- Email: [email protected]
Evidence of temperature dependent activation barriers for near-threshold aqueous photoionization of 2′-deoxyguanosine and tryptophan
28 September 2001
Chemical Physics Letters 346 (2001) 97±102
www.elsevier.com/locate/cplett
Evidence of temperature dependent activation barriers for near-threshold aqueous photoionization of 20-deoxyguanosine and tryptophan George A. Papadantonakis a, Kenneth L. Stevenson b, Pierre R. LeBreton a,* a
Department of Chemistry (m/c 111), University of Illinois at Chicago, 845 Wext Taylor Street, Room 4500 SES, Chicago, IL 60607, USA b Department of Chemistry, Indiana University Purdue University at Fort Wayne, Fort Wayne, IN 46805, USA Received 19 April 2001; in ®nal form 1 June 2001
Abstract Alkaline 20 -deoxyguanosine undergoes one- and two-photon laser photoionization at 266 nm. Similar to tryptophan and indole, the one-photon quantum yield increases ®ve-fold between 296 and 362 K. Earlier Arrhenius analyses of tryptophan and indole photoionization rate constants, yielded unreasonably large pre-exponential factors (1013 ± 1018 s 1 ). An analysis employing pre-exponential factors for diusion controlled reactions (1010 ±1011 s 1 ) provides evidence of activation barriers that decrease with increasing temperature over a range where water structure undergoes signi®cant changes. In the context of Marcus theory, this analysis suggests that curvatures of the electron-transfer free energy surfaces decrease as temperature increases. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Direct photoionization of guanine and tryptophan (Trp) are implicated in mechanisms associated with UV damage of DNA [1±3] and proteins [4,5]. In the gas-phase and in aqueous environments, guanine has the smallest ionization energy of all the DNA bases [1,6±9]. However, little is known about near-threshold aqueous DNA photoionization. Earlier investigations [10] indicate that the 248 nm, one-photon quantum yields for aqueous photoionization of guanine and of the RNA nucleoside, guanosine, are 0.061
*
Corresponding author. Fax: +1-312-996-0431. E-mail address: [email protected] (P.R. LeBreton).
and 0.016 at 288 K and pH 11.0±11.5. For guanine and guanosine at neutral pH, only twophoton 248 nm ionization is reported [2,10]. Recently, a surprisingly large value (0.083) was reported for the 266 nm one-photon ionization quantum yield of guanosine (Guo), at neutral pH [11]. The low-energy aqueous photoionization of Trp, and of its chromophore, indole, has been the focus of numerous experiments employing both conventional and laser light sources [12±16]. Interestingly, photoionization quantum yields of Trp and indole increase four-fold or more over the temperature range 278±348 K [12,13,15], indicating the occurrence of an activation barrier. Barriers also occur in attachment reactions of solvated electrons with indole radical cations and H3 O [16,17]. Earlier analyses of the temperature
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 8 6 8 - 5
98
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
dependence of rates for indole and Trp photoionization using the Arrhenius equation ke T Ae
DGz =RT
1
and based on plots of ln ke versus 1=T yield values of activation barriers heights in the range 50.0± 65.0 kJ/mol and values of pre-exponential factors in the range 1013 ±1018 s 1 [13,15,18]. An earlier evaluation of these results [15] concluded that the pre-exponential factors, which this analysis provides are unreasonably large. While Trp and indole aqueous photoionization have not been considered in the context of the Marcus [19] theory of electron transfer, the theory provides a framework in which to analyze photoionization as an electron-transfer process. In this framework, the transfer is from an excited donor to a solvation environment that accepts the photoejected electron. The goal of the present investigation is to examine photoionization of alkaline 20 -deoxyguanosine (dGuo ) at near-threshold energy, to compare dGuo results with earlier results for Trp, and to reconsider the temperature dependence of near-threshold aqueous photoionization.
dition of 2 M NaOH. Photolysis of 1.0 ml aqueous solutions of 20 -dGuo was carried out in a 1.0 cm ¯ow-through cuvette. In experiments at pH 11.4, (1:2 10 4 M) the absorbance of dGuo at 266 nm was 1.42 and remained unchanged over the temperature range examined. Before photolysis, O2 was removed from 200.00 ml of each solution by purging for 30 min with Ar gas. Sample temperature was controlled by immersing a stock solution in a VWR Model 1131 constant temperature bath, and pumping the thermostated sample into the cuvette immediately before irradiation. Photoionization quantum yields (/e ) were calculated [12] by employing the relationship /net 0:52AdGuo =ANa4 Fe CN6 :
2
Here, AdGuo and ANa4 Fe CN6 are the transient absorbances at 670 nm of the hydrated electrons produced immediately (i.e. at t 0) following 266 nm laser photoionization of dGuo and 0.001 M Na4 Fe CN6 , respectively. Laser pulse energies were determined from a calculation [12] employing values of ANa4 Fe CN6 and the value (0.52) [20] of the monophotonic photoionization quantum yield of Na4 Fe CN6 .
2. Experimental
3. Results
Photoionization experiments were carried out with unfocused 266 nm, 7 ns pulses from a Surelite Nd-YAG laser. The 266 nm light was generated by twice doubling the frequency of the 1064 nm fundamental laser pulses in KDP crystals. Transient absorption spectra were measured with a pulsed 75 W Xe arc lamp (Photon Technology International), triggered by the laser pulse and controlled with a laser diode driver (Analog Modules). The analyzing beam was collinear with the laser beam and dispersed with a monochromator (Photon Technology International) equipped with a Hammamatsu R936 photomultiplier. The photoionization experimental procedure was the same as described earlier [12]. 20 -deoxyguanosine, 99±100% pure, was obtained from Sigma, and solutions were prepared in doubly distilled water. The natural pH of dGuo is 6.3; a pH of 11.4 was obtained by drop-wise ad-
Fig. 1 shows transient absorption spectra obtained after 266 nm irradiation of 20 -dGuo at
Fig. 1. Transient absorption spectra of Guo at 296 K, and pH 11.4 () and 6.3 (N) measured at a delay time of 200 ns after irradiation with 266 nm, 13 mJ, 7 ns laser pulses.
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
296 K, and pH 11.4 and 6.3. The pH 11.4 data exhibits a broad absorption band with a maximum (700±720 nm) associated with hydrated electrons [21]. At pH 6.3, there is no evidence of electron production under the conditions of these experiments. For the measurements at high pH, electron absorbance at 670 nm does not increase linearly with increasing laser power. A double logarithmic plot of the 670 nm absorbance versus laser pulse energy has a slope of 1.63. This value is between values of 1.0 and 2.0 associated with mono- and biphotonic processes, respectively [22], and indicates that both processes occur. Trp photoionization at 266 nm exhibits similar behavior [12]. Fig. 2, shows quantum yields obtained from Eq. (2) for 266 nm photoionization of dGuo at pH 11.4 for laser powers between 3 and 13 mJ. Results are given at 296, 330 and 362 K. At each temperature, the biphotonic component increases with the laser energy [22], while the monophotonic component is independent of laser energy and equals the intercept [23]. The monophotonic quantum yield is 0:011 0:002 at 296 K and increases with increasing temperature. Fig. 3 shows the monophotonic quantum yields for 266 nm photoionization of dGuo at pH 11.4 over the temperature range 296±362 K. The results indicate that the quantum yield increases ®ve-fold as the temperature increases. In addition to the
99
Fig. 3. Plots of monophotonic quantum yields for the 266 nm photoionization of dGuo at pH 11.4 () as a function of temperature. Results were obtained with 13 mJ, 7 ns pulses. Error bars correspond to standard deviations in graphical determination of the intercept from three separate measurements of plots of quantum yield versus laser energy. The ®gure also shows similar results taken from [12] for the 266 nm one-photon ionization of Trp () at pH 5.9. The scales of the dGuo and Trp quantum yields are given on the left and right, respectively. The insert shows plots of the ratio of the rate constants for onephoton ionization at temperature, T, and at 296 K (ke T =ke (296)) versus temperature for Trp at pH 11.0 () and pH 7.0 (D), indole at pH 3.5 (j), and tyrosine at pH 12.0 (}).
data for dGuo , Fig. 3 shows previously reported quantum yields for the one-photon ionization of Trp [12]. While the quantum yield for Trp at 296 K is four times larger than that of dGuo , the increases in the relative quantum yields of dGuo and Trp over the temperature range examined are similar. Fig. 3 also contains an insert which shows previously reported ratios of the rate constants for one-photon ionization at varying temperature to the rate constant at 296 K (ke T =ke 296) for Trp [13], indole [15] and tyrosine [24,25]. 4. Discussion
Fig. 2. Plot of the photoionization quantum yields (/e ) of dGuo at pH 11.4 versus pulse energy measured with 266 nm, 7 ns pulses at 296 (j), 330 () and 362 K (.).
The results in Fig. 1 demonstrate that the electron absorption spectrum from dGuo at pH 11.4 is easily measured while, under the present conditions, no absorbance associated with transient electrons is observed from dGuo at pH 6.3. The results in Fig. 1, together with results in Figs. 2 and 3, provide evidence that the one-photon quantum yield of dGuo at neutral pH and room tempera-
100
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
ture is signi®cantly smaller than the value of 0.083 reported from other recent 266 nm laser experiments on guanosine [11]. 4.1. Ionization over a temperature dependent activation barrier Earlier descriptions of the temperature dependence of rate constants for Trp and indole photoionization, that yield anomalously large preexponential factors, employed energy barriers that are not dependent on temperature [13,15]. Another possibility is that the barriers are temperature dependent, and decrease in height as the temperature increases. This is shown in Fig. 4a. Here, the ratio of the photoionization rate constants (ke ) at two temperatures, T1 and T2 , is given by
ke T1 A T1 e ke T2 A T2 e
DGz T1 =RT1 DGz T2 =RT2
:
3
Using a previously reported ke value of 8 107 s 1 for the 264 nm photoionization of Trp at pH 11.0 and 291 K [13], and pre-exponential factors associated with diusion controlled reactions (1010 ±1011 s 1 [26]) yields values of 12.0±18.0 kJ/ mol for DGz . Assuming that, over the temperature range examined, the pre-exponential factor remains nearly constant [26], an analysis of the data in the insert of Fig. 3 using Eq. (3) indicates that, as the temperature increases by 30 K, DGz decreases to 9.0 to 15.0 kJ/mol. Similar results are obtained from an analysis of the photoionization rate constant of indole at 254 nm [15]. Here, the barrier height decreases between 1.0 and 2.0 kJ/
Fig. 4. (a) Schematic diagram showing the decrease in the activation barrier height (DGz ) for dGuo one-photon ionization, which occurs as the temperature increases from T1 to T2 and (b) Free energy curves for dGuo and dGuo eaq , showing the decrease in the curvatures, reorganization energy (k), and barrier height (DG ) as the temperature increases from T1 to T2 .
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
mol when the temperature increases from 298 to 313 K. The large pre-exponential factor for indole ionization, obtained when a temperature independent barrier is employed, was previously attributed, in an unspeci®ed way, to indole±water interactions [15]. The present consideration of a temperature dependent barrier also suggests that water plays a key role in determining the barrier properties for near-threshold photoionization. This is consistent with the observation that the relative increases in the rate constants are similar for the dierent molecules in the insert in Fig. 3. For tyrosine at pH 12.0, and Trp at pH 11.0 and pH 7.0, the rate constants increase 3±11 times in the temperature range 296±333 K. Speculation about the importance of water structure leads to a description of near-threshold photoionization in which electron separation occurs over fragile barriers that are disrupted at elevated temperatures. In this description, the barrier heights (9.0±12.0 kJ/ mol) and the changes that occur over the temperature range examined are similar in magnitude to the free energy of a hydrogen bond in liquid water (10.6±11.0 kJ/mol [27,28]). Investigations of the temperature dependence of hydrogen bond strength [29] and of the structure of liquid water employing quantum mechanical calculations, and IR and NMR results [29±32] provide evidence of signi®cant water structure changes over the temperature range examined in the Trp, indole and dGuo photoionization experiments. 4.2. Marcus electron-transfer theory A simple application of Marcus electron-transfer theory provides a similar picture. Within the Marcus framework, the energies associated with the initial and ®nal states are shown schematically in Fig. 4b. Under normal conditions [19], the barrier for electron transfer, DG , increases as the reorganization energy, k, increases. Generally, the major contribution to k is associated with solvent relaxation [33]. In the systems examined here, where solvation energies are large, the increase in the photoionization quantum yield with temperature is depicted in Fig. 4 as a decrease in the curvature of the free energy surfaces which results in
101
decreases in k and G . The decrease in curvature is caused by a decrease in water orientational polarization in the vicinity of dGuo , in the initial and ®nal states, that occurs with increasing temperature. The relationship between solvent polarization and curvature of the free energy surfaces, considered here, is similar to that examined in earlier investigation of the in¯uence of electrostatics on electron-transfer rate constants [34]. Acknowledgements Support by the Petroleum Research Fund of the American Chemical Society and by the American Cancer Society (RPG-91-024-09-CNE), and the help of Mr. Manuel Arroyo-Vega in carrying out these experiments is gratefully acknowledged. References [1] T. Douki, T. Delatour, R. Martini, J. Cadet, J. Chim. Phys. Chim. Biol. 96 (1999) 138. [2] L.P. Candeias, S. Steenken, J. Am. Chem. Soc. 114 (1992) 699. [3] D. Angelov, A. Spassky, M. Berger, J. Cadet, J. Am. Chem. Soc. 119 (1997) 11373. [4] L.I. Grossweiner, Curr. Eye Res. 3 (1984) 137. [5] J.A. Shauerte, A. Gafni, Biochem. Biophys. Res. Commun. 212 (1995) 900. [6] H.S. Kim, P.R. LeBreton, J. Am. Chem. Soc. 118 (1996) 3694. [7] H. Fernando, N.S. Kim, G.A. Papadantonakis, P.R. LeBreton, in: N.B. Leontis, J. SantaLucia Jr. (Eds.), Molecular Modeling of Nucleic Acids, ACS Symposium Series 682, Washington, DC, 1997, p. 18. [8] N.S. Kim, Q. Zhu, P.R. LeBreton, J. Am. Chem. Soc. 121 (1999) 11516. [9] H. Fernando, G.A. Papadantonakis, N.S. Kim, P.R. LeBreton, Proc. Natl. Acad. Sci. USA 95 (1998) 5550. [10] Q. Song, S. Yao, W. Wang, N. Lin, J. Photochem. Photobiol. A: Chem. 102 (1997) 197. [11] C.E. Crespo-Hernandez, S. Flores, C. Torres, I. Negr onEncarnaci on, R. Arce, Photochem. Photobiol. 71 (2000) 544. [12] K.L. Stevenson, G.A. Papadantonakis, P.R. LeBreton, J. Photochem. Photobiol. A: Chem. 133 (2000) 159. [13] R.J. Robbins, G.R. Fleming, G.S. Beddard, G.W. Robinson, P.J. Thistlethwaite, G.J. Wolfe, J. Am. Chem. Soc. 102 (1980) 312. [14] D.V. Bent, E. Hayon, J. Am. Chem. Soc. 97 (1975) 2612. [15] J. Feitelson, Photochem. Photobiol. 13 (1971) 87.
102
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
[16] J. Peon, G.C. Hess, J.-M.L. Pecourt, T. Yuzawa, B. Kohler, J. Phys. Chem. A 103 (1999) 2460. [17] T. Goulet, J.-P. Jay-Gerin, J. Chem. Phys. 96 (1992) 5076. [18] M.C. Chang, J.W. Petrich, D.B. McDonald, G.R. Fleming, J. Am. Chem. Soc. 105 (1983) 3819. [19] R.A. Marcus, J. Chem. Phys. 24 (1956) 966. [20] M. Shirom, G. Stein, J. Chem. Phys. 55 (1971) 3372. [21] E.J. Hart, J.W. Boag, J. Am. Chem. Soc. 84 (1962) 4090. [22] S.H. Lin, Y. Fujimura, H.J. Neusser, E.W. Schlag, Multiphoton Spectroscopy of Molecules, Academic Press, New York, 1984 (Chapter 4). [23] D.N. Nikogosyan, H. G orner, J. Photochem. Photobiol. B: Biol. 13 (1992) 219. [24] D.V. Bent, E. Hayon, J. Am. Chem. Soc. 97 (1975) 2599. [25] J.A. Gally, G.M. Edelman, Biochim. Biophys. Acta 60 (1962) 499.
[26] P. Atkins, Physical Chemistry, sixth edn., Freeman, San Francisco, CA, 1997, p. 823. [27] D.M. Carey, G.M. Korenowski, J. Chem. Phys. 108 (1998) 2669. [28] O.D. Bonner, G.B. Woolsey, J. Phys. Chem. 72 (1968) 899. [29] R.C. Dougherty, J. Chem. Phys. 109 (1998) 7372. [30] U. Peskin, A. Edlund, I. Bar-On, M. Galperin, A. Nitzan, J. Chem. Phys. 111 (1999) 7558. [31] R. Ludwig, T.C. Farrar, M.D. Zeidler, J. Phys. Chem. 98 (1994) 6684. [32] R. Ludwig, F. Weinhold, T.C. Farrar, J. Chem. Phys. 103 (1995) 6941. [33] J.R. Bolton, M.D. Archer, in: J.R. Bolton, N. Mataga, G. McLendon (Eds.), Electron Transfer in Inorganic, Organic, and Biological Systems, Advances in Chemistry Series 228, Washington, DC, 1991, p. 7. [34] T. Kakitami, N. Mataga, J. Phys. Chem. 89 (1985) 8.
Chemical Physics Letters 346 (2001) 97±102
www.elsevier.com/locate/cplett
Evidence of temperature dependent activation barriers for near-threshold aqueous photoionization of 20-deoxyguanosine and tryptophan George A. Papadantonakis a, Kenneth L. Stevenson b, Pierre R. LeBreton a,* a
Department of Chemistry (m/c 111), University of Illinois at Chicago, 845 Wext Taylor Street, Room 4500 SES, Chicago, IL 60607, USA b Department of Chemistry, Indiana University Purdue University at Fort Wayne, Fort Wayne, IN 46805, USA Received 19 April 2001; in ®nal form 1 June 2001
Abstract Alkaline 20 -deoxyguanosine undergoes one- and two-photon laser photoionization at 266 nm. Similar to tryptophan and indole, the one-photon quantum yield increases ®ve-fold between 296 and 362 K. Earlier Arrhenius analyses of tryptophan and indole photoionization rate constants, yielded unreasonably large pre-exponential factors (1013 ± 1018 s 1 ). An analysis employing pre-exponential factors for diusion controlled reactions (1010 ±1011 s 1 ) provides evidence of activation barriers that decrease with increasing temperature over a range where water structure undergoes signi®cant changes. In the context of Marcus theory, this analysis suggests that curvatures of the electron-transfer free energy surfaces decrease as temperature increases. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Direct photoionization of guanine and tryptophan (Trp) are implicated in mechanisms associated with UV damage of DNA [1±3] and proteins [4,5]. In the gas-phase and in aqueous environments, guanine has the smallest ionization energy of all the DNA bases [1,6±9]. However, little is known about near-threshold aqueous DNA photoionization. Earlier investigations [10] indicate that the 248 nm, one-photon quantum yields for aqueous photoionization of guanine and of the RNA nucleoside, guanosine, are 0.061
*
Corresponding author. Fax: +1-312-996-0431. E-mail address: [email protected] (P.R. LeBreton).
and 0.016 at 288 K and pH 11.0±11.5. For guanine and guanosine at neutral pH, only twophoton 248 nm ionization is reported [2,10]. Recently, a surprisingly large value (0.083) was reported for the 266 nm one-photon ionization quantum yield of guanosine (Guo), at neutral pH [11]. The low-energy aqueous photoionization of Trp, and of its chromophore, indole, has been the focus of numerous experiments employing both conventional and laser light sources [12±16]. Interestingly, photoionization quantum yields of Trp and indole increase four-fold or more over the temperature range 278±348 K [12,13,15], indicating the occurrence of an activation barrier. Barriers also occur in attachment reactions of solvated electrons with indole radical cations and H3 O [16,17]. Earlier analyses of the temperature
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 8 6 8 - 5
98
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
dependence of rates for indole and Trp photoionization using the Arrhenius equation ke T Ae
DGz =RT
1
and based on plots of ln ke versus 1=T yield values of activation barriers heights in the range 50.0± 65.0 kJ/mol and values of pre-exponential factors in the range 1013 ±1018 s 1 [13,15,18]. An earlier evaluation of these results [15] concluded that the pre-exponential factors, which this analysis provides are unreasonably large. While Trp and indole aqueous photoionization have not been considered in the context of the Marcus [19] theory of electron transfer, the theory provides a framework in which to analyze photoionization as an electron-transfer process. In this framework, the transfer is from an excited donor to a solvation environment that accepts the photoejected electron. The goal of the present investigation is to examine photoionization of alkaline 20 -deoxyguanosine (dGuo ) at near-threshold energy, to compare dGuo results with earlier results for Trp, and to reconsider the temperature dependence of near-threshold aqueous photoionization.
dition of 2 M NaOH. Photolysis of 1.0 ml aqueous solutions of 20 -dGuo was carried out in a 1.0 cm ¯ow-through cuvette. In experiments at pH 11.4, (1:2 10 4 M) the absorbance of dGuo at 266 nm was 1.42 and remained unchanged over the temperature range examined. Before photolysis, O2 was removed from 200.00 ml of each solution by purging for 30 min with Ar gas. Sample temperature was controlled by immersing a stock solution in a VWR Model 1131 constant temperature bath, and pumping the thermostated sample into the cuvette immediately before irradiation. Photoionization quantum yields (/e ) were calculated [12] by employing the relationship /net 0:52AdGuo =ANa4 Fe CN6 :
2
Here, AdGuo and ANa4 Fe CN6 are the transient absorbances at 670 nm of the hydrated electrons produced immediately (i.e. at t 0) following 266 nm laser photoionization of dGuo and 0.001 M Na4 Fe CN6 , respectively. Laser pulse energies were determined from a calculation [12] employing values of ANa4 Fe CN6 and the value (0.52) [20] of the monophotonic photoionization quantum yield of Na4 Fe CN6 .
2. Experimental
3. Results
Photoionization experiments were carried out with unfocused 266 nm, 7 ns pulses from a Surelite Nd-YAG laser. The 266 nm light was generated by twice doubling the frequency of the 1064 nm fundamental laser pulses in KDP crystals. Transient absorption spectra were measured with a pulsed 75 W Xe arc lamp (Photon Technology International), triggered by the laser pulse and controlled with a laser diode driver (Analog Modules). The analyzing beam was collinear with the laser beam and dispersed with a monochromator (Photon Technology International) equipped with a Hammamatsu R936 photomultiplier. The photoionization experimental procedure was the same as described earlier [12]. 20 -deoxyguanosine, 99±100% pure, was obtained from Sigma, and solutions were prepared in doubly distilled water. The natural pH of dGuo is 6.3; a pH of 11.4 was obtained by drop-wise ad-
Fig. 1 shows transient absorption spectra obtained after 266 nm irradiation of 20 -dGuo at
Fig. 1. Transient absorption spectra of Guo at 296 K, and pH 11.4 () and 6.3 (N) measured at a delay time of 200 ns after irradiation with 266 nm, 13 mJ, 7 ns laser pulses.
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
296 K, and pH 11.4 and 6.3. The pH 11.4 data exhibits a broad absorption band with a maximum (700±720 nm) associated with hydrated electrons [21]. At pH 6.3, there is no evidence of electron production under the conditions of these experiments. For the measurements at high pH, electron absorbance at 670 nm does not increase linearly with increasing laser power. A double logarithmic plot of the 670 nm absorbance versus laser pulse energy has a slope of 1.63. This value is between values of 1.0 and 2.0 associated with mono- and biphotonic processes, respectively [22], and indicates that both processes occur. Trp photoionization at 266 nm exhibits similar behavior [12]. Fig. 2, shows quantum yields obtained from Eq. (2) for 266 nm photoionization of dGuo at pH 11.4 for laser powers between 3 and 13 mJ. Results are given at 296, 330 and 362 K. At each temperature, the biphotonic component increases with the laser energy [22], while the monophotonic component is independent of laser energy and equals the intercept [23]. The monophotonic quantum yield is 0:011 0:002 at 296 K and increases with increasing temperature. Fig. 3 shows the monophotonic quantum yields for 266 nm photoionization of dGuo at pH 11.4 over the temperature range 296±362 K. The results indicate that the quantum yield increases ®ve-fold as the temperature increases. In addition to the
99
Fig. 3. Plots of monophotonic quantum yields for the 266 nm photoionization of dGuo at pH 11.4 () as a function of temperature. Results were obtained with 13 mJ, 7 ns pulses. Error bars correspond to standard deviations in graphical determination of the intercept from three separate measurements of plots of quantum yield versus laser energy. The ®gure also shows similar results taken from [12] for the 266 nm one-photon ionization of Trp () at pH 5.9. The scales of the dGuo and Trp quantum yields are given on the left and right, respectively. The insert shows plots of the ratio of the rate constants for onephoton ionization at temperature, T, and at 296 K (ke T =ke (296)) versus temperature for Trp at pH 11.0 () and pH 7.0 (D), indole at pH 3.5 (j), and tyrosine at pH 12.0 (}).
data for dGuo , Fig. 3 shows previously reported quantum yields for the one-photon ionization of Trp [12]. While the quantum yield for Trp at 296 K is four times larger than that of dGuo , the increases in the relative quantum yields of dGuo and Trp over the temperature range examined are similar. Fig. 3 also contains an insert which shows previously reported ratios of the rate constants for one-photon ionization at varying temperature to the rate constant at 296 K (ke T =ke 296) for Trp [13], indole [15] and tyrosine [24,25]. 4. Discussion
Fig. 2. Plot of the photoionization quantum yields (/e ) of dGuo at pH 11.4 versus pulse energy measured with 266 nm, 7 ns pulses at 296 (j), 330 () and 362 K (.).
The results in Fig. 1 demonstrate that the electron absorption spectrum from dGuo at pH 11.4 is easily measured while, under the present conditions, no absorbance associated with transient electrons is observed from dGuo at pH 6.3. The results in Fig. 1, together with results in Figs. 2 and 3, provide evidence that the one-photon quantum yield of dGuo at neutral pH and room tempera-
100
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
ture is signi®cantly smaller than the value of 0.083 reported from other recent 266 nm laser experiments on guanosine [11]. 4.1. Ionization over a temperature dependent activation barrier Earlier descriptions of the temperature dependence of rate constants for Trp and indole photoionization, that yield anomalously large preexponential factors, employed energy barriers that are not dependent on temperature [13,15]. Another possibility is that the barriers are temperature dependent, and decrease in height as the temperature increases. This is shown in Fig. 4a. Here, the ratio of the photoionization rate constants (ke ) at two temperatures, T1 and T2 , is given by
ke T1 A T1 e ke T2 A T2 e
DGz T1 =RT1 DGz T2 =RT2
:
3
Using a previously reported ke value of 8 107 s 1 for the 264 nm photoionization of Trp at pH 11.0 and 291 K [13], and pre-exponential factors associated with diusion controlled reactions (1010 ±1011 s 1 [26]) yields values of 12.0±18.0 kJ/ mol for DGz . Assuming that, over the temperature range examined, the pre-exponential factor remains nearly constant [26], an analysis of the data in the insert of Fig. 3 using Eq. (3) indicates that, as the temperature increases by 30 K, DGz decreases to 9.0 to 15.0 kJ/mol. Similar results are obtained from an analysis of the photoionization rate constant of indole at 254 nm [15]. Here, the barrier height decreases between 1.0 and 2.0 kJ/
Fig. 4. (a) Schematic diagram showing the decrease in the activation barrier height (DGz ) for dGuo one-photon ionization, which occurs as the temperature increases from T1 to T2 and (b) Free energy curves for dGuo and dGuo eaq , showing the decrease in the curvatures, reorganization energy (k), and barrier height (DG ) as the temperature increases from T1 to T2 .
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
mol when the temperature increases from 298 to 313 K. The large pre-exponential factor for indole ionization, obtained when a temperature independent barrier is employed, was previously attributed, in an unspeci®ed way, to indole±water interactions [15]. The present consideration of a temperature dependent barrier also suggests that water plays a key role in determining the barrier properties for near-threshold photoionization. This is consistent with the observation that the relative increases in the rate constants are similar for the dierent molecules in the insert in Fig. 3. For tyrosine at pH 12.0, and Trp at pH 11.0 and pH 7.0, the rate constants increase 3±11 times in the temperature range 296±333 K. Speculation about the importance of water structure leads to a description of near-threshold photoionization in which electron separation occurs over fragile barriers that are disrupted at elevated temperatures. In this description, the barrier heights (9.0±12.0 kJ/ mol) and the changes that occur over the temperature range examined are similar in magnitude to the free energy of a hydrogen bond in liquid water (10.6±11.0 kJ/mol [27,28]). Investigations of the temperature dependence of hydrogen bond strength [29] and of the structure of liquid water employing quantum mechanical calculations, and IR and NMR results [29±32] provide evidence of signi®cant water structure changes over the temperature range examined in the Trp, indole and dGuo photoionization experiments. 4.2. Marcus electron-transfer theory A simple application of Marcus electron-transfer theory provides a similar picture. Within the Marcus framework, the energies associated with the initial and ®nal states are shown schematically in Fig. 4b. Under normal conditions [19], the barrier for electron transfer, DG , increases as the reorganization energy, k, increases. Generally, the major contribution to k is associated with solvent relaxation [33]. In the systems examined here, where solvation energies are large, the increase in the photoionization quantum yield with temperature is depicted in Fig. 4 as a decrease in the curvature of the free energy surfaces which results in
101
decreases in k and G . The decrease in curvature is caused by a decrease in water orientational polarization in the vicinity of dGuo , in the initial and ®nal states, that occurs with increasing temperature. The relationship between solvent polarization and curvature of the free energy surfaces, considered here, is similar to that examined in earlier investigation of the in¯uence of electrostatics on electron-transfer rate constants [34]. Acknowledgements Support by the Petroleum Research Fund of the American Chemical Society and by the American Cancer Society (RPG-91-024-09-CNE), and the help of Mr. Manuel Arroyo-Vega in carrying out these experiments is gratefully acknowledged. References [1] T. Douki, T. Delatour, R. Martini, J. Cadet, J. Chim. Phys. Chim. Biol. 96 (1999) 138. [2] L.P. Candeias, S. Steenken, J. Am. Chem. Soc. 114 (1992) 699. [3] D. Angelov, A. Spassky, M. Berger, J. Cadet, J. Am. Chem. Soc. 119 (1997) 11373. [4] L.I. Grossweiner, Curr. Eye Res. 3 (1984) 137. [5] J.A. Shauerte, A. Gafni, Biochem. Biophys. Res. Commun. 212 (1995) 900. [6] H.S. Kim, P.R. LeBreton, J. Am. Chem. Soc. 118 (1996) 3694. [7] H. Fernando, N.S. Kim, G.A. Papadantonakis, P.R. LeBreton, in: N.B. Leontis, J. SantaLucia Jr. (Eds.), Molecular Modeling of Nucleic Acids, ACS Symposium Series 682, Washington, DC, 1997, p. 18. [8] N.S. Kim, Q. Zhu, P.R. LeBreton, J. Am. Chem. Soc. 121 (1999) 11516. [9] H. Fernando, G.A. Papadantonakis, N.S. Kim, P.R. LeBreton, Proc. Natl. Acad. Sci. USA 95 (1998) 5550. [10] Q. Song, S. Yao, W. Wang, N. Lin, J. Photochem. Photobiol. A: Chem. 102 (1997) 197. [11] C.E. Crespo-Hernandez, S. Flores, C. Torres, I. Negr onEncarnaci on, R. Arce, Photochem. Photobiol. 71 (2000) 544. [12] K.L. Stevenson, G.A. Papadantonakis, P.R. LeBreton, J. Photochem. Photobiol. A: Chem. 133 (2000) 159. [13] R.J. Robbins, G.R. Fleming, G.S. Beddard, G.W. Robinson, P.J. Thistlethwaite, G.J. Wolfe, J. Am. Chem. Soc. 102 (1980) 312. [14] D.V. Bent, E. Hayon, J. Am. Chem. Soc. 97 (1975) 2612. [15] J. Feitelson, Photochem. Photobiol. 13 (1971) 87.
102
G.A. Papadantonakis et al. / Chemical Physics Letters 346 (2001) 97±102
[16] J. Peon, G.C. Hess, J.-M.L. Pecourt, T. Yuzawa, B. Kohler, J. Phys. Chem. A 103 (1999) 2460. [17] T. Goulet, J.-P. Jay-Gerin, J. Chem. Phys. 96 (1992) 5076. [18] M.C. Chang, J.W. Petrich, D.B. McDonald, G.R. Fleming, J. Am. Chem. Soc. 105 (1983) 3819. [19] R.A. Marcus, J. Chem. Phys. 24 (1956) 966. [20] M. Shirom, G. Stein, J. Chem. Phys. 55 (1971) 3372. [21] E.J. Hart, J.W. Boag, J. Am. Chem. Soc. 84 (1962) 4090. [22] S.H. Lin, Y. Fujimura, H.J. Neusser, E.W. Schlag, Multiphoton Spectroscopy of Molecules, Academic Press, New York, 1984 (Chapter 4). [23] D.N. Nikogosyan, H. G orner, J. Photochem. Photobiol. B: Biol. 13 (1992) 219. [24] D.V. Bent, E. Hayon, J. Am. Chem. Soc. 97 (1975) 2599. [25] J.A. Gally, G.M. Edelman, Biochim. Biophys. Acta 60 (1962) 499.
[26] P. Atkins, Physical Chemistry, sixth edn., Freeman, San Francisco, CA, 1997, p. 823. [27] D.M. Carey, G.M. Korenowski, J. Chem. Phys. 108 (1998) 2669. [28] O.D. Bonner, G.B. Woolsey, J. Phys. Chem. 72 (1968) 899. [29] R.C. Dougherty, J. Chem. Phys. 109 (1998) 7372. [30] U. Peskin, A. Edlund, I. Bar-On, M. Galperin, A. Nitzan, J. Chem. Phys. 111 (1999) 7558. [31] R. Ludwig, T.C. Farrar, M.D. Zeidler, J. Phys. Chem. 98 (1994) 6684. [32] R. Ludwig, F. Weinhold, T.C. Farrar, J. Chem. Phys. 103 (1995) 6941. [33] J.R. Bolton, M.D. Archer, in: J.R. Bolton, N. Mataga, G. McLendon (Eds.), Electron Transfer in Inorganic, Organic, and Biological Systems, Advances in Chemistry Series 228, Washington, DC, 1991, p. 7. [34] T. Kakitami, N. Mataga, J. Phys. Chem. 89 (1985) 8.