Conformational preferences of proline analogues with a fused benzene ring

Abstract

The intrinsic conformational preferences of indoline-2-carboxylic acid (Inc) and its R-methylated derivative (RMeInc) have been investigated using quantum mechanical calculations. Specifically, the behavior of their N-acetyl-N′-methylamide derivatives, Ac-L-Inc-NHMe and Ac-L-RMeInc-NHMe, has been explored at the B3LYP/6-31+G(d,p) level. Such amino acids are analogues of proline and (R-methyl)proline, respectively, bearing a benzene ring fused to the Cγ-Cδ bond of the five-membered pyrrolidine ring. The additional aromatic group has been shown to significantly restrict the conformational space available to these residues by reducing the flexibility of both the five-membered cycle and the peptide backbone. The fused benzene ring also plays a critical role in determining the cis-trans arrangement of the amide bond involving the pyrrolidine nitrogen, which is also modulated by the presence of the R-methyl group in the RMeInc derivative. Furthermore, the influence of the environment on the conformational propensities of these compounds has been evaluated by using both a self-consistent reaction field model and a recently developed interface in a hybrid QM/MM scheme, in which the solvent molecules are treated explicitly with classical mechanics while the solute is described by quantum mechanics at the density functional theory level.