Molecular structure and conformations of caramboxin, a natural neurotoxin from the star fruit: A computational study

Using density functional theory calculations we investigate the molecular structure and conformations of caramboxin, a neurotoxin recently isolated from the star fruit Averroha carambola. Among the seven conformers that exist within an energy window of ∼16.0 kcal/mol, two of them are the most favored ones with an energy difference of less than 2.0 kcal/mol. The computed chemical shifts of these two low-energy conformers are in good agreement with the experimental values determined in deuterated dimethylsulfoxide thus confirming the 2D chemical structure assigned to the neurotoxin. A topological analysis of the theoretical electronic charge density of four caramboxin conformers reveals the existence of intramolecular CH⋯O/N interactions which, in addition to the classical OH⋯O/N H-bonding interactions, contribute to decrease the conformational freedom of the neurotoxin.