ß-Sheets are characterized
by the orientation of individual strands with respect to each
other. In a parallel ß-sheet, both strands run in the
same direction from N- to C-terminus. In anti-parallel ß-sheets,
one strand runs N- to C-terminal while its partner strand
runs in the opposite direction. ß-Sheets
exhibit different hydrogen-bonding patterns depending on the
relative orientation of the strands. Both types of ß-sheets
exhibit a characteristic right-handed twist.
Beta-Hairpin
Turn Development
The
minimal unit of antiparallel beta sheet is the beta hairpin,
composed of a tight turn connecting two beta strands. The development
of appropriate turn promoting segments has been the key to designing
stable beta hairpin model systems.
Nature provide insight into the turn segments
necessary for antiparallel sheet formation. By analogy, turn
segments incorporating strand reversing elements (a diamine
and a diacid equivalent) where constructed to generate parallel
beta hairpins. The incorporation of an unnatural D-proline
in the turn strongly promotes hairpin formation, simply changing
the chirality of this residue abolishes turn hairpin formation
creating a valuable zero percent folded reference compound.
The D vs L switching
also works in the parallel turns.
Anti-parallel
turn units incorporating D- or L-proline
Parallel
turn segments incorporating D-proline for connecting C (with
diamine) and N (with urea) termini.
Sheet
Co-operativity
We have
studied the cooperativity of folding in hairpin and three stranded
sheet model systems. These types of studies are impossible in
larger protein systems because the overall folding cooperativity
completely masks the contributions of individual secondary structure
elements.
Co-operativity
of folding in a three stranded sheet.
Co-operativity
in the strand direction of a beta-hairpin.
