Stereoelectronic effects on polyproline conformation

Publisher Website
Google Scholar
Abstract
The polyproline type II (PPII) helix is a prevalent conformation in both folded and unfolded proteins, and is known to play important roles in a wide variety of biological processes. Polyproline itself can also form a type I (PPI) helix, which has a disparate conformation. Here, we use derivatives of polyproline, (Pro)10, (Hyp)10, (Flp)10, and (flp)10, where Hyp is (2S,4R)‐4‐hydroxyproline, Flp is (2S,4R)‐4‐fluoroproline, and flp is (2S,4S)‐4‐fluoroproline, to probe for a stereoelectronic effect on the conformation of polyproline. Circular dichroism spectral analyses show that 4R electron‐with‐drawing substituents stabilize a PPII helix relative to a PPI helix, even in a solvent that favors the PPI conformation, such as n‐propanol. The stereochemistry at C4 ordains the relative stability of PPI and PPII helices, as (flp)10 forms a mixture of PPI and PPII helices in water and a PPI helix in n‐propanol. The conformational preferences of (Pro)10 are intermediate between those of (Hyp)10/(Flp)10 and (flp)10. Interestingly, PPI helices of (flp)10 exhibit cold denaturation in n‐propanol with a value of Ts near 70°C. Together, these data show that stereoelectronic effects can have a substantial impact on polyproline conformation and provide a rational means to stabilize a PPI or PPII helix.