Molecular Rotation Spectrum of Tetracyclic Quinolizidines: Observation of trans-Matrine and the Elusive cis-Matrine

We characterized the bis-quinolizidine tetracyclic alkaloid (5S, 6S, 7R, 11R)-matrine in a supersonic jet expansion, using chirped-pulsed broadband microwave spectroscopy. Previous crystal diffraction analyses suggested 16 diastereoisomers associated with matrine's four carbon stereocenters but were inconclusive whether lactamic nitrogen atom would additionally produce separated trans-/cis- or if both species may interconvert through low potential barriers. Our experiment simultaneously detected trans- and cis-matrine their rotational spectrum, confirming possibility of conformational rearrangement matrine alkaloids. The two conformers mainly differ envelope half-chair ring, as evidenced by experimental nuclear quadrupole coupling parameters. Molecular orbital calculations ab initio (MP2) density functional methods (B3LYP-D3(BJ) MN15) tested against experiment, offering an estimation cis-/trans- barrier 24.9-26.9 kJ mol-1. illustrates structural spectroscopy for high-resolution studies biomolecules range 20-40 atoms.