Characterizing the properties of anticancer silibinin and silybin B complexes with UV–Vis, FT-IR, and Raman spectroscopies: A combined experimental and theoretical study

Abstract

We present a combined experimental and theoretical study dedicated to analyze the structure, optical properties, and vibrational behavior of the anticancer silibinin complex and one of its molecular constituents, namely; silybin B. A comparison of the UV–Vis and FT-IR characterization of these samples reveals the presence of almost identical features. In both cases, the existence of four absorption maxima located at 204, 230, 287, and 330 nm are observed, as well as vibrational bands that show a complex distribution in the 300–1700 cm−1 frequency domain together with three infrared excitations placed at ∼3180, 3450, and 3600 cm−1 typically associated with different CH and OH bond vibrations. For the first time, we present the Raman spectra of these samples. Interestingly, the Raman response reveals more notable differences between the two molecular complexes, mainly in the 300–900 cm−1 frequency range. Furthermore, we obtain that the wavelength of the excitation laser plays a fundamental role in revealing precise features of the spectra. Based on density functional theory (DFT) calculations we also simulate the UV–Vis, circular dichroism, infrared, and Raman spectra of model silybin A and silybin B (C25H22O10) molecules in both isolated and aggregated forms. An excellent agreement is found when contrasting experimental and theoretical data. Based on this comparison, we identify the main infrared- and Raman-active vibrational modes of silybin B, and infer those frequencies that seems to be unique to silybin A. Wavelength dependent calculations of the Raman spectra also reveal the crucial role played by the energy of the incident laser in determining the intensities of the Raman active modes, allowing for a better comparison between theory and experiment. Finally, simulations of the circular dichroism spectra for isolated silybin A and B species reveal clear differences between the two molecules, being thus an appropriate tool to distinguish the presence of specific isomers in a sample.