Desolvation of L-histidine and α-ketoisocaproic acid complex from ethanolate crystals under humidified conditions and influence of crystallinity on its desolvation

Abstract

Desolvation of L-histidine and α-ketoisocaproic acid complex from ethanolate crystals was investigated. The ethanolate crystals were obtained from ethanol aqueous solutions of above 60 wt% of ethanol. It was difficult to remove ethanol molecules from the crystals by vacuum drying. However, it was found that ethanol molecules in the crystal lattice could be released under humidified conditions, for example, 313 K and 60 % relative humidity, accompanied by transformation to non-solvated crystals. When the peak of 2θ=9.0°(CuKα radiation) in powder X-ray diffraction pattern of the ethanolate crystals was weak, ethanol molecules (about 1 wt%) remained in the crystals at the end of transformation, and then the residual ethanol decreased slowly. A controlled moderate cooling process, where the supersaturation is released slowly, is the key point to obtain ethanolate crystals having high "crystallinity" (defined as peak height of 2θ=9.0°) which shows quick desolvation rather than adding ethanol for a rapid increase of supersaturation in crystallization.