Quantum-chemical approach to determining the high potency of clorgyline as an irreversible acetylenic monoamine oxidase inhibitor

Density functional theory calculations were employed to investigate the nature of chemical bond formation between the flavin co-factor of the enzyme monoamine oxidase (MAO) and its irreversible acetylenic inhibitor clorgyline in its terminally deprotonated anionic form. Since MAOs regulate the level of neurotransmitters in living cells, this reaction is pharmacologically relevant for treating depression and other mood disorders. The results revealed that this pathway is associated with the activation free energy of ΔG _act ^#  = 17.4 kcal mol⁻¹, which, together with our previous results, suggests that clorgyline is intrinsically a more effective MAO inhibitor than antiparkinsonian drugs rasagiline and selegiline considering the preferred MAO isoforms in each case, thus displaying a trend in agreement with experimental data. The reaction is facilitated by the pronounced electrophilic character of the flavin moiety, due to its ability to efficiently accommodate excess negative charge from the approaching anionic inhibitor through resonance effect. The investigated mechanism was additionally validated by the inspection of the geometry of the flavin moiety in the formed adduct, which exhibit distortion from planarity consistent with experimental observations. These results offer valuable insight for mechanistic studies on other flavoenzymes and for the design of new antidepressants and antiparkinsonian drugs.