- Email: [email protected]
Characterization of reactive intermediates in laser photolysis of nucleoside using of sodium salt anthraquinone-2-sulfonic acid as photosensitizer
PERGAMON
Radiation Physics and Chemistry Radiation Physics and Chemistry 54 (1999) 491±497
Characterization of reactive intermediates in laser photolysis of nucleoside using of sodium salt anthraquinone-2-sulfonic acid as photosensitizer Jianhua Ma, Weizhen Lin, Wenfeng Wang, Zhenhui Han, Side Yao, Nianyun Lin * Laboratory of Radiation Chemistry, Shanghai Institute of Nuclear Research, Chinese Academy of Science, P.O. Box 800-204, Shanghai 201800, China Accepted 7 July 1998
Abstract The interaction of triplet state of sodium salt of anthraquinone-2-sulfonic acid (AQS) with nucleosides has been investigated in CH3CN using KrF(248 nm) laser ¯ash photolysis. The transient absorption spectra and kinetics obtained from the interaction of triplet AQS and nucleoside demonstrated that the primary ionic radical pair, radical cation of nucleosides and radical anion of AQS has been detected simultaneously for the ®rst time. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Laser photolysis; Anthraquinone-2-sulfonate; Triplet state; Nucleoside; Electron transfer
1. Introduction The photochemistry of anthraquinone derivatives, in particular that of sulfates derivatives of 9,10 anthraquinone has received much attention (Loe et al., 1983, 1984, 1991, 1993) because their con®guration isomers, such as 1 and 2 sulfonates derivatives of anthraquinone have been used as simple model compounds representing ``weak'' and ``strong'' sensitizers. Besides, anthraquinone derivatives may be also used as radiosensitizer (Infante et al., 1982) and cleaving agent for duplex DNA (Armitage et al., 1994; Breslin and Schuster, 1996; Breslin et al., 1997). The behavior of triplet state of ``strong'' sensitizers such as AQS is complicated due to its decay associated with long-lived intermediates produced from triplet AQS-water reaction (Loe et al., 1983). Thus, in order to investigate the photosensitized oxidation of DNA constituents by
* Corresponding author.
AQS CH3CN was used instead of water as solvent, because no long-lived intermediate in CH3CN could be detected. In this work, the electron transfer reactions between triplet AQS and nucleoside in CH3CN have been reported for the ®rst time. The formation and decay rate constants of the AQS radical anion and nucleoside radical cations have been determined simultaneously. The elucidated mechanism of the electron transfer reaction has oered that AQS may be used as a model of photonuclease for selective cleavage of backbone of DNA.
2. Experimental section AQS was obtained from Fluka. Guanosine (dG), cytidine (dC), adenosine (dA), thymidine (T) and deoxyribose were obtained from Sigma. NaNO2, HCl, HClO4, NaOH (analytical grade reagent) and CH3CN (chromatagraphic grade) were used as received.
0969-806X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 9 8 ) 0 0 3 0 0 - 4
492
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
Laser ¯ash photolysis experiments were performed using a KrF excimer laser which provided a 248 nm light pulse with a duration of 20 ns. The maximum laser energy was 50 mJ per pulse. The monitoring light source was a high pressure xenon lamp. The signals were collected using an HP54510B 300 MHz transient recorder and then processed with a PC-486. Detailed descriptions of equipment were given previously (Zuo et al., 1992). All experiments were carried out in CH3CN at room temperature. The sample solutions were deaerated by high-purity nitrogen (99.99%) by bubbling for 20 min. Electrochemical measurement of reduction potential of AQS was carried out in a medium of CH3CN (97%):H2O (3%) by cyclic voltammeter with a sweep rate of 100 mVsÿ1 at Electrochemical Laboratory of Chemistry Department of Fudan University. A large-area Pt was used as counter electrode and Ag+/Ag (SCE) was used as the reference electrode. 3. Results and discussion 3.1. Triplet AQS in CH3CN Fig. 1 shows the transient absorption spectra from laser excitation of 0.3 mM AQS in deaerated CH3CN. At 0.07 ms after laser pulse an absorption band in the
wavelength region 360±580 nm displayed a strong absorption maximum at 380 nm, two weaker peaks around 470 and 580 nm. The spectrum characterized by signals at 380, 470 and 580 nm as a whole can be quenched by Mn2+ or O2 and decayed following ®rstorder kinetics. Thus, it should be assigned to triplet AQS as described in our previous work (Ma et al., 1998). 3.2. Electron transfer reactions between triplet AQS and nucleosides The transient absorption spectrum characterized by lmax at 510 nm from interaction of triplet AQS with NOÿ 2 is shown in Fig. 2 which is in good agreement with that of AQS radical anion reported previously (Loe et al., 1984). Fig. 3 shows transient absorption spectra from laser photolysis of 0.07 mM dG in CH3CN containing 0.3 mM AQS. The absorption spectrum with lmax at 510 nm and 5 ms after the pulse displayed the radical anion from interaction of triplet AQS and dG. Afterward, a slow decaying absorption spectrum also shown in Fig. 4 appeared subsequently at 40 ms after laser pulse. This long-lived species with absorption maximum at 310 nm and another weak absorption maximum at 390 nm decayed following the secondorder rate law, which is very similar to that produced from the electron transfer reaction between dG and tri-
Fig. 1. Transient absorption spectra obtained from laser photolysis of 0.3 mM AQS CH3CN solution at: (r) 0.07 ms; (w) 1 ms; (Q) 10 ms after the laser pulse. Insert: plot of the logarithm of the absorbance of 3AQS at 380 nm vs time.
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
493
Fig. 2. Absorption spectra of radical anion of AQS from laser photolysis of 0.3 mM AQS and 0.1 M NaNO2 in aqueous solution.
plet acetone in CH3CN as shown in Fig. 4 and consistent with that from interaction of triplet acetone with dG in aqueous solution (Jian et al., 1991). Thereafter, it should be assigned to the radical cation of dG. Similarly, transient absorption spectra from interaction of triplet AQS and dA displayed an absorption maximum at 360 nm which is consistent with that from the interaction of triplet acetone with dA and should be assigned to radical cation of dA (not shown). However, the transient absorption spectra of long-lived
transient species from interaction of triplet AQS and dT or dC revealed weak, characterless absorptions, as shown in Fig. 5 from dT. Fig. 6 shows the transient absorption traces from the photolysis of dG in the presence of AQS. It is evident that the growth of a long-lived transient species at 310 nm is synchronous with decay of triplet AQS at 380 nm. Similar absorption time pro®les have also been obtained in the laser photolysis of dA, dT and dC in the presence of AQS (not shown).
Fig. 3. Transient absorption spectra obtained from laser photolysis of 0.07 mM dG in CH3CN solution, containing 0.3 mM AQS: (w) 5 ms; (.) 40 ms after the laser pulse.
494
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
Fig. 4. Transient absorption spectra obtained from laser photolysis of 0.07 mM dG in CH3CN solution, containing: (*) 0.3 mM AQS; (w) 200 mM acetone at 40 ms after the laser pulse.
Additionally, the long-lived radical spectra from dT or dC should also be assigned to the radical cations of pyrimidine nucleoside, because the absorption times pro®les from interaction of triplet AQS with dT or dC and the transient absorption spectrum of radical anion of AQS from interaction of triplet AQS and pyrimidine nucleoside are quite similar to that
from interaction of triplet AQS with dG or dA, respectively. The pseudo-®rst order quenching rate constants of triplet AQS by nucleosides have been determined from the decay trace at 380 nm and the pseudo-®rst order rate constants for formation of radical cations of nucleosides as well as radical anion of AQS have also
Fig. 5. Transient absorption spectra obtained from laser photolysis of 0.07 mM dT in CH3CN solution, containing 0.3 mM AQS: (w) 5 ms; (.) 40 ms after the laser pulse.
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
495
Fig. 6. Transient absorption growth and decay traces obtained by photolysis of 0.3 mM AQS in CH3CN solution containing 0.07 mM dG, growth traces at 310 nm; decay traces at 380 nm.
been determined by the growth trace at 310 and 510 nm, respectively, which are listed in Table 1. From Table 1, it is evident that the rate constants of formation (kf , 310 nm) of radical cations of nucleosides and radical anion of AQS (kf , 510 nm) are nearly equal to those of the decay of triplet AQS (kd, 380 nm). However, the rate constant of decay of radical anion (kd, 510 nm) is higher than that of radical cation (kd, 310 nm).
be detected. Thereafter, both the transient absorption spectra of radical anion of AQS and radical cations of nucleoside from the interaction of triplet AQS with nucleoside have been observed simultaneously, since the lifetime of AQS radical anion is shorter than that of radical cation of nucleoside in CH3CN. The reaction mechanism of quenching of triplet AQS by nucleosides via electron transfer producing ion pair can be described as: 3
4. Discussion The behavior of triplet state of AQS is complicated due to its reaction with water producing long-lived intermediates including the hydroxylation product of AQS. Thus, the interaction of triplet AQS with nucleoside should be inhibited by the reaction between AQS and water. However, the photolysis of AQS is rather simple in CH3CN, because long-lived transient absorption could
AQS pur; pyr4AQS .ÿ pur. ; pyr. :
Furthermore, in order to examine the possibility of hydrogen abstraction from sugar moiety of nucleoside, quenching of triplet AQS by deoxyribose in CH3CN has been performed. As shown in Fig. 7, the decay of transient species with and without deoxyribose displayed the same rate. Thus, the reaction mechanisms for neat electron transfer has been elucidated unambiguously. Moreover, the free energy changes (DG, kJmolÿ1) for triplet AQS reacting with nucleosides were calculated
Table 1 Rate constants for reaction of triplet AQS with nucleoside in CH3CN Nucleoside
kf /105 sÿ1 310 nm
Adenosine Guanosine Cytidine Thymidine
2.7 4.2 1.7 1.3
510 nm
kd/105 sÿ1 380 nm
kd/103 sÿ1, 104 sÿ1 310 nm 510 nm
2.2 3.9 1.9 1.1
2.3 3.9 1.9 1.4
3.9 3.1 2.7 2.4
8.1 9.3 6.7 6.1
496
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
Fig. 7. Transient absorption decay traces at 380 nm obtained by photolysis of: (a) 0.3 mM AQS containing 0.1 mM deoxyribose in CH3CN; (b) 0.3 mM AQS in CH3CN.
according to the Rehm±Weller equation (Rehm and Weller, 1970) DG 96:48 Eox ÿ Ered ÿ e2 =ed ÿ DE0;0 While Eox and Ered are the half-wave potential in volts for the oxidation of the donor and reduction of the acceptor, respectively, DE0,0 is the electronic excitation energy of the triplet AQS (2.68 eV) (Loe et al., 1993). The Ered values of AQS is ÿ0.81 (SCE); The coulombic term e2 /ed is 0.06 eV (Fox and Chanon, 1988). The Eox values (SCE) of DNA bases were calculated based on the empirical equation Eox=0.92 IP ÿ 6.20 (SCE) (Miller et al., 1972), the corresponding IP values were adopted from a previous paper (Nikogosyan, 1990). Thus, the free energy changes for electron transfer reaction for AQS and nucleosides, are shown in Table 2.
Table 2 DG of electron transfer reactions for nucleosides and triplet AQS Nucleosides
Eox (SCE)
DG (kJ molÿ1)
Adenosine Guanosine Cytidine Thymidine
1.53 1.16 1.71 1.80
ÿ38.8 ÿ74.3 ÿ21.0 ÿ12.2
On the basis of comparison of dierence in reactivities between pyrimidine and purine nucleosides with that in DG, it is concluded that the electron transfer reaction should proceed in the ``normal'' free energy region (Bensasson et al., 1993).
Acknowledgements This project was supported by the National Natural Science Commission and National Natural Science Foundation of China.
References Armitage, B., Yu, C., Devadoss, C., Schuster, G.B., 1994 J. Am. Chem. Soc. 116, 9847. Bensasson, R.V., Land, E.J., Truscott, T.G., 1993. Excited states and free radicals in biology and medicine: contributions from ¯ash photolysis and pulse radiolysis. Oxford University Press, Oxford. Breslin, D.T., Schuster, G.B., 1996 J. Am. Chem. Soc. 118, 2311. Breslin, D.T., Coury, J.E., Anderson, J.R., McFail-Isom, L., Kan, Y., Williams, L.D., Bottomley, L.A., Schuster, G.B., 1997 J. Am. Chem. Soc. 119, 5043. Fox, M.A., Chanon, M., 1988. Photoinduced electron transfer. Elsevier, Amsterdam. Infante, G.A.P., Gonzalez, D., Cruz, J., Correa, J.A., Myers, M.F., Ahmad, W.L., Whitter, A., Santos, A.S., Neta, P., 1982 Rad. Res. 92, 307.
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497 Jian, L., Wang, W.F., Zheng, Z.D., Yao, S.D., Zhang, J.S., Lin, N.Y., 1991 Res. Chem. Intermed. 15, 283. Loe, I., Treinin, A., Linschitz, H., 1983 J. Phys. Chem. 87, 2536. Loe, I., Treinin, A., Linschitz, H., 1984 J. Phys. Chem. 88, 4931. Loe, I., Goldstein, S., Treinin, A., Linschitz, H., 1991 J. Phys. Chem. 95, 4423. Loe, I., Rabani, J., Treinin, A., Linschitz, H., 1993 J. Am. Chem. Soc. 115, 8933.
497
Ma, J.H., Lin, W.Z., Wang, W.F., Yao, S.D., Lin, N.Y., 1998. . Radiat. Phys. Chem. To be published. Miller, L.L., Nordblom, G.D., Mageda, E.A., 1972 J. Org. Chem. 37, 916. Nikogosyan, D.N., 1990 Int. J. Radiat. Biol. 57, 233. Rehm, D., Weller, D., 1970 Isr-J. Chem. 8, 259. Zuo, Z.H., Yao, S.D., Luo, J., Wang, W.F., Zhang, J.S., Lin, N.Y., 1992 J. Photochem. Photobiol. B: Biol. 15, 215.
Radiation Physics and Chemistry Radiation Physics and Chemistry 54 (1999) 491±497
Characterization of reactive intermediates in laser photolysis of nucleoside using of sodium salt anthraquinone-2-sulfonic acid as photosensitizer Jianhua Ma, Weizhen Lin, Wenfeng Wang, Zhenhui Han, Side Yao, Nianyun Lin * Laboratory of Radiation Chemistry, Shanghai Institute of Nuclear Research, Chinese Academy of Science, P.O. Box 800-204, Shanghai 201800, China Accepted 7 July 1998
Abstract The interaction of triplet state of sodium salt of anthraquinone-2-sulfonic acid (AQS) with nucleosides has been investigated in CH3CN using KrF(248 nm) laser ¯ash photolysis. The transient absorption spectra and kinetics obtained from the interaction of triplet AQS and nucleoside demonstrated that the primary ionic radical pair, radical cation of nucleosides and radical anion of AQS has been detected simultaneously for the ®rst time. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Laser photolysis; Anthraquinone-2-sulfonate; Triplet state; Nucleoside; Electron transfer
1. Introduction The photochemistry of anthraquinone derivatives, in particular that of sulfates derivatives of 9,10 anthraquinone has received much attention (Loe et al., 1983, 1984, 1991, 1993) because their con®guration isomers, such as 1 and 2 sulfonates derivatives of anthraquinone have been used as simple model compounds representing ``weak'' and ``strong'' sensitizers. Besides, anthraquinone derivatives may be also used as radiosensitizer (Infante et al., 1982) and cleaving agent for duplex DNA (Armitage et al., 1994; Breslin and Schuster, 1996; Breslin et al., 1997). The behavior of triplet state of ``strong'' sensitizers such as AQS is complicated due to its decay associated with long-lived intermediates produced from triplet AQS-water reaction (Loe et al., 1983). Thus, in order to investigate the photosensitized oxidation of DNA constituents by
* Corresponding author.
AQS CH3CN was used instead of water as solvent, because no long-lived intermediate in CH3CN could be detected. In this work, the electron transfer reactions between triplet AQS and nucleoside in CH3CN have been reported for the ®rst time. The formation and decay rate constants of the AQS radical anion and nucleoside radical cations have been determined simultaneously. The elucidated mechanism of the electron transfer reaction has oered that AQS may be used as a model of photonuclease for selective cleavage of backbone of DNA.
2. Experimental section AQS was obtained from Fluka. Guanosine (dG), cytidine (dC), adenosine (dA), thymidine (T) and deoxyribose were obtained from Sigma. NaNO2, HCl, HClO4, NaOH (analytical grade reagent) and CH3CN (chromatagraphic grade) were used as received.
0969-806X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 9 8 ) 0 0 3 0 0 - 4
492
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
Laser ¯ash photolysis experiments were performed using a KrF excimer laser which provided a 248 nm light pulse with a duration of 20 ns. The maximum laser energy was 50 mJ per pulse. The monitoring light source was a high pressure xenon lamp. The signals were collected using an HP54510B 300 MHz transient recorder and then processed with a PC-486. Detailed descriptions of equipment were given previously (Zuo et al., 1992). All experiments were carried out in CH3CN at room temperature. The sample solutions were deaerated by high-purity nitrogen (99.99%) by bubbling for 20 min. Electrochemical measurement of reduction potential of AQS was carried out in a medium of CH3CN (97%):H2O (3%) by cyclic voltammeter with a sweep rate of 100 mVsÿ1 at Electrochemical Laboratory of Chemistry Department of Fudan University. A large-area Pt was used as counter electrode and Ag+/Ag (SCE) was used as the reference electrode. 3. Results and discussion 3.1. Triplet AQS in CH3CN Fig. 1 shows the transient absorption spectra from laser excitation of 0.3 mM AQS in deaerated CH3CN. At 0.07 ms after laser pulse an absorption band in the
wavelength region 360±580 nm displayed a strong absorption maximum at 380 nm, two weaker peaks around 470 and 580 nm. The spectrum characterized by signals at 380, 470 and 580 nm as a whole can be quenched by Mn2+ or O2 and decayed following ®rstorder kinetics. Thus, it should be assigned to triplet AQS as described in our previous work (Ma et al., 1998). 3.2. Electron transfer reactions between triplet AQS and nucleosides The transient absorption spectrum characterized by lmax at 510 nm from interaction of triplet AQS with NOÿ 2 is shown in Fig. 2 which is in good agreement with that of AQS radical anion reported previously (Loe et al., 1984). Fig. 3 shows transient absorption spectra from laser photolysis of 0.07 mM dG in CH3CN containing 0.3 mM AQS. The absorption spectrum with lmax at 510 nm and 5 ms after the pulse displayed the radical anion from interaction of triplet AQS and dG. Afterward, a slow decaying absorption spectrum also shown in Fig. 4 appeared subsequently at 40 ms after laser pulse. This long-lived species with absorption maximum at 310 nm and another weak absorption maximum at 390 nm decayed following the secondorder rate law, which is very similar to that produced from the electron transfer reaction between dG and tri-
Fig. 1. Transient absorption spectra obtained from laser photolysis of 0.3 mM AQS CH3CN solution at: (r) 0.07 ms; (w) 1 ms; (Q) 10 ms after the laser pulse. Insert: plot of the logarithm of the absorbance of 3AQS at 380 nm vs time.
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
493
Fig. 2. Absorption spectra of radical anion of AQS from laser photolysis of 0.3 mM AQS and 0.1 M NaNO2 in aqueous solution.
plet acetone in CH3CN as shown in Fig. 4 and consistent with that from interaction of triplet acetone with dG in aqueous solution (Jian et al., 1991). Thereafter, it should be assigned to the radical cation of dG. Similarly, transient absorption spectra from interaction of triplet AQS and dA displayed an absorption maximum at 360 nm which is consistent with that from the interaction of triplet acetone with dA and should be assigned to radical cation of dA (not shown). However, the transient absorption spectra of long-lived
transient species from interaction of triplet AQS and dT or dC revealed weak, characterless absorptions, as shown in Fig. 5 from dT. Fig. 6 shows the transient absorption traces from the photolysis of dG in the presence of AQS. It is evident that the growth of a long-lived transient species at 310 nm is synchronous with decay of triplet AQS at 380 nm. Similar absorption time pro®les have also been obtained in the laser photolysis of dA, dT and dC in the presence of AQS (not shown).
Fig. 3. Transient absorption spectra obtained from laser photolysis of 0.07 mM dG in CH3CN solution, containing 0.3 mM AQS: (w) 5 ms; (.) 40 ms after the laser pulse.
494
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
Fig. 4. Transient absorption spectra obtained from laser photolysis of 0.07 mM dG in CH3CN solution, containing: (*) 0.3 mM AQS; (w) 200 mM acetone at 40 ms after the laser pulse.
Additionally, the long-lived radical spectra from dT or dC should also be assigned to the radical cations of pyrimidine nucleoside, because the absorption times pro®les from interaction of triplet AQS with dT or dC and the transient absorption spectrum of radical anion of AQS from interaction of triplet AQS and pyrimidine nucleoside are quite similar to that
from interaction of triplet AQS with dG or dA, respectively. The pseudo-®rst order quenching rate constants of triplet AQS by nucleosides have been determined from the decay trace at 380 nm and the pseudo-®rst order rate constants for formation of radical cations of nucleosides as well as radical anion of AQS have also
Fig. 5. Transient absorption spectra obtained from laser photolysis of 0.07 mM dT in CH3CN solution, containing 0.3 mM AQS: (w) 5 ms; (.) 40 ms after the laser pulse.
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
495
Fig. 6. Transient absorption growth and decay traces obtained by photolysis of 0.3 mM AQS in CH3CN solution containing 0.07 mM dG, growth traces at 310 nm; decay traces at 380 nm.
been determined by the growth trace at 310 and 510 nm, respectively, which are listed in Table 1. From Table 1, it is evident that the rate constants of formation (kf , 310 nm) of radical cations of nucleosides and radical anion of AQS (kf , 510 nm) are nearly equal to those of the decay of triplet AQS (kd, 380 nm). However, the rate constant of decay of radical anion (kd, 510 nm) is higher than that of radical cation (kd, 310 nm).
be detected. Thereafter, both the transient absorption spectra of radical anion of AQS and radical cations of nucleoside from the interaction of triplet AQS with nucleoside have been observed simultaneously, since the lifetime of AQS radical anion is shorter than that of radical cation of nucleoside in CH3CN. The reaction mechanism of quenching of triplet AQS by nucleosides via electron transfer producing ion pair can be described as: 3
4. Discussion The behavior of triplet state of AQS is complicated due to its reaction with water producing long-lived intermediates including the hydroxylation product of AQS. Thus, the interaction of triplet AQS with nucleoside should be inhibited by the reaction between AQS and water. However, the photolysis of AQS is rather simple in CH3CN, because long-lived transient absorption could
AQS pur; pyr4AQS .ÿ pur. ; pyr. :
Furthermore, in order to examine the possibility of hydrogen abstraction from sugar moiety of nucleoside, quenching of triplet AQS by deoxyribose in CH3CN has been performed. As shown in Fig. 7, the decay of transient species with and without deoxyribose displayed the same rate. Thus, the reaction mechanisms for neat electron transfer has been elucidated unambiguously. Moreover, the free energy changes (DG, kJmolÿ1) for triplet AQS reacting with nucleosides were calculated
Table 1 Rate constants for reaction of triplet AQS with nucleoside in CH3CN Nucleoside
kf /105 sÿ1 310 nm
Adenosine Guanosine Cytidine Thymidine
2.7 4.2 1.7 1.3
510 nm
kd/105 sÿ1 380 nm
kd/103 sÿ1, 104 sÿ1 310 nm 510 nm
2.2 3.9 1.9 1.1
2.3 3.9 1.9 1.4
3.9 3.1 2.7 2.4
8.1 9.3 6.7 6.1
496
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497
Fig. 7. Transient absorption decay traces at 380 nm obtained by photolysis of: (a) 0.3 mM AQS containing 0.1 mM deoxyribose in CH3CN; (b) 0.3 mM AQS in CH3CN.
according to the Rehm±Weller equation (Rehm and Weller, 1970) DG 96:48 Eox ÿ Ered ÿ e2 =ed ÿ DE0;0 While Eox and Ered are the half-wave potential in volts for the oxidation of the donor and reduction of the acceptor, respectively, DE0,0 is the electronic excitation energy of the triplet AQS (2.68 eV) (Loe et al., 1993). The Ered values of AQS is ÿ0.81 (SCE); The coulombic term e2 /ed is 0.06 eV (Fox and Chanon, 1988). The Eox values (SCE) of DNA bases were calculated based on the empirical equation Eox=0.92 IP ÿ 6.20 (SCE) (Miller et al., 1972), the corresponding IP values were adopted from a previous paper (Nikogosyan, 1990). Thus, the free energy changes for electron transfer reaction for AQS and nucleosides, are shown in Table 2.
Table 2 DG of electron transfer reactions for nucleosides and triplet AQS Nucleosides
Eox (SCE)
DG (kJ molÿ1)
Adenosine Guanosine Cytidine Thymidine
1.53 1.16 1.71 1.80
ÿ38.8 ÿ74.3 ÿ21.0 ÿ12.2
On the basis of comparison of dierence in reactivities between pyrimidine and purine nucleosides with that in DG, it is concluded that the electron transfer reaction should proceed in the ``normal'' free energy region (Bensasson et al., 1993).
Acknowledgements This project was supported by the National Natural Science Commission and National Natural Science Foundation of China.
References Armitage, B., Yu, C., Devadoss, C., Schuster, G.B., 1994 J. Am. Chem. Soc. 116, 9847. Bensasson, R.V., Land, E.J., Truscott, T.G., 1993. Excited states and free radicals in biology and medicine: contributions from ¯ash photolysis and pulse radiolysis. Oxford University Press, Oxford. Breslin, D.T., Schuster, G.B., 1996 J. Am. Chem. Soc. 118, 2311. Breslin, D.T., Coury, J.E., Anderson, J.R., McFail-Isom, L., Kan, Y., Williams, L.D., Bottomley, L.A., Schuster, G.B., 1997 J. Am. Chem. Soc. 119, 5043. Fox, M.A., Chanon, M., 1988. Photoinduced electron transfer. Elsevier, Amsterdam. Infante, G.A.P., Gonzalez, D., Cruz, J., Correa, J.A., Myers, M.F., Ahmad, W.L., Whitter, A., Santos, A.S., Neta, P., 1982 Rad. Res. 92, 307.
J. Ma et al. / Radiation Physics and Chemistry 54 (1999) 491±497 Jian, L., Wang, W.F., Zheng, Z.D., Yao, S.D., Zhang, J.S., Lin, N.Y., 1991 Res. Chem. Intermed. 15, 283. Loe, I., Treinin, A., Linschitz, H., 1983 J. Phys. Chem. 87, 2536. Loe, I., Treinin, A., Linschitz, H., 1984 J. Phys. Chem. 88, 4931. Loe, I., Goldstein, S., Treinin, A., Linschitz, H., 1991 J. Phys. Chem. 95, 4423. Loe, I., Rabani, J., Treinin, A., Linschitz, H., 1993 J. Am. Chem. Soc. 115, 8933.
497
Ma, J.H., Lin, W.Z., Wang, W.F., Yao, S.D., Lin, N.Y., 1998. . Radiat. Phys. Chem. To be published. Miller, L.L., Nordblom, G.D., Mageda, E.A., 1972 J. Org. Chem. 37, 916. Nikogosyan, D.N., 1990 Int. J. Radiat. Biol. 57, 233. Rehm, D., Weller, D., 1970 Isr-J. Chem. 8, 259. Zuo, Z.H., Yao, S.D., Luo, J., Wang, W.F., Zhang, J.S., Lin, N.Y., 1992 J. Photochem. Photobiol. B: Biol. 15, 215.