Multiphoton-gated cycloreversion reaction of a fluorescent diarylethene derivative as revealed by transient absorption spectroscopy.

The one- and two-photon cycloreversion reactions of a fluorescent diarylethene derivative with oxidized benzothiophene moieties were investigated by means of ultrafast laser spectroscopy. Femtosecond transient absorption spectroscopy under the one-photon excitation condition revealed that the excited closed-ring isomer is simply deactivated into the initial ground state with a time constant of 2.6 ns without remarkable cycloreversion, the results of which are consistent with the very low cycloreversion reaction yield (<10-5) under steady-state light irradiation. On the other hand, an efficient cycloreversion reaction was observed under irradiation with a picosecond laser pulse at 532 nm. The excitation intensity dependence of the cycloreversion reaction indicates that a highly excited state attained by the stepwise two-photon absorption is responsible for the marked increase of the cycloreversion reaction, and the quantum yield at the highly excited state was estimated to be 0.018 from quantitative analysis, indicating that the reaction is enhanced by a factor of >1800.

A cyclobutane thymine-N4-methylcytosine dimer is resistant to hydrolysis but strongly blocks DNA synthesis.

Exposure of DNA to ultraviolet light produces harmful crosslinks between adjacent pyrimidine bases, to form cyclobutane pyrimidine dimers (CPDs) and pyrimidine(6-4)pyrimidone photoproducts. The CPD is frequently formed, and its repair mechanisms have been exclusively studied by using a CPD formed at a TT site. On the other hand, biochemical analyses using CPDs formed within cytosine-containing sequence contexts are practically difficult, because saturated cytosine easily undergoes hydrolytic deamination. Here, we found that N-alkylation of the exocyclic amino group of 2'-deoxycytidine prevents hydrolysis in CPD formation, and an N-methylated cytosine-containing CPD was stable enough to be derivatized into its phosphoramidite building block and incorporated into oligonucleotides. Kinetic studies of the CPD-containing oligonucleotide indicated that its lifetime under physiological conditions is relatively long (∼ 7 days). In biochemical analyses using human DNA polymerase η, incorporation of TMP opposite the N-methylcytosine moiety of the CPD was clearly detected, in addition to dGMP incorporation, and the incorrect TMP incorporation blocked DNA synthesis. The thermodynamic parameters confirmed the formation of this unusual base pair.

Photoswitchable fluorescent diarylethene derivatives with short alkyl chain substituents.

The sulfone derivatives of 1,2-bis(2-alkyl-6-phenyl-1-benzothiophen-3-yl)perfluorocyclopentene having various short alkyl chain substituents at reactive carbons were prepared and the effect of alkyl substitution on the fluorescence property of the closed-ring isomers was studied. Upon irradiation with ultraviolet (UV) light the derivatives exhibit a brilliant green fluorescence under irradiation with visible (> 400 nm) light, while the fluorescence disappears upon irradiation with visible (> 400 nm) light alone. The fluorescence quantum yield of the methyl substituted derivative (1b) dramatically decreases from 0.84 to 0.15 when the solvent is changed from hexane to acetonitrile, while the changes of ethyl, n-propyl and n-butyl substituted derivatives (2b)­(4b) are moderate. The quantum yields of (2b)­(4b) are kept to values close to 0.7 even in polar acetonitrile. The fluorescence lifetime measurement revealed that efficient non-radiative decay processes took place in (1b) in polar solvent, while their contribution to the deactivation was not so large in (2b)­(4b). The neighboring short alkyl chains at the connecting carbons are considered to defend the sulfone units against the attack of polar solvent molecules and weaken the solvent polarity effect.