ABSTRACT

The transient absorption spectra and kinetics of excited triplet state of anthraquinone derivatives 2-anthraquinonesulfonatesodium (AQS) and 2- deoxythymidine (dT) have been investigated in CH₃CN-H₂O (97:3) using the time-resolved laser flash photolysis technique (KrF, 248 nm). The absorption spectra of dT radical cation and the radical anion of AQS have been observed. From dynamic and thermodyrnamic analysis, the mechanism of this transient reaction has been initially analysed.

KEYWORDS

2-Anthraquinonesulfonatesodium (AQS); 2-Deoxythymidine; Transient Reaction; Laser Flash Photolysis

1. INTRODUCTION

Quinones play central roles in aerobic respiration and energy-yielding photosynthesis [1]. The “strong sensitizer” 2-anthraquinonesulfonatesodium (AQS), has received much attention because of its relevance to some important photosensitizing effects induced by anthraquinones, phototendering of cellulosic materials. Recently, interest in photobiology area has been heightened by suggestions for utilizing anthraquinones as photonuclease [2]. AQS is a strong sensitizer and for numerous investigations concerning on it [3-5], only a few studies have been performed on the reaction of AQS with biological substances [5]. Here we report that time-resolved spectroscopic and kinetic evidences for the interaction of AQS with dT in CH₃CN-H₂O solvent mixture were investigated using KrF laser flash photolysis. The transient absorption spectra and kinetics obtained from electron transfer oxidation of dT by triplet AQS indicated that the produced radical ion pairs, radical cations of dT and radical anion of AQS, were identified simultaneously.

2. EXPERIMENTAL

2-Anthraquinonesulfonatesodium (AQS) (Fluka, >98%) was recrystallized twice from triply distilled water before use. 2-Deoxythymidine (dT) was purchased from sigma. All the samples were prepared in triply distiled water and deoxygenated by bubbling with high purity nitrogen (99.99%). All experiments were carried at room temperature. Laser flash photolysis experimental methods and the specifications of equipment (KrF excimer laser: 248 nm; pulse wide: 20 ns, 50 mJ) has been described previously [6].

3. RESULTS AND DISCUSSION

3.1. The Characteristic Transient Absorption Spectra of the H-Abstraction Radicals of Deoxyribonucledeotide Bases

Figure 1 shows the transient absorption spectra at 1 μs after laser photolysis of N₂ saturate CH₃CN-H₂O (97:3) solution containing 1 × 10⁻⁴ mol·dm⁻³ deoxyribonucledeotide bases (TMP, dCMP, dGMP, dAMP) and 2 × 10² mol·dm⁻³ K₂S₂O₈ and 2 × 10⁻² mol·dm⁻³ t-BuOH, respectively. These characterized transient absorption peaks are very similarity that reported previously [5-7],

should be reasonably ascribed to the radical cation of Habstraction radicals of deoxyribonucledeotide bases. It is clear that the same radical cation species were produced in different experiments. For example, the one of generation of dT cation radical can be illustrated as below:

t-BuOH + ·OH → CH₂C(CH₃)₂OH + H₂O     (1)

e_-aq + →+          (2)

dT + → dT⁺ + + H⁺       (3)

3.2. The Transient Absorption Spectra of the Interaction of AQS with dT

In our present paper, the characteristic transient absorption spectra of radical anion of AQS (AQS⁻) and triplet AQS (³AQS^*) are from laser photolysis in deaerated CH₃CN-H₂O (97:3) solution have been repored, and the spectrum characterized at 510 nm and 380 nm, 470 nm, 580 nm have assigned to radical anion of AQS (AQS⁻) and triplet AQS (³AQS^*), respectively [8].

Figure 2 shows transient absorption spectra from laser photolysis of 0.028AQS mmol·dm⁻³ and 0.05 mmol·dm⁻³ dT in CH₃CN-H₂O (97:3) solution, saturated with N₂O. After the pulse，an absorption band characterized at ~510 nm and decays by second order kinetics, which is similar to that of the radical anion of AQS reported previously [8], and should be assigned to AQS⁻. The late transient absorption characterized rang 350 nm - 450 nm，should be reasonably ascribed to the radical cation of dT-H due to its similarity to that the radical cation of dT in Figure 1 and photoinization of deoxyribonucleotide [8,9]. The growth trace of transient species at 510 nm was occurred exactly in the same time interval as did the AQS triplet decay at 580 nm (Figure 3), which decayed following first-order kinetics, and it is the net contribution of triplet AQS (³AQS^*), as shown Figure 4.

So we implying that the AQS triplet state is the precursor of the radical anion and transient species at 350 nm - 450 nm, revealing the feature of dT⁺ characterized. It is evident that ³AQS^* was quenched by dT via electron transfer producing long lived dT⁺ species.

3.3. The kinetic Parameters of Transient Species and Free Energy Change (ΔG) of Electron Transfer between AQS and dT

In the reaction of electron transfer between AQS and dT, the formation trace of dT⁺ at 360 nm can be obtained by subtracting the absorbance at 580 nm multiplied by A³⁶⁰/A⁵⁸⁰ from that at 360 nm. At mean time, the formation trace of AQS⁻ at 510 nm can be obtained by subtracting the absorbance at 580 nm multiplied by A⁵¹⁰/ A⁵⁸⁰ from that at 510 nm too. The corresponding kinetic parameters of transient species growth at 360 nm, 510 nm and triplet AQS decay at 580 nm are 1.2 × 10⁹ dm³·mol⁻¹·s⁻¹, 1.6 × 10⁹ dm³·mol⁻¹·s⁻¹ and 1.5 × 10⁹ dm³·mol⁻¹·s⁻¹, respectively. Obviously, the formation rate constants of radical cations of dT and radical anion of AQS are nearly equal to that of decay of triplet AQS respectively. On the other hand, The ΔG (free energy changes) for the electron transfer of between triplet AQS and dT can be calculated according to the Rehm-Weller equation and the E_ox values of dT was calculated via an

empirical equation E_ox = 0.89IP - 6.04 [10], and the E_red (AQS, vs. SCE), ΔE_0,0, ΔG (energy of ³AQS^*), e²/εd and the coulombic term are published results [11-14], respectively. From these, free energy change (ΔG) value (KJ/ mol) was calculated as dT—68.3 KJ/mol. It means that the electron transfer oxidation reactions between triplet AQS and dT is exothermic.

From the above results, these experimental findings provide reliable evidence for the electron transfer oxidation reactions between triplet AQS and dT. The predominant initial species from electron transfer oxidation of dT is dT radical cation. The proposed reaction mechanism of generation of the dT radical cation and the radical anion of AQS can be presented as follows:

hv       ISC AQS → AQS^* → ³AQS^*

³AQS^* + dT → AQS⁻ + dT⁺

4. CONCLUSION

The transient absorption spectra of interaction of triplet AQS with dT were observed and the rate constants for formation of radical cation of dT and those of radical anion of AQS and the decay of triplet AQS were also determined. The time resolved evidence of dynamic and thermo-dynamic of laser spectra and kinetics of radical ion pairs from electron transfer oxidation of dT by triplet AQS were provided.

ACKNOWLEDGEMENTS

This project was supported by the Education Committee of Fujian Province, China (JA13593S).

REFERENCES