Stereoelectronic effects on polyproline conformation

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-withdrawing 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 T-c near 70 degrees 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.