Complexes of damirone A/C, batzelline A/D, makaluvamine O and makaluvone with guanidinium and magnesium cations: a theoretical study

Among physiologically active compounds isolated from marine organisms, the pyrroloquinoline alkaloids (PQAs) family consisting of damirone A/C, batzelline A/D, makaluvamine O and makaluvone exhibit numerous biological activities derived from the inimitable highly-fused structures. Their structural and electronic properties were investigated through intramolecular interactions, electron affinities (EA), reduction potentials (E°) and complexation of compounds with magnesium and guanidinium cations (ionic and hydrogen bond interactions) using quantum mechanical calculations in the gas phase and solution. For both series of complexes, the most and least stable ones correspond to damirone A and batzelline D, respectively. The energy data, geometrical parameters, and the minima of electrostatic potentials (Vmin) are in good correlation with the results of population analyses. The localized molecular orbital energy decomposition analyses (LMO-EDA) demonstrate that the electrostatic interaction is the most important stabilizing component. The EA values are positive in the gas phase and solution, and are evaluated with the increase in the dielectric constant of solvent. Increase in the EA values and decrease in the E° values are observed after complexation with guanidinium and magnesium cations. Those changes are higher for electron-rich compounds compared to electron-deficient ones, containing halogen substituents.