Mechanism of Phase Transition in dl-Methionine: Determining Cooperative and Molecule-by-Molecule Transformations.

The solid-state phase transition in dl-methionine has been extensively studied because of its atypical behavior. The transition occurs through changes in the molecular conformation and 3D packing of the molecules. Phase transitions in racemic aliphatic amino acid crystals are known to show different behaviors depending on whether conformational changes or packing changes are involved, where the former is thought to proceed through a nucleation-and-growth mechanism in a standard molecule-by-molecule picture, and the latter through a cooperative mechanism. The phase transition of dl-methionine resembles the thermodynamic, kinetic, and structural features of both categories: a conformational change and relative shifts between layers in two directions. The present paper presents molecular dynamics simulations of the phase transition to examine the underlying mechanism from two perspectives: (i) analysis of the scaling behavior of the free energy barriers involved in the phase transition and (ii) a structural inspection of the phase transition. Both methods can help to distinguish between a concerted phase change and a molecule-by-molecule or zip-like mechanism. The free energy predominantly scales with the system size, which suggests a cooperative mechanism. The structural changes draw, however, a slightly more complex picture. The conformational changes appear to occur in a molecule-by-molecule fashion, where the rotational movement is triggered by movement in the same layer. Conformational changes occur on a time scale nearly twice as long as the shifts between layers. Shifts in one direction appear to be less concerted than shifts in the perpendicular direction. We relate this to the edge-free energy involved in these shifts. We believe that the behavior observed in dl-methionine is likely applicable to phase transitions in other layered systems that interact through aliphatic chains as well.