Physical and chemical evaluation of insulin and curcumin inclusion complexes in cyclodextrins: study with Raman, FTIR and photoacoustic spectroscopies

The aim of this work was the study of the physico chemical properties of inclusion complexes between hydroxypropyl-beta-cyclodextrin and insulin (HPβCD-I) and of complexes between beta-cyclodextrin and curcumin (β-CD-C). The techniques were FT-Raman, FTIR, FTIR-ATR, Photoacoustic Spectroscopy, besides complementary methods like Nuclear Magnetic Resonance (¹HRMN) and X-ray diffraction. The FT-Raman spectra suggested that the complex formation occurred through the inclusion of aromatic amino acids in the HPβCD cavity, especially tyrosine and phenylalanine. Changes were observed in the band assigned to the (C-O-C) stretching vibration, involving the (α 1-4) bonds of complexing agent. This bond is directed into the HPβCD cavity, suggesting that there was insertion of a guest in the CD cavity, confirming the formation of the inclusion complex. As a result of this interaction there was greater stability of the disulfide bonds, in addition to changes in secondary structure of the protein. The results with the ¹HNMR technique showed that the presence of cyclodextrin induces structural modification in the aromatic region of insulin, suggesting the interaction of insulin with HPβCD in this region of the protein, validating the results of the Raman spectroscopy. However, it was concluded that the data presented by Raman spectroscopy provides more information about the complexation process between molecules, compared to ¹HRMN. The technique showed changes suffered by insulin in different regions of its chain, due to complexation with the CD, in addition to being non-destructive and no need to use solvents. The curcumin results obtained with the FTIR technique showed the absence of the bands in the most significant carbonyl region indicated that curcumin was in the keto-enol form. The FT-Raman and FTIR techniques presented good evidence of the complex formation between β-CD and curcumin. The spectra revealed changes in the bands associated with the vibrational modes of the aromatic rings of curcumin, suggesting that there was insertion of aromatic rings of curcumin into the CD cavity. The measures with the photoacoustic technique confirmed this observation. The X-ray diffractograms of curcumin and β-CD exhibited a series of thin and intense lines revealing the crystalline structure of the material. Changes were observed in the complex diffractogram with the disappearance of some spectral lines of the curcumin and formation of new lines characteristic of a crystalline solid phase, which corresponds to an inclusion complex of the same nature. Thus, the X-ray diffraction con-firmed the results which were obtained from FTIR, FT-Raman and photoacoustic spectroscopy. In conclusion, the results of this work suggest that the used techniques are important tools to evaluate the physico chemical properties of inclusion complexes involving cyclodextrin, which is an important scientific and technological area with great possibilities of applications in the development of novel medicines and foodstuffs. (author)