Polymorphism and modulation of para-substituted L-Phenylalanine

Abstract

The crystal structure of para-methyl-L-phenylalanine at 230 K resembles that of the para-fluorinated analogue from the literaturebut is commensurately modulated with seven molecules in the asymmetric unit (Z′ = 7). At 100 K, the superstructure loses its modulation, leading to a unit cell with Z′ = 1, with clear disorder in the phenyl ring orientations. The methyl-substituent in para-methyl-Lphenylalanine 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.