Theoretical mechanistic insight into the gabapentin lactamization by an intramolecular attack: Degradation model and stabilization factors

Gabapentin is degraded directly into a high toxicity form known as gabapentin lactam (gaba-L) with a maximizing desire in mild pH and low humidity. This study reports the lactamization process of gabapentin, along with a detailed analysis of the energy landscape, geometry, and thermodynamic and kinetic preference of the process. To investigate the effect of the acidic/basic conditions on the energy landscape, the energy profiles were investigated for both protonation and deprotonation forms of gabapentin.All the calculations were performed by using the density functional theory (DFT) and the G4MP2 levels of theory in the conductor-like polarizable continuum model, CPCM, and water as the solvent.The lactamization process is an intramolecular cyclization which results in formation of gabapentin-lactam. The chemically intact gabapentin exists in two forms of a stable, R, and a relatively disordered form, R*. The conversion of stable crystalline form R to the intact unstable isomer R* is considered as the primary step in the gabapentin degradation. The results exhibited that near the unstable geometry, R*, a transition state (TS), is 41.3 kcal/mol higher in energy than the optimized ground state, R* (4.1 kcal/mol). From the intrinsic reaction coordinates (IRC) computations, it can be concluded that this transition state led to the unstable R* in one direction and to gabapentin-lactam in the other.The thermodynamic stability of the lactam form (-13.63 kcal/mol) clarifies the more thermal stability of gaba-L than its related gabapentin form and the experimental preference for the lactamization. The corresponding energy profile on protonation/deprotonation forms of gabapentin indicates the pH-dependent of the process and the rate reduction in out of the mild pH.