An examination of the steric effects of 5-tert-butylproline on the conformation of polyproline and the cooperative nature of type II to type I helical interconversion

The influence of steric effects on the helical geometry and the interconversion of type II to type I polyproline in water was examined by the synthesis and analysis of proline dimers and hexamers containing up to three (2S,5R)‐5‐tert‐butylproline residues. In the dimers, the bulky 5‐tert‐butyl substituent was found to exert a significant influence on the local prolyl amide geometry such that the predominant trans‐isomer in N‐(acetyl)prolyl‐prolinamide (1) was converted to 63% cis isomer in N‐(acetyl)prolyl‐5‐tert‐butylprolinamide (2) as measured by 1H‐nmr spectroscopy. Similarly, the presence of a 5‐tert‐butyl group on the C‐terminal residue in the polyproline hexamer Ac–Pro5–t‐BuPro–NH2 (4) produced a local 5‐tert‐butylprolyl amide population containing 61% cis isomer in water. In spite of the presence of a local prolyl cis amide geometry, the downstream prolyl amides in 4 remained in the trans isomer as determined by NOESY spectroscopy. Conformational analysis by 13C‐nmr and CD spectroscopy indicated that Ac–Pro6–NH2 (3) adopted the all‐trans amide polyproline type II helix in water. As the amount of 5‐tert‐butylproline increased from one to three residues in hexamers 4–6, a gradual destabilization of the polyproline type II helical geometry was observed by CD spectroscopy in water; however, no spectrum was obtained, indicative of a complete conversion to a polyproline type I helix. The implications of these results are discussed with respect to the previously proposed theoretical mechanisms for the helical interconversion of polyproline, which has been suggested to occur by either a cooperative C‐ to N‐terminal isomerization of the prolyl amide bonds or via a conformational intermediate composed of dispersed sequences of prolyl amide cis and trans isomers. © 2000 John Wiley & Sons, Inc. Biopoly 53: 249–256, 2000