Structural Properties
Traditionally, the relative rigid structure of PPII has been used as a "molecular ruler" in structural biology, e.g., to calibrate FRET efficiency measurements. However recent experimental and theoretical studies have called into question this picture of polyproline peptide as a "rigid rod". Interconversions between the PPII and PPI helix forms of poly-proline are slow, due to the high activation energy of X-Pro cis-trans isomerization (Ea ≈ 20 kcal/mol); however, this interconversion may be catalyzed by specific isomerases known as prolyl isomerases or PPIases.
Read more about this topic: Polyproline Helix
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