Stereoelectronic effects on the transition barrier of polyproline conformational interconversion

There has been growing interest in polyproline type II (PPII) helices since PPII helices have been found in folded and unfolded proteins and involved in a variety of biological activities. Polyproline can also form type I helices (PPI) which are very different from PPII conformation and only exist in certain organic solvents. Recent studies have shown that stereoelectronic effects play a critical role in stabilizing a PPI or PPII helix. Here, we have synthesized a series of host-guest peptides with an electron-withdrawing substituent at the 4R or 4S position of proline and used a kinetic approach to further explore stereoelectronic effects on the transition barrier of the interconversion between PPI and PPII conformations. Time-dependent circular dichroism measurements revealed that the rates of PPII -> PPI conversion were reduced upon incorporating the hydroxyl-, fluoro-, and methoxy-groups at the 4R position while the rates would be increased if these substituents were at the 4S position. We quantified the changes in transition free energy by comparing their rate constants. (4R,2S)-4-Fluoroproline and (4S,2S)-4-fluoroproline have the largest effect on the transition energy barrier for PPII -> PPI conversion. Our results provide important insights into the role of stereoelectronic effects on the PPII -> PPI transition state barrier, which has not been reported in past thermodynamic studies.