Protein Folding

[Alan J. Robinson]

The Ramachandran plot shows the allowable values for the phi and psi 
angles at each alpha carbon in a protein chain.

What puzzles me is that there are 3 distinct, unconnected areas on the 
plot, for right and left handed helices, and for beta sheets.  All 
other regions are claimed to be sterically impossible.  But this would 
imply that smooth variation of phi and psi during folding may be 
impossible, because the random coil values would all presumably be 
somewhere in the beta sheet region, but some residues would need to 
move to the right and left hand alpha helix regions, moving through 
disallowed "no man's land" values.

Obviously there are assumptions built into this whole model which 
are invalid, but I'm not sure where the problem lies.  Any 
suggestions, please!  (The Ramachandran plot would make more sense if 
the allowed regions were merely preferred regions - energetically 
favorable.)

Alan J. Robinson
robin073 at tc.umn.edu