To identi
fy basic local backbone motions in un
folded chains, simulations are per
formed
for avariety o
f peptide systems using three popular
force
fields and
for implicit and explicit solvent models.A dominant "cranksha
ft-like" motion is
found that involves only a localized oscillation o
f the plane o
fthe peptide group. This motion results in a strong anticorrelated motion o
f the
![](/images/gi<font color=)
fchars/Phi.gi
f" BORDER=0 > angle o
f the
ith residue(
![](/images/gi<font color=)
fchars/Phi.gi
f" BORDER=0 >
i) and the
![](/images/gi<font color=)
fchars/Psi.gi
f" BORDER=0 > angle o
f the residue
i - 1 (
![](/images/gi<font color=)
fchars/Psi.gi
f" BORDER=0 >
i-1) on the 0.1 ps time scale. Only a slight correlation is
found between the motions o
f the two backbone dihedral angles o
f the same residue. Aside
from thespecial cases o
f glycine and proline, no correlations are
found between backbone dihedral angles that areseparated by more than one torsion angle. These short time, correlated motions are
found both in equilibrium
fluctuations and during the transit process between Ramachandran basins, e.g.,
from the
![](/images/gi<font color=)
fchars/beta2.gi
f" BORDER=0 ALIGN="middle"> to the
![](/images/gi<font color=)
fchars/alpha.gi
f" BORDER=0> region.A residue's complete transit
from one Ramachandran basin to another, however, occurs in a mannerindependent o
f its neighbors' con
formational transitions. These properties appear to be intrinsic becausethey are robust across di
fferent
force
fields, solvent models, nonbonded interaction routines, and mostamino acids.