Effect of High Pressure on the Crystal Structures of Polymorphs of l-Histidine

The effect of pressure on the crystal structures of the two ambient-pressure polymorphs of the amino acid l-histidine has been investigated. Single-crystal diffraction measurements, up to 6.60 GPa for the orthorhombic form I (P2₁2₁2₁) and 6.85 GPa for the monoclinic form II (P2₁), show their crystal structures undergo isosymmetric single-crystal-to-single-crystal first-order phase transitions at 4.5 and 3.1 GPa to forms I′ and II′, respectively.