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-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 s 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. Published by Cold Spring Harbor Laboratory Press. Copyright © 2006 The Protein Society.