Crystal Structures of Peptide Enantiomers and Racemates: Probing Conformational Diversity in Heterochiral Pro-Pro Sequences

Multiple conformational states in heterochiral diproline sequences have been characterized in the solid state by the determination of the crystal structures of seven tripeptides in enantiomeric and racemic forms. The sequences of the type Piv- $^D Pro$- $^L Pro$ - $^D Xxx$ -NHMe (D-L-D) $[^D Xxx = ^DVal 1, ^D Leu 3, and ^D Phe 5]$ and their corresponding enatiomeric $L-D-L$ sequences $[^L Xxx = ^L Val 2, ^L Leu 4, and ^L Phe 6]$ have been investigated.Single crystals have been obtained for the pure enantiomers 1, 2, 3, 4 and for the racemates 1/2, 3/4, and 5/6. For Xxx = Leu, mirror image conformations (type II/II' \beta -turns) at Pro-Leu segment are obtained. For $Xxx = Val, a ^L Pro- ^D Pro$ type II \beta -turn in 2 and an open/extended structure is obtained in the solvated form of the enantiomer 1. For Xxx = Phe, suitable crystals could not be obtained for enatiomeric peptides. The racemate 5/6 revealed a cis peptide bond between the diproline segment with the absence of any intramolecular hydrogen bonds.Crystal structures of enantiomers and racemates prove useful in characterizing the multiple conformational states that are accessible to Pro-Pro segments