Conformational Behavior of β-Proline Oligomers

Conformations of the monomer, dimer, and hexamer of β-proline ((S) pyrrolidine-3-carboxylic acid) were determined using ab initio molecular orbital calculations at the RHF/6-31G* level of theory. The calculated minima are in good agreement with experimental data for the system and imply that the conformations could be controlled through chemical modification at Cα, Cγ, or Cδ. The monomer and dimer are small and flexible with many low-energy minima. In the hexamer, two forms of regular secondary structure are preferred:  left-handed helices with cis-peptide bonds and right-handed helices with trans-peptide bonds. This is similar to the behavior of α-proline helices, except that the relationship between the peptide rotamer and the handedness of the helix is reversed. Therefore, helices of the enantiomer of β-proline ((R)-pyrrolidine-3-carboxylic acid) should exhibit the same behavior as α-proline helices. Through understanding the conformational behavior of β-proline in various environments, it may be possible to use these protein mimics to inhibit various protein−protein recognition events. To estimate these effects, SCRF energies for the conformers were determined in dielectrics corresponding to water, methanol, and chloroform. It appears that the cis helices are more favorably solvated than the trans helices, but the cause is not clear.