 9 also exists. A variety of reports
have subsequently appeared that are in agreement with the conformation we described in this work.
A similar C-terminal turn structure was observed in an analogous NMR study of a first-generation kinin
antagonist, NPC 567 (DArg 0-Arg 1-Pro 2-Hyp 3-Gly 4-Phe 5-Ser 6-DPhe 7-Phe 8-Arg 9), although the type of
turn was not the same. Our initial speculation was that this slight structural difference might partially
account for the functional differences of bradykinin and NPC 567. These solution conformations, one of
an agonist and the other of an antagonist, were subsequently used to focus the design and synthesis of
conformationally constrained peptide analogues of NPC 567.
B. Conformationally Constrained Bradykinin Antagonist Peptides
The ligand-based approach of conformationally constrained peptides has been widely used. The process
involves the incorporation of conformational constraints into known peptides, either agonist or
antagonist, which enforce a
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