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 interconver sion of type II to type I polyproline in water was examined by the synthesi s and analysis of proline dimers and hexamers containing rip to three (2S,5 R)-5-tert-butylproline residues. In the dimers, the bulky 5-tert-butyl subs tituent was found to exert a significant influence oil the local prolyl ami de geometry such that the predominant trans-isomer in N-(acetyl)prolyl-prol inamide (1) was converted to 63% cis isomer in N-(acetyl)prolyl-5-tert-buty lprolinamide (2) as measured by H-1-nmr spectroscopy. Similarly, the presen ce of aa 5-tert-butyl group on the C-terminal residue ill the polyproline h examer Ac-Pro(5)-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 remain ed in the trans isomer as determined by NOESY spectroscopy. Conformational analysis by C-13-nmr and CD spectroscopy indicated that Ac-Pro(6)-NH2 (3) a dopted the ail-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 wa s observed by CD spectroscopy in water; however, no spectrum was obtained i ndicative of a complete conversion to a polyproline type I helix. The impli cations of these results are discussed with respect to the previously! prop osed 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 intermedia te composed of dispersed sequences of prolyl amide cia and trans isomers. ( C) 2000 John Wiley & Sons, Inc.