5,7-Dimethoxycoumrin(DMC) is highly active as a photosensitizer in $\mbox{\underline{Bacillus}}$ $\mbox{\underline{Subtilis}}$. It is suggested that the photosensitizing effect of DMC is caused by the formation of $C_4$-cycloadduct between a pyrimidine base in DNA and the pyrone double bond of DMC on irradiation with ultraviolet light. The photosplitting of $C_4$-cyclodimers of DMC is studied in order to elucidate the repair mechanism of the photomodified DNA by DMC and/or psoralens.
The $C_4$-cyclodimers of DMC are photosplitted into DMC by 300nm or shorter wavelength ultraviolet light. Photorebersion product DMC was detected by silica gel thin layer chromatography and UV absorption spectra ($λ_{max}$ of DMC is 324 nm in acetonitrile.)
In the presence of triplet sensitizers such as benzophenone and acetophenone, these dimers are also splitted into DMC by 350 nm ultraviolet light excitation.
The direct photosplitting of $\underline{anti}$-dimer of DMC by 300 nm UV light was quenched by $NiSO_4 ㆍ6H_2O$ while that of $\underline{syn}$ head-to-tail dimer was not affected.
It is well-known that $S_1\leadsto{T}_1$ intersystem crossing and $S_0 +h ν → T_n$ transitions enhanced by heavy atoms in the compounds such as potassium iodide and alkyl halides. The photosplitting of $\underline{anti}$-dimer is increased in the presence of potassium iodide, but that of $\underline{syn}$-dimer is slightly decreased.
It is thus concluded that the $\underline{syn}$ head-to-tail dimer is photosplitted into DMC through the singlet excited state while $\underline{anti}$-dimer through the excited triplet state.
DMC는 $\underline{Bacillus}$ $\underline{Subtilis}$에 감광제로써 매우 큰 활성을 가지고 있다. 이런 DMC의 감광작용은 DNA의 피리미린 염기와 $C_4$ -고리 화합물을 형성하기 때문이라 제안했다. 본 연구는 DMC나 소랄렌에 의해 손상된 DNA를 수선(repair)하며, 그에 따른 반응 메카니즘을 알아 보기 위해서다.
DMC의 $C_4$ - 광이합체는 300nm이하의 자외선에 의해 DMC로 쪼개지며, 벤조페논과 같은 삼중상태 증감제에 의해서도 잘 쪼개진다. Anti이합체는 삼중상태 증감제 없이 300nm에서 광쪼갬 반응 시킬 때, 상자성체인 $NiSO_4 ㆍ6H_2O$에 의해 ？칭되나, syn 이합체는 ？칭되지 않았다. 한편, 중원자 염인 요오드와 칼륨에 의해 syn 이합체의 광 쪼갬 반응은 거의 변화가 없으나, anti 이합체는 그 반응이 증가하였다. 그러므로, syn이합체는 여기된 단일상태에서, anti 이합체는 들뜬 삼중상태에서 쪼개짐을 알았다.
