Raman and solid state 13C-NMR investigation of the structure of the 1 : 1 amorphous piroxicam : β-cyclodextrin inclusion compound

The results of a Raman and solid state ¹³C-NMR spectroscopic investigation aimed at studying the conformation of piroxicam (P) and its interaction with β-cyclodextrin (βCD) in 1 : 1 amorphous PβCD inclusion compound are reported. The 1700–1200 cm⁻¹ FT-Raman and the ¹³C CP/MAS NMR spectra of 1 : 1 PβCD inclusion compound are discussed and assigned in comparison with those of the three main modifications of piroxicam (α, β, and monohydrate). The FT-Raman and ¹³C-NMR results show that in 1 : 1 PβCD inclusion compound piroxicam mainly assumes the zwitterionic structure typical of monohydrate, even if the presence of a different structure, that is, β form, is not excluded. Piroxicam monohydrate, differently from α and β forms, is characterized by a zwitterionic structure with an internal proton transfer and an increased charge delocalization, as shown by our spectroscopic results. The charge delocalization characteristic of this zwitterionic structure gives rise to the interaction with βCD via electrostatic and hydrogen bonds. The possibility of a host–guest interaction between piroxicam and βCD is not excluded; the guest molecule can be accommodated in βCD cavity by interaction via hydrophobic bonds. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 243–251, 1999