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a b s t r a c tIntercalation complexes of psoralen and 8-methoxypsoralen with DNA-bases (G,C) and Watson-Crick (GC) base pair were studied using quantum chemical methods. The HOMO and LUMO energies, structures and properties of isolated psoralen and 8-methoxypsoralen have been computed. Their interactions with DNA-bases and base pair were studied aiming to shade more light on the nature of binding in DNA intercalation complexes. The geometry of adducts of psoralens with cytocine and guanine as well as WatsonCrick (GC) base pair have been optimized in two main orientations, planar and stacked, by means of HF, DFT and MP2 levels of computation employing different basis sets. The different computational methods have been compared and the effect of the basis set has been discussed. Interaction energy of the studied complexes were calculated and corrected for BSSE. Effect of vertical distance and rotational angle between the stacked molecules on the interaction energy were investigated by the above methods in gas phase and water media. Of the computational methods used, only higher correlated theories have been proven effective in describing these interactions. Ab initio methods which account for the electron correlation effects are the minimum level for studying the non-covalent interactions.

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Quantum chemical calculations were used to study the production of ethylene and keto/enol tautomers from ethoxyquinoline (2-EQ) and ethoxyisoquinoline (1-EisoQ and 3-EisoQ) in the gas phase and ethanol at the MP2/6-311++G(2d,2p)//BMK/6-31+G(d,p) level. The obtained data indicate that the elimination of ethylene from 1-EisoQ and 2-EQ is slightly more favorable than from 3-EisoQ. Formation of quinolone and isoquinolone (2-EQO, 1-EisoQO, and 3-EisoQO) is kinetically favored compared to their enols. Decomposition of 2-EQ and 1-EisoQ to ethylene and keto forms is thermodynamically and kinetically preferable more stable than the corresponding enols. However, the hydroxy form of 3-EisoQ is more stable than its keto tautomer in the gas phase and ethanol. The enol tautomers cost less energy when formed from their keto forms rather than from the parent ethoxyquinolone and ethoxyisoquinoline. K E Y W O R D SBMK and MP2, ethoxyisoquinoline, ethoxyquinoline, ethylene formation, potential energy surface 604