AM1 studies of hydrogen bonded adducts between 2,5-dihydroxy-p-quinone and N-bases

Semiempirical AM1 calculations were used to optimize the geometry of the adducts formed by 2,5-dihydroxy-p quinone (DHpQ) with substituted pyridines or imidazole. Hydrogen bond formation was seen to be responsible for the stability of the adducts. From the study of the total energy function, obtained by imposing fixed positions to the proton, two structures were found to be possible for each adduct, one schematically indicated as -OH … N-base and the proton transfer one, indicated as -O⁻ … HN⁺base. The proton transfer energy was found to vary linearly with the base pK_a value. A study was also carried out on the system formed by DHpQ and a cluster of three water molecules acting as proton acceptors. The implications were discussed relating to possible hydrogen bond formation between trihydroxyphenylalanine quinone, the coenzyme of bovine serum amine oxidase, and some basic residue or water molecule, lying close to the protein active site.