title: Stereoelectronic effects on polyproline conformation.
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 T(s) 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.
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison, 433 Babcock Drive, Madison, WI 53706-1544, USA.
authors.forename: Jia-Cherng
authors.initials: JC
authors.lastname: Horng
authors.forename: Ronald T
authors.initials: RT
authors.lastname: Raines
mesh_terms: D002942:Circular Dichroism; D003080:Cold Temperature; D004583:Electrons; D006358:Hot Temperature; D007700:Kinetics; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry*; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary
chemical_list: D010455:Peptides; C011083:polyproline