Cyclic voltammetric and EPR spectroscopic studies of benzodiazepines: loprazolam and flunitrazepam

Abstract

The present paper reports on the electrochemical characterization by cyclic voltammetry of two 1,4-benzodiazepines, loprazolam and flunitrazepam in protic, aprotic and mixed media. In protic media (ethanol + 15 mM citrate buffer pH 8.0) both drugs were irreversibly reduced at HDME involving 4 electrons to give the hydroxylamine derivative. In the optimal mixed media conditions (DMF + 15 mM citrate buffer, 0.1 M TBAI, pH 9.0) and in aprotic media (DMF + 0.1 M TBAI), the one-electron reduction product corresponding to the nitro radical anion was successfully isolated for both drugs. In both electrolytic media (mixed or aprotic media) loprazolam was reduced at less negative potentials compared with flunitrazepam. Radical decay follows second order kinetics. In mixed media the following second order decay constants and half-lives (for a 5 mM concentration) were found: k 2 = 1573.6 ± 35.7 l mol −1 s −1 , t 1 2 = 0.13 s for flunitrazepam and k 2 = 455.5 ± 16.6 l mol −1 s −1 , t 1 2 = 0.44 s , for loprazolam. EPR spectra recorded in situ using DMF as an electrolytic solvent showed well-resolved spectra, confirming the reduction of both benzodiazepine derivatives to their corresponding nitro radical anions. The experimental and the simulated hyperfine constant values obtained by INDO calculations are in agreement. The magnitude of such hyperfine splitting constants permits us to conclude that these radical anions are mainly restricted to the benzene ring of the molecules.