Polymorphism and Modulation of Para-Substituted l-Phenylalanine.

The crystal structure of -methyl-l-phenylalanine at 230 K resembles that of the para-fluorinated analogue from the literature but is commensurately modulated with seven molecules in the asymmetric unit (' = 7). At 100 K, the superstructure loses its modulation, leading to a unit cell with ' = 1, with clear disorder in the phenyl ring orientations. The methyl-substituent in -methyl-l-phenylalanine has, in contrast to fluorine, no polar interactions with protons of neighboring molecules, which might allow for the well-defined modulation of the crystal structure at 230 K.