Quantum chemical studies of morphine-like opiate narcotics. Effect of N-substituent variations

Quantum chemical calculations including extensive conformational variations are performed on three morphine-like analgesics with varying N-substituents using the PCILO and INDO methods. The three compounds, morphine, nalorphine, and N-phenethylmorphine, have been shown experimentally to exemplify opiate narcotic agonism, antagonism, and increased agonism, respectively. In this study, these properties are correlated with the electronic and conformational results. The electronic properties of the fused ring skeleton including specifically the cationic region around the nitrogen are relatively unaffected by varying N-substituents. The properties studied include net charges, bond polarities, and the nature and energy of the highest filled and lowest empty molecular orbitals. The conformational behavior appears to be the main cause of differing receptor binding and interaction with the active site and is discussed in these terms.