Quantum chemical studies of manganese centers in biology.

During the past five years, hybrid density functional theory has been used to study mechanisms for redox-active enzymes containing complexes with a variety of different transition metals. In this paper, suggested mechanisms of some manganese enzymes are described. For photosystem II, a mechanism is proposed leading to an oxyl radical in the S(3)-state, which is the precursor for the O-O bond formation. For manganese catalase, the suggested mechanism instead leads to the formation of a hydroxyl radical after the O-O bond of hydrogen peroxide is cleaved. This radical is immediately quenched by a manganese center. Parallels between these enzymes are highlighted. Jahn-Teller and trans effects are emphasized.