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A detailed structural analysis and spectral behavior of the glucoalkaloid strictosidine, a precursor of all monoterpene indole alkaloids, are discussed. The experimental NMR, FTIR, and UV results were compared to the theoretical DFT spectra calculated by Becke using the three-parameter Lee-Yang-Parr (B3LYP) function with 6-31G(d) and 6-311++G(2d,p) basis sets. The theoretical geometry optimization data were compared with the X-ray data for precursors and similar structures in the associated literature. The similarity between the theoretical and experimental coupling constants values made it possible to affirm the values of dihedral angles and their configuration, reinforcing findings from previous stereochemical studies. Theoretical UV analysis agreed well with the measured experimental data, with bands assigned. Calculated HOMO/LUMO gaps show low excitation energy for strictosidine, justifying its stability and reaction kinetics. The molecular electrostatic potential map shows opposite potentials regions that form hydrogen bonds that stabilize the dimeric form, which were confirmed by excellent agreement of the dimeric form theoretical wavenumbers with the experimental IR spectrum. ESI-MS/MS data revealed patterns for the fragmentation of the protonated strictosidine molecule outlined by an NBO study.

A theoretical and experimental DFT study of the vibrational, structural and quantum properties of annomontine (1) and N-hydroxyannomontine (2) alkaloids using the B3LYP exchange-correlation functional with 6-311G (2d, p) basis set is presented. The theoretical geometry optimization data of the two structures were compared with the X-ray data of (1) in the associated literature and a conformational study is presented for both molecules, providing a good comprehension of the conformational stability. In addition, natural bond orbitals (NBOs), HOMO-LUMO energy gap and mapped molecular electrostatic potential surface (MEPS) calculations were also performed at the same calculation approach. The calculated UV spectra agreed well with the measured experimental data, with transitions assigned. The comparative IR studies confirmed the intramolecular hydrogen bonds of the conformations and the intermolecular hydrogen bonds of dimeric forms and also revealed several characteristic vibrations for the structures. Molecular docking studies with DNA Topoisomerase II-DNA complex showed binding free energies of À11.5 and À10.6 kcal/mol for 2 and 1 respectively, while for amsacrine, used for the treatment of leukemia, and doxorubicin, used for the treatment of breast cancer, bladder cancer, Kaposi's sarcoma, and acute lymphocytic leukemia, the presented binding free energies values are À10.0 and À9.9 kcal/mol respectively, revealing good bind affinities of the tested alkaloids with the complex. In vitro cytotoxicity assay revealed an expressive antitumor activity of N-hydroxyannomontine against human hepatocellular carcinoma cell line HepG2.

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