SEMIEMPIRICAL AND AB-INITIO STUDY OF CLOSED AND OPEN-SHELL DERIVATIVES OF 10-METHYLISOALLOXAZINE - A MODEL OF FLAVIN REDOX STATES

The structures of a series of neutral and charged derivatives of 10-me thylisoalloxazine in three different redox states have been optimized with the semiempirical PM3 method and with ab initio methods at Hartre e-Fock level. The isoalloxazine ring is planar in the oxidized and one electron reduced state, except for the cations with a proton at N1, a nd it is folded in the two electron reduced state. Single point MP2 an d self consistent reaction field calculations have been used for the a nalysis of the energies in the gas phase and in a polar medium. The en ergy difference between the cations with a proton at N1 and O2 is smal l for each redox state and depends on the solvent. Protonation and dep rotonation energies have been calculated.