J. Photochem. Photobiol. B: BioL, 15 (1992) 215-222

215

Laser photolysis of cytosine, cytidine and dCMP in aqueous solution Zhi-hua Zuo, Side and Nian-yun Lin

Yao, Jian Luo, Wen-feng

Wang, Jia-shan

Zhang

Laboratory of Radiation Chemistv, Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204, Shanghai 201800 (China) (Received

January

22, 1992; accepted

March 30, 1992)

Abstract Laser photolysis of cytosine and cytosine derivatives (cytidine and 2’-deoxycytidine-5’monophosphate) using acetone as a sensitizer have been carried out in aqueous solution using a KrF laser. The absorption spectra of triplet states of cytosine and cytosine derivatives have been observed for the first time. From detailed kinetic analyses, the reaction mechanisms including triplet-triplet excitation transfer, and intramolecular excitation transfer within the exciplex arising from the interaction of triplet acetone with cytosine and cytosine derivatives, have been suggested. Accordingly, a series of kinetic parameters have also been obtained.

Keywords: Laser transfer, excited

photolysis, cytosine, triplet states.

cytosine

derivatives,

excimer

laser,

excitation

1. Introduction The excited states of DNA bases have been acknowledged to play an important role in the primary process of radiation and photo-induced damage of DNA. Both their emission and absorption spectra at low and room temperatures have been studied [l-3]. The excited singlet states of pyrimidine bases and their derivatives have been studied by picosecond laser photolysis [4]. The transient absorption spectra of excited triplet states of some pyrimidine bases and their derivatives [5-121 as well as of purine bases and their derivatives [13-171 were observed by nanosecond laser photolysis. In addition, similar transient absorption spectra of thymine [18, 191 and guanosine [19, 201 were obtained via excitation transfer from triplet acetone to their ground states. Information about the excited states of cytosine and cytosine derivatives is relatively scarce. The transient absorption spectra of their triplet states have not yet been reported, because of their low intersystem-crossing quantum yields, short lifetimes, and small molar absorption coefficients at room temperature. In the present work, the novel transient species, triplet states of cytosine and cytosine derivatives produced from the laser photolysis of their aqueous solutions using acetone as sensitizer, are observed for the first time.

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2. Experimental

details

Cytosine (Tokyo Kasei), cytidine (Fluka) and dCMP (2’-deoxycytidine-5’-monophosphate) (Sigma) were obtained commercially and used without further purification. Acetone (analytic grade) was distilled before use. MnC12 was recrystallized from triply distilled water. All solutions were made with triple-distilled water just before the experiment, and deaerated by high purity nitrogen (99.99%) bubbling for 20 min. All experiments were carried out at room temperature. Laser photolysis experiments were performed using a home-made excimer laser which provided 248 nm (KrF) pulses with a duration of 20 ns. The maximum energy was 50 mJ per pulse. The source of analysing light was a 500 W Xenon lamp; its intensity was increased about 100 times during the detection of transient absorption. The laser and analysing light beam passed perpendicularly through a quartz cell with an optical path length of 10 mm. The transmitted light entered a monochromator equipped with a lP28 photomultiplier. The signals were collected using a 100 MHz transient recorder, then processed with an IBM-286 personal computer.

3. Results 3.1. Identification of transient species No signals of transient absorption could be observed from the photolysis of the aqueous solution of cytosine or its derivatives, since the intensity of the laser was too low (50 mJ) to produce hydrated electrons. However, as shown in Fig. 1, transient spectra were observed from photolysis of 2~ 10e4 M cytosine in aqueous solution containing 0.13 M acetone. At the end of the pulse, an absorption band appears in the wavelength region of 300-380 nm, similar to that from photolysis of 0.13 M acetone in aqueous solution. This spectrum can be quenched by Mn”. Thus, it is assigned

0.04

0.03 : 5 $0.02 :: 4

i

::::

1

300

350

10

Wavelength

Fig. 1. Transient absorption spectra from the laser photolysis of 2~10~~ M cytosine aqueous solution containing 0.13 M acetone saturated with nitrogen at pH 7 at: 0 /JS (0); 1 ps (0); 5 ps (A); 10 ps (A). Insert: characteristic absorption spectrum of triplet state of cytosine at 1 ps after the pulse, according to eqn. (2).

217

to the triplet state of acetone 3A *. The half-life of 3A* determined in this work is 2.2 ps, at the concentration used (see below). From laser photolysis of aqueous solution containing both cytosine and acetone, a transient absorption band appears in the wavelength region of 300-450 nm which is different from that stemming from aqueous solution of acetone alone (Fig. 1). It is reasonable to suppose that the novel transient species have been induced by 3A*. The transient species decay following first-order kinetics and can be quenched by Mn*’ ions. Thus, the novel transient species should be identified as the triplet states of cytosine (3Cyt*). Similar transient absorptions, as shown in Figs. 2 and 3, have been recorded from laser photolysis of 2~ lop4 M cytidine and dCMP respectively in the presence of 0.13 M acetone in aqueous solution. Based on the above results and kinetic analyses of the decay traces, they are assigned to the triplet states of cytidine and dCMP respectively. 3.2. Characteristic absorption spectra As shown in Fig. 1, as triplet acetone disappears the absorbance of triplet cytosine appears. The experiments indicated that the 71n of 3A* in the presence of cytosine is shorter than that of acetone alone; thus the absorption spectrum at 5 ps after the pulse could be attributed to that of triplet cytosine. In order to confirm the above prediction, we have resolved the composite spectrum at 1 11s after the pulse. It is obvious that before 3A* has disappeared completely the absorbance in the region of 300-380 nm is the overlap of the absorbance of 3A* and 3Cyt*, and the absorbance in the wavelength region longer than 380 nm is a result of the 3Cyt* alone. Let (A,“), and (&“), represent the absorbance of 3Cyt* at Ax (3OO

Laser photolysis of cytosine, cytidine and dCMP in aqueous solution.

Laser photolysis of cytosine and cytosine derivatives (cytidine and 2'-deoxycytidine-5'-monophosphate) using acetone as a sensitizer have been carried...
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