Order–Disorder Phase Transition between High- and Low- Z ′ Crystal Structures of the P 1 Space Group
Résumé
We report a crystal structure of Boc-l-methionyl glycine methyl ester (MGP) with eight molecules in the asymmetric unit (Z′ = 8), which is the highest number in dipeptide compounds. Interestingly, its order–disorder phase transition to the crystal structure of Z′ = 4 was observed at −113 °C. The reversible order–disorder character of the phase transition was revealed by single-crystal X-ray diffraction and temperature-dependent second harmonic generation (SHG) signal measurements. The disorder of the methionine moiety of the high-temperature phase reflected the conformation of the low-temperature phase, implying that these crystal structures were essentially identical and that atomic displacement due to thermal fluctuation triggered the phase transition. Although differential scanning calorimetry (DSC) analysis and heat capacity (Cp) measurements did not provide clear signals to reveal and detect the character of phase transition, the SHG technique was able to detect the order–disorder structural change. The theoretical calculation clarified the intermolecular interaction energy together with the conformational freedom of the molecule involved in the high-Z′ crystal structure of MGP. The comparison study of the lattice energy with the virtually generated low-Z′ crystal structures derived from the observed molecular conformations provided quantitative analysis for the general problem in the field of crystallization, namely, why does a molecule crystallize in a high-Z′ crystal structure? This study also revealed the contribution of the entropy term, which plays a critical role in understanding the origin of an unusual high-Z′ crystal structure.