Analysis of electronic structures of physostigmine analogs

To elucidate the action mechanism and structural prerequisites of 21 physostigmine analogs as acetylcholinesterase inhibitors at the molecular level, and help the rational design of these dihydroindoline inhibitors.

Initial structures of these compounds were built and minimized by SYBYL 6.2 molecular modeling software. Conformations of those molecules with the highest predictive abilities in the Comparative Molecular Field Analysis model were chosen to the semiempirical quantum chemical calculations.

(1) The highest occupied molecular orbital (HOMO) consisted mainly of the orbitals in phenyl group and N1 atom; the lowest unoccupied molecular orbital (LUMO) of the molecules was contributed from phenyl group and C11 atom. While the HOMO energies did not show any recognizable relationship with activity, the LUMO energies showed a decreased tendency with increasing activity. The active compounds showed lower LUMO energies. (2) The carbon atom (C11) had the most positive net atom charge. The most active compound had the most positive charge on this carbon, but had the lower charges on the carbonyl oxygen (O12) which was the most negative charge atom. (3) The bond order of carbon-oxygen bond (C11-O10) was invariant across the series of the compounds. (4) Compounds with too high or too low total dipole moment had lower activities, while the most active one had a lower molecular polarizability.

A molecular model was suggested to explain the possible mode of action by which these compounds inhibit acetylcholinesterase.