Vibrational and thermal analyses of multicomponent crystal forms of the anti-HIV drugs lamivudine and zalcitabine

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The vibrational and thermal characterizations of four multicomponent molecular crystals of lamivudine, namely, lamivudine hydrochloride anhydrate (1), lamivudine hydrochloride monohydrate (2), lamivudine duplex I (3), with a 8:2:2:1:4 lamivudine:maleic acid:HCl:(CH3)2CHOH:H2O stoichiometry, being all three more soluble in water than the commercial solid form of lamivudine, and lamivudine maleate (4), have been performed here by infrared (IR) and Raman spectroscopy, differential scanning calorimetry (DSC), and thermogravimetry (TG). Furthermore, the vibrational spectra of zalcitabine hydrochloride (5), isostructural to 1 but with a methylene moiety in the 3′-position of the five-membered ring instead of sulfur in lamivudine, have also been measured in order to point out the role of this molecular substitution and conformation in the vibrational modes of the salts. In fact, scattering bands at the high frequency range relative to CH stretching modes are not superimposable in the Raman spectra of 1 and 5, even though these crystal forms are assembled with the same molecular conformation and intermolecular packing. At the same time, the structural similarity between 1 and 5 can be reflected in their IR spectra, as in the carbonyl and iminium stretching bands shifted to lower frequencies as consequence of their hydrogen bonding engagement. Furthermore, a scattering band at 3057 cm−1 is observed only in the Raman spectra of crystal forms present with their 5′-CH2OH moiety in–gauche conformation, namely, 2–4. It is absent in the Raman spectra of 1 and 5 whose 5′-CH2OH moiety adopts +gauche conformation. In–gauche conformation, the 5′-OH oxygen is pointed toward one of the two aromatic CH hydrogens. Consequently, there is formation of an intramolecular hydrogen bond between them, shifting the aromatic CH stretching band to a lower frequency. The DFT calculations have also revealed in-phase and out-of-phase couplings of the two aromatic CH stretchings in the Raman spectra of 1, which is without intramolecular hydrogen bond due to +gauche conformation of 5′-CH2OH moiety. Both coupled vibrational modes are observed in the corresponding experimental spectrum as a single peak because of their similar frequencies. On contrary, aromatic CH stretching modes are not coupled in 2 due to the intramolecular hydrogen bond, resulting in resolution of the Raman bands. Thermal events in DSC and TG curves of 1 and 2 are also in agreement with crystal stoichiometry as observed from single-crystal X-ray diffraction analysis.

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