Terahertz Spectroscopy of Histidine Enantiomers and Polymorphs
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Terahertz Spectroscopy of Histidine Enantiomers and Polymorphs Alan B. True & Konstanze Schroeck & Timothy A. French & Charles A. Schmuttenmaer
Received: 25 January 2010 / Accepted: 20 April 2010 / Published online: 22 May 2010 # Springer Science+Business Media, LLC 2010
Abstract We have measured terahertz and powder x-ray diffraction spectra of D-histidine, L-histidine, and DL-histidine. The as-received D and L material exists in two different polymorphs: D-histidine is in the metastable monoclinic form, while L-histidine is in the stable orthorhombic form. For both the L and D enantiomers, recrystallization of the asreceived material results in a mixture of the monoclinic and orthorhombic forms. Keywords Histidine . Polymorphism . THz
1 Introduction Polymorphism has long been an important issue in solid state chemistry and the pharmaceutical industry. Polymorphs are different crystal structures of the same molecular material. Within the pharmaceutical industry, important physical properties such as solubility and bioavailability are dependent on the product’s morphology [1–6]. In turn, the manufacturing process is governed by the physical properties of the product’s morphology in terms of its solubility, melting point and reactivity. Terahertz time-domain spectroscopy (THz-TDS) has become a useful tool for morphology determination. Although other techniques exist, including x-ray crystallography and Raman spectroscopy, THz spectroscopy is a simple and effective method that probes the spectral region of 0.1–3 THz where the crystal librations arising from the lattice structure are found [7, 8]. These are in addition to modes that are of mixed intermolecular A. B. True : K. Schroeck : T. A. French : C. A. Schmuttenmaer (*) Department of Chemistry, Yale University, P.O. Box 208107, 225 Prospect St., New Haven, CT 06520-8107, USA e-mail: [email protected] Present Address: T. A. French Department of Chemistry and Chemical Biology, Harvard University, 1 Oxford Street, Cambridge, MA 02138, USA
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J Infrared Milli Terahz Waves (2011) 32:691–698
and intramolecular character, particularly at in the higher frequency range. Despite the fact that the THz spectrum is a direct description of the phonon modes of a crystalline state, only limited work has been done with THz spectroscopy to explore polymorphism [9–12]. Applying THz spectroscopy to some important and well studied polymorphs demonstrates the technique’s ability to quickly and clearly differentiate between them. The general conclusions of previous work with amino acids are that crystals of D and L enantiomers of a particular molecule have identical spectra while the racemate has a unique spectrum [13]. This is because the optical isomers can crystallize in the same configuration (with opposite handedness) and the DL racemate necessarily has a different lattice structure. This also demonstrates conclusively that the modes found in these spectra are significantly intermolecular in nature as any intramolecular modes would be necessarily common to both
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