ALA carbonyl flip in alpha helix protein

 Dear all,
Working on MD simulations (AMBER6, parm96.dat) of a small alpha helix protein (gramicidin A) i've noticed that some ALA carbonyls of the backbone flip over (~180 degrees) along the helix. I am wondering if it can take place in a real life or it's a drawback of the force field/parametrisation/methodology. I will appreciate any comments/references.
 Artem Mamonov
 Graduate Student
 Department Of Chemistry
 University Of Pittsburgh
 email: artem[ AT ]