The electronic structure of photoskinsensitizing 8-azapsoralen and 4,4',5'-trimethyl-8-azapsoralen has been investigated by the semiempirical (PM3-CI-UHF, etc) methods. The formation of molecular complexes between ground thymine and excited azapsoralen is discussed in terms of charge transfer interaction. The results indicated that the most probable orientation through C4-cycloaddition of 3,4-double bond of azapsoralen and 5,6-double bond of thymine bases, expecially photoadducts were inferred to be a trans-anti azapsoralen(3,4) < > Thymine(5,6) and trans-anti 4,4',5'-trimethyl-8-azapsoralen (3,4) < > Thymine(5,6).
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