Ab initio and DFT cation affinity study of selected neurosteroids

Full 3N-6 optimization electronic structure calculations using the HF 6-31G(d)//HF 6-31G(d) and B3LYP 6-31G(d)//B3LYP 6-31G(d) levels of theory have been employed to determine the theoretical gas-phase cation affinities of 5-pregnen-3β-ol-20-one, 5-androsten-3β-ol-17-one, 4-pregnen-3,20-dione, 5α-pregnan-3, 20-dione, 5α-pregnan-3α-ol-20-one and 5α-pregnan-3β-ol-20-one. For all the selected neurosteroids both theoretical methods predicted the trend of cation affinity as: H+≫Li+>Na+>K+. The bonding interactions occur at the carbonyl group of the neurosteroids. The large cation affinities of the neurosteroid-proton adduct was found to be indicative of the formation of a stable complex via a relatively strong polar-covalent mechanism, while the lower cation affinity of the other adducts reflect the weaker ion-dipole mechanism.