b-sheet vs. a-helix folding
The time scale for formation of elements of secondary
structure is a fundamental problem in the study of protein
folding. Several studies have shown that a-helices fold
on a time scale of 200 ns. Moreover, an a-helix model
system appears to follow a two step kinetic model.
There are fewer studies of b-sheet folding due to the lack
of solubility of many b -sheet peptides. The folding appears
much slower. This is likely due to the requirement for
chain diffusion over greater length scales. The nucleation
of an a-helix can occur any place along the length of the
chain that a hydrogen bond forms between residue i and i+4.
b-sheet folding requires proper registry at a unique location.