The geometries and electronic structures in the lowest triplet (T 1) states of psoralen, 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP), and their monoadducts were studied by performing density functional theory (DFT) calculations. It was shown that the pyrone ring of 8-MOP is cleaved in the T1 state and the cleaved pyrone ring loses the ability to bond to the thymine residue. Therefore, 8-MOP in the open-ring structure does not form a diadduct, which is thought to cause side effects. DFT calculations on 20 kinds of substituted psoralens showed that the cleavage of the pyrone ring in the T1 state can be controlled by substituting an electron-donating group, especially one having a strong electron-donating resonance effect, at the 8-position of psoralen.
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