Elucidation of the Dexter‐Type Energy Transfer in DNA by Thymine–Thymine Dimer Formation Using Photosensitizers as Artificial Nucleosides

Abstract

C-nucleosides of 4-methylbenzophenone, 4-methoxybenzophenone, and 2'-methoxyacetophenone were synthetically incorporated as internal photosensitizers into DNA double strands. This structurally new approach makes it possible to study the distance dependence of thymidine dimer formation because the site of photoinduced triplet energy transfer injection is clearly defined. The counterstrands to these modified strands lacked the phosphodiester bond between the two adjacent thymidines that are supposed to react with each other. Their dimerization could be evidenced by gel electrophoresis because the covalent connection by cyclobutane formation between the two thymidines changes the mobility. A shallow exponential distance dependence for the formation of thymidine dimers over up to 10 A-T base pairs was observed that agrees with a Dexter-type triplet-triplet energy transfer mechanism. Concomitantly, a significant amount of photoinduced DNA crosslinking was observed.

Generation of the Thymine Triplet State by Through‐Bond Energy Transfer

“…[4] However, important issues such as how triplet excitation travels within DNA, or the maximum distance for efficient triplet energy transfer before reaching aT hy (the lowest-energy triplet nucleobase) have only recently attracted attention. [5] In contrast, related processes, such as hole [6] or excess electron transfer [7] along DNA, have been thoroughlyi nvestigated.…”

Thermally induced hopping model for long-range triplet excitation energy transfer in DNA

“…8 Given that for super-exchange HT and ET in DNA, b HT E b ET Z 0.7 Å À1 , we estimate b TEET Z 1.4 Å À1 , which is much bigger than the b values of 0.13 and 0.17 Å À1 reported for systems 1 and 3. 7 In principle, a singlestep Dexter mechanism could be reconciled with small b values assuming significant delocalization of triplet excitations over several bases. However, theoretical analysis of TEET in DNA 28 shows that triplet excited states in DNA are strongly localized on single bases.…”

“…This would also help to elucidate whether superexchange TEET steps over a few bases can occur with different sequences than the one studied in ref. 7. For instance, it seems plausible that intrastrand hops mediated by an intermediate base will also occur when A is substituted by G.…”

Wie weit wandert Energie in der DNA und verursacht Schäden? Nachweis des langreichweitigen Photoschadens in DNA

“…Wir zeigten kürzlich, dass die Übertragung von Triplettenergie mit einer exponentiellen Distanzabhängigkeit stattfindet und das Limit bei 10 A-T-Paaren (37.4 ) liegt. [11] Dies kann durch einen Hüpfmechanismus der Triplettenergie und geringer Delokalisierung der intermediären Excitonen erklärt werden. [12] Hier präsentieren wir eine neue DNA-Architektur, mit der wir die Frage der CPD-Bildung über große Reichweiten adressieren (Abbildung 1).…”

“…Das erlaubt die PAGE-Analyse der chemischen CPD-Ausbeuten. [11] Mithilfe dieser DNA-Architektur kann die Distanzabhängigkeit untersucht werden, weil sowohl die Stelle X, wohin die Photoenergie injiziert wird, als auch die Stelle der Beschädigung klar definiert sind und in bestimmten Distanzen angeordnet werden kann. Entsprechend synthetisierten wir die Hybride DNAn-TT, die sich in der Zahl n der Paare von A-T-Paaren unterscheiden, wobei n zwischen 0 (direkte Nachbarschaft) in DNA0-TT und 15 (30 dazwischenliegende A-T-Paare) in DNA15-TT liegt.…”