Polarization Switching of Crystal Structure in the Nonphotochemical Laser-Induced Nucleation of Supersaturated Aqueous l-Histidine

Polarization switching of polymorphs has been observed in the nonphotochemical laser-induced nucleation of aqueous l-histidine in the supersaturation range 1.40–1.60. Exposure of solutions to 0.24 GW/cm2, 7 ns pulses of 532 nm light induced the nucleation of different polymorphs of l-histidine depending on polarization state of the light. Circularly polarized laser pulses tended to nucleate the orthorhombic A polymorph, whereas linearly polarized pulses tended to nucleate a mixture of the orthorhombic A and monoclinic B polymorphs. Higher supersaturation also favors the formation of mixed polymorphs. These observations support the hypothesis that the laser is organizing hydrogen-bonded groups of histidine molecules through an optical Kerr alignment.