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We have studied the dynamic properties of human (h) and salmon (s) calcitonin
(CT) in solution. For both hormones, distance geometry in torsion-angle space
has been used to generate three-dimensional structures consistent with NMR data
obtained in sodium dodecyl sulfate micelles. For sCT and hCT we used,
respectively, 356 and 275 interproton distances together with hydrogen-bonds as
restraints. To better characterize their flexibility and dynamic properties two
fully unrestrained 1100-ps molecular dynamics (MD) simulations in methanol were
performed on the lowest-energy structures of both hormones. Statistical analyses
of average geometric parameters and of their fluctuations performed in the last
1000 ps of the MD run show typical helical values for residues 9-19 of sCT
during the whole trajectory. For hCT a shorter helix was observed involving
residues 13-21, with a constant helical region in the range 13-19. Angular order
parameters S(phi) and S(psi) indicate that hCT exhibits a higher flexibility,
distributed along the whole chain, including the helix, while the only flexible
amino acid residues in sCT connect three well-defined domains. Finally, our
study shows that simulated annealing in torsion-angle space can efficiently be
extended to NMR-based three-dimensional structure calculations of helical
polypeptides. Furthermore, provided that a sufficient number of NMR restraints
describes the system, the method allows the detection of equilibria in solution.
This identification occurs through the generation of 'spurious' high-energy
structures, which, for right-handed alpha-helices, are likely to be represented
by left-handed alpha-helices.
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