Structural changes of β-carotene and some retinoid pharmaceuticals induced by environmental factors

Four pharmaceuticals (β-carotene (1), retinoic acid (2), isotretinoin (3), and etretinate (4)) were analyzed using Raman spectroscopy and quantum-chemical calculations followed by potential energy distribution (PED) analysis to gain deeper insight into the experimental vibrational spectra. Small shifts of characteristic bands of (4) upon change of solvent and pH were interpreted as a result of molecular aggregation. Temperature-dependent studies on the Raman spectrum of (1) were performed in the temperature region of −150 °C to +150 °C. The observed small shifts in the experimental spectra upon heating were explained by increase of the high-energy conformers (of the trans type) in the population of (1) related to weakening of the intermolecular interactions that enables rotation of the terminal rings with respect to the polyene chain. Deconvolution of the ν1 band showed changes in intensity and position of the deconvoluted bands with the increase of temperature.