L.C. Sögütoglu, M. Lutz, H. Meekes, R. de Gelder, E. Vlieg,
Polymorphism and Modulation of Para-Substituted L‑Phenylalanine
Cryst. Growth Des. 17 (2017) 6231-6238

Abstract

The crystal structure of para-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 (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-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.