PHOTOCHEMIE ANOMALER NUCLEINSAUREBAUSTEINE‐VI. ZUM PHOTOCHEMISCHEN REAKTIONSMECHANISMUS DES 6‐AZAURACILS†

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Abstract
Abstract— The photohydration reaction of 6‐azauracil has been investigated in the presence of paramagnetic ions, at different pH and in the presence of benzophenone, acetophenone and acetone as sensitizers. Paramagnetic ions inhibit photohydrate formation and there is a linear relation between the magnetic moment of the added metal ions and the half lifetime of the photoreaction. By reactivation experiments it was shown that the hydration reaction is mainly quenched. From the pH dependence of the photoreaction of 6‐azauracil and its N‐methyl‐derivatives it follows that the properties at the N3‐atom are important. Thus, the reaction was decreased strongly if the N3‐position is protonated, while methyl substitution at the same position causes an increase. The small differences between the pKs of the ground state and the photochemical ly active state as well as the quenching experiments indicate that the triplet state is involved in the hydration reaction. Strong evidence for this was also the triplet‐triplet transfer which increases in going from benzophenone to acetophenone and acetone. Thus the photochemically active state of 6‐azauracil leading to hydration differs from that of uracil where the hydrate forming reaction occurs via the excited singlet state.