The intrinsic conformational preferences of indoline-2-carboxylic acid (Inc) and its alpha-methylated derivative (alpha MeInc) have been investigated using quantum mechanical calculations. Specifically, the behavior of their N-acetyl-N'-methylamide derivatives, Ac-L-Inc-NHMe and Ac-L-alpha MeInc-NHMe, has been explored at the B3LYP/6-31+G(d,p) level. Such amino acids are analogues of proline and (alpha-methyl)proline, respectively, bearing a benzene ring fused to the C-gamma-C-delta 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 alpha-methyl group in the alpha MeInc 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.