5180
J. Phys. Chem. 1992,96, 5180-5183
Pulse Radlolysis of Dextran-Water Solutions Marsha M. Glezen: Anita C. Chernovitz,’ and Charles D. Jonah* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: November 5, 1991; In Final Form: February 26, 1992)
The yield and rates of the reaction of the hydrated electron, e, -,and the precursor of eaq-with nitromethane were measured in 5%-50% dextran solutions in water. The results show no decrease in the initial yield of ea; nor in its reactivity. These results suggest that the dextran divides the solution into regions of water that are separated from each other by polymer and that the electron can move facilely within a water region but not across the polymer-water boundary. The results also suggest that the electron is more strongly hydrated than is the dextran molecule.
Introduction Recently, there has been considerable effort to determine the mechanism of radiation damage in biological materials. These discussions have questioned the relevance of radiation chemical studies in dilute solution studies to the processes that occur in biological systems. Possible differences between dilute solutions and the biological system could be caused both by the direct ionization of biological compounds (“direct effects”) and by the modification of the chemistry in water caused by the association of water molecules with the biological compounds. It is the latter effect that we wish to address. It has been postulated that large molecules can influence the orientation of the water molecules near them and thereby organize the water molecules present in solution.’ Studies have been made of the structure of water around DNA molecules.la The concept “organized water” has been postulated to occur specifically around the sugar phosphate backbone of DNA and can affect radiation damage.lb Also, the mobility of the electron can be modified in polymer-water solutions.Ic It has also been suggested that this organization will change the reactivity of radicals and electrons created by the ionizing radiation and make rate constants determined in lowconcentration solutions irrelevant.Ib To measure such an effect, one needs either an unusual reaction that will take place only in the localized, organized region or a solution where the entire solution is organized. It is the latter approach that we have chosen to use. The hydrated electron’, eq-, was chosen as a probe of the environment of the water because it can be created cleanly and efficiently in water and its properties are well-characterized. Polysaccharide solutions such as dextran in water provide a system that has the following favorable properties: (1) the solubility of the dextran is high; (2) the solution is optically transparent, and (3) eaq-does not react with the dextran. The high concentration of dextran was necessary to have most of the water molecules interact with the polysaccharide. Previous experiments with dextran suggest that it would be a profitable system to study. Koulkes-Pujo and Tran-Thi observed a decrease in the initial yield of ey- in solutions with high dextran concentrations? In addition, thew experimentsshowed a decrease in the ability of certain scavengers for the electron precursor to inhibit the formation of the electron. In other experiments, an increase in the solvation time of the electron was seen.3 The reactivity of the hydrated electron in dextran solutions was studied by the addition of nitromethane. The use of an uncharged molecule m i n i ionic strength effects on the reaction rate and on the structure of the solution. Nitromethane reacts with eaqat or near the diffusion-controlled limit, and it also reacts with the precursor of the hydrated electron so that we could measure the competition for the solvation of the electron. We report a detailed account of the kinetic behavior of the hydrated electron with and without scavenger (CH3N03)over a large concentration range of dextran (0-50 wt % dextran). Present address: Dow Chemical, Midland, MI. $Present address: Chemistry Department, Rochester Institute of Technology, Rochester, NY
TABLE I: Reaction of the Hydrated Electroll with Nitromethane
viscosity (P) dextran/H20, % 5 20 30 40 43.5 50
k, M-’ 4.2 4.2 2.6 1.9 1.2
X X X X
x
s-’
yield of ea’:
1O1O 10” 1O1O 10’O 10’0
0.73 1.oo 1.09 1.10 1.15 1.11
from lit.” 0.6 3.1 15.8 32 (43)< 187
Relative to ea; in H20. From ref 7 unless noted. e Recorded in our laboratory; see Experimental Section.
Experimental Section The pulse radiolysis equipment used in this study has previously been described in detail4and uses the pump-probe technique f i t applied to pulse radiolysis by Hunt and co-~orkers.~Each pulse contained 20-22-MeV electrons with a repetition rate of 60 Hz. From this pulse 30% of the beam is passed through a xenon cell (1 atm), generating Cerenkov light, which was used as the probe beam, to measure the absorption of the transient species. The electron pulse width is approximately 25 ps, and the dose deposited is determined by measuring the absorption of the hydrated electron (ea;) at 600 nm. Dextran (pyrogen-free), with a molecular weight range of 100000-200 OOO, was purchased from Accurate Chemical and Serva Chemical suppliers and was used as received. Solutions were prepared using triply distilled water and were deoxygenated by purging with helium. Nitromethane (Aldrich, 99+%) was used as purchased. Because the OH radical reacts with the dextran to create a semisolid gel, a new type of flow cell was designed for these experiments. The rectangular cell was 20 cm long and was translated through the beam. In addition, the cell was modified to maximize the flow of the sdution near the quartz windows to minimize the buildup of polymeric materials on the window. The windows were also easily demountable to facilitate cleaning. Viscosities were measured using a Brookfield Synchro-lectric viscometer with a No. 2 spindle. All polysaccharide solutions were allowed to equilibrate with the rotating spindle for 1 h before the readings were recorded. Results and Discussion Initial Yield of the Electron. Table I shows the initial yield of the electron as a function of dextran concentration as well as the second-order decay rates at high concentrations of nitromethane (0.1-0.5 M). These yields are corrected for the density of the solution. Previous studies have indicated that the absorption spectrum of the hydrated electron in dextran solutions is very similar to that in waters2 Therefore, a change in yield is probably not due to a change in the extinction coefficients. At 20%, the yield increases slightly and then remains essentially constant up to 50% dextran. This increase can be explained on the basis that carbohydrates have proven to be very good OH’ scavengers>and thus when the OH is scavenged, the yield of the electron will be slightly increased.
0022-365419212096-5180$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5181
Pulse Radiolysis of Dextran-Water Solutions 0.0
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Time ns for clarity. Our measurements of the electron yield differ from those previously reported by Koulkes-Pujo and Tran-Thi? Their results indicated a decrease in the electron yield in 43.5%dextran solutions. This was thought to OCCUT due to the presence of "perturbed" or organized water molecules at high concentrations. This correlated well with other physical measurements that showed a break in the dielectric constant of aqueous dextran solutions occurring at 43.5%.' Kiaetic Analysis of Decay Rates. Figure 1 compares both the growth and decay of the e,; in H 2 0 with that in 50%dextran (measured at 600 nm). The curves have been normalized and displaced in time to facilitate comparison. The growth of the absorption is identical in the two solutions. The decay in the presence of the dextran is slightly slower. This may be due to the increased viscosity or to the reaction of the OH radical with dextran; the decreased OH then slows the rate of the e; reaction. Surprisingly, the second-order decay rates given in Table I for nitromethane remain very fast even at high concentrations of dextran and are on the order of those recorded for nitromethane in water.8 Because the measured viscosity of the dextran solution is virtually unchanged when nitromethane is added to the solution, the high rates do occur in highly viscous solutions. While there is a slight decrease in the rate as one increases the viscosity, the decrease in rate is not as large as one would expect. The Stokes-Einstein relationship states that the diffusion coefficient is inversely proportional to the viscosity. Essentially, when the moving particle is smaller than the solvent molecules, D = kT/ 4uqr where k is the Boltzmann constant, q is the viscosity, and r is the radius of the spherical p a r t i ~ l e . ~If the viscosity effect followed the Einstein equation, one would see a decrease of 4000 in the decay rate going from 0% dextran to 43.5% dextran. However, the rate only diminishes by a factor of 4. Previous studiesl0of the diffusion properties of macromoleculesin aqueous dextran solutions indicated a direct correlation between viscosity and diffusion coefficents. The larger the particle, the more likely its dynamics will be influenced by the bulk properties of the solution. However, the behavior of the small hydrated electron (e, -) is only slightly affected by these bulk properties. &her studies of ionic reactions in polysaccharide matrices and other gelatins have also indicated that the behavior of the electron or other small ions seem to be uninhibited by the bulk properties of the matrix." Conductivity data on gelatins have been mixed; however, in most cases for very small ions very little difference in conductivity is observed.12 Decay rates for viscous polyanionic sols of K-carrageenan have also been measured for the electron with various solutes." The viscosities of these polysaccharide solutions were in the region 1-250 cP. Second-order rate constants for the electron with various scavengers were not reduced. In fact, in some cases the rates were faster than in pure water. This effect was explained in terms of facilitated solute movement that can occur in ionic solutions. This has been observed previously in other polyanionic polysaccharide matrices as well.13 However, our results in dextran, which is a neutral medium, are very similar to these in the respect that the viscosity does not affect the rata significantly. The range
*/
I
Figure 1. Time dependence of the absorption of the hydrated electron at 600 nm in pure water and 50% dextran. The two curves are displaced
.I
/
0.5 0
0.1
0.2 0.3
0.4
0.5 0.6
Conc CH3N02
Figure 2. Experimental second-order rate constants for the reaction of the hydrated electron with nitromethane as a function of concentration for four different dextran concentrations. The lines are drawn to guide the eye.
of the viscosities employed in this study is also 100-fold greater. Other pulsed radiolysis studies of the hydrated electron in glycerol/H20 mixtures have indicated that the second-order rate constant, k (e,; S),is proportional to q4-0.66.14This effect was seen for various scavengers such as nitrobenzene, carbon tetrachloride, and acetone. However, our data (Table I) do not show this trend either since one should see at least 5 times the rate for the 30% as the 43.5%solutions. The second-order rate constant is not a constant but depends strongly on the concentration of scavenger present as shown in Figure 2. This large concentration dependence has not been seen in either H2015or glycerol/H20 mixtures.I4 Nitromethane was chosen for this study due to its high reactivity and the fact that it is electrically neutral. Clearly the rate constant, which should increasewith concentration, reaches a maximum in approximately 0.3 M nitromethane for both dextran solutions considered. The accuracy of the rate constants decreases markedly at the high concentrations because (1) the decays become much faster and thus overlap the experimental resolution of the system and (2) the yield of the electron decream markedly at higher concentration of the electron, as discussed below. From experiments that measure the limiting rate constant at long times (greater than a few hundred picoseconds), one can only extract information about the product of the reaction radius and diffusion constant. However, the dependence of the rate constant on time or concentrationcan give insight into these separate terms. The observed increase in the rate constant as a function of concentration is suggestive of "time-dependent" rate constants. We used the Smoluchowski equation for the rate constant as shown in the equation16
+
where a = 4uRDN/1@ and 0 = R(rD)-ll2. k, is the rate constant at time t in M-' sd, R is the reaction radius, and D is the diffusion coeMicient for the ew- + CH3N02reaction. If one measures only the limiting rate constant a,only the product of the reaction radius and the diffusion constant can be determined. However, because a and 0 have different dependences on R and D,both R and D can be determined if one measures the rate constant as a function of time. A very wide dynamic range for the signal is needed to extract the time dependence of a rate constant from experimental data where the concentration is measured as a function of time. Such
5182 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 22.0 ?
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TABLE I 1 Reaction of the Precursor of the Solvated Electron with sample EtOH 5% dextran/H20 20% dextran/H20 30% dextran/H20 40% dextran/H20 43.5% dextran/H20
= !
~
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0.01
Glezen et al.
0.1
reaction radius,
Q37
3.87 4.02 5.21 3.76 3.07 2.35
A
11.6 12.7 11.4 10.6 9.8
1
Conc CH3N02, M
Figure 3. Experimental measurements of the second-order rate constant vs concentration in 30% dextran solution. The line is the theoretical rate constant for a diffusionantrolled rate with a reaction radius of 22 A and a diffusion constant of 3.2 X lo4 cm2/s.
a wide dynamic range is often attainable in a time-correlated photon counting experiment but is not normally attainable in an absorption measurement. The time range that is probed by a decay curve will depend on the concentration of the reactant. That is, if there is a high concentration of a reactant, the time range will be a short time; if the concentration is low, the time scale probed will be at a long time. The time that is being probed can be approximated by the half-time for the decay for an absorption measurement. (The appropriate time scale would be different for an emission experiment because of the different weighting that is done for the decay curve.) At very high concentrations, one can no longer probe shorter times because one is limited by the time resolution of the experimental system. Figure 2 shows the rate constant as a function of the concentration of the electron scavenger. The lines are included for visualization only. The values of a and B were adjusted so that good agreement with the experimental data was obtained at all concentrations. Then from the values of a and 8, one can then obtain a value for R and D at a given concentration. Figure 3 compares the experimental data with the theoretical fit for the 30% dextran solution, where D = 3.2 X 10“ cm2/s and R = 22 A. Using a similar analysis on the 43.5% dextran solutions, D was calculated as 3.4 X 10” cm2/s and R = 15 A. Changes greater than approximately 5% clearly modify the agreement between the experimental and calculated data. These two constants are considerably different from those seen in water. The diffusion constants are approximately 20 times smaller than the diffusion constant of the electron in water, and the reaction radii are much larger than those calculated for the same reaction in water (6 These results can be explained by considering the structure of the water-dextran solution. It is known that the dextran does not tend to associate, but instead dextran chains tend to remain separate from one another. Many polysaccharides do form aggregates; however, those based entirely on 1,6-linkages such as dextran do not associate in aqueous s01utions.l~ The physical model proposed by Flory indicates that dextran, a 1,6-glucopyranose, is an isotropic solution of rodlike chains in dilute solutions.I* However, at high concentrations these chains do tend to line up to a large degree because of the limited space and form a ‘tactoidal anisotropic solution phase”.17 This picture shows some order in one direction due to the fact that the long-chained molecules are stacked one on top of each other with regions of water in between. Thus, dextran associates with the water molecules nearby and partitions the water into regions or pools of water. If one assumes that an electron can diffuse quickly through a pool but only slowly between pools, then the electron would appear to react with a very large reaction radius (the size of the pool) but diffuse slowly (the time between pools). The concentration regime where this mechanism would be important is where the probability of an electron and an electron scavenger being in the same pool initially is small. Then the time scale for the reaction within the same pool would be so small compared to the diffusion time and ultimate reaction time that one could not observe the time dependence of the reaction within a pool. Once the solution is broken up into pools, the time to go from one
pool to another will be independent of the size of the pool. If we increase the concentrationof dextran, the pool size will get smaller but the diffusion constant will remain the same. This leads to what is observed-the reaction radius is smaller at 43.5% dextran as compared with 30% dextran while the diffusion constant remains about the same. At lower concentrations of dextran, the regions of water will not be strongly separated and thus the solution will appear to react similarly to what is seen in pure water. This is what was seen in the polysaccharide solution studies cited above.” This is also the behavior seen in lower concentration dextran solutions. Yield of tbe sdvated Electron ia tbe Presenceof Nihmethme. One of the probes of solvation has been the inhibition of the yield of the solvated electron or dry electron scavenging. For example, the existence of two different solvation mechanisms in alcohols has been demonstrated by precursor scavenging studies.19 Because the experiments of Koulkes-hjo and Tran-Thi showed a strong decrease in the dry electron scavenging? we repeated their experiments using a different electron scavenger. Table I1 gives the results of our experiments. The results are given for both C3,(the concentration necessary to reduce the initial yield to l / e of the original value) and Q37(the quenching constant and the inverse of C37). The Q37values were determined from the slope of the log of the yield of the electron as a function of scavenger concentration. Sufficiently high concentrations were used to reduce the yield of the electron to l/e of its initial value. The reaction radius for the electron-nitromethane is calculated assuming a static quenching mechanism; that is, if the electron is formed within the reaction radius of a nitromethane molecule, it reacts instaneously; if the electron is formed outside the reaction radius, it solvates. It is well-known that the reaction radius will be described by the equation 6.02 x 1023 4*r) 3
- I
The reactivity is maximal in 20% dextran solutions and drops considerably at higher concentrations; however, the drop appears less signifcant when looking at the static quenching reaction radii. The decrease in static quenching reaction radius is consistent with the above data. As the concentration of the dextran increases, the water pools decrease in size. Therefore, the probability that the electron will be localized in a water pool with a nitromethane molecule present decreases. Because the electron will not probe across the dextran chain, the fraction of the electrons that will solvate in the presence of nitromethane will increase. This will make the reaction probability appear to be smaller. Comparisoa with M o m E.nrperimentalWork. Above we have mentioned several previous experimental studies in which the characteristics of water have been altered using polymeric materials. We shall attempt to summarize, compare, and contrast our results with these previous experimental data. Because both the characteristics of the different techniques for altering the solution properties and the experimental techniques are greatly different, only general trends appear to be discernible. The closest experiments to those discussed here were done by Koulkes-Pujo and Tran-Thi and their co-~orkers.~J These experiments used dextran-water solutions in the same general concentration regime than we have done. As discussed above, there are several major differences between their results and ours that are presently unexplained. As was mentioned above, we saw no decrease in the electron yield at high dextran concentrations nor did we see a strong change in the ability of an electron pre-
Pulse Radiolysis of Dextran-Water Solutions cursor scavenger to inhibit the formation of the solvated electron. Also, the formation rate of the electron in high dextran concentrations was the same as in pure water. One possible explanation was the use of different dextrans. These dextrans are purified from bacterial fermentations, and differences have been observed among dextrans isolated from fermentations from different microorganisms.20 We have carried out experiments using dextrans from Accurate and Serva (the same source as in ref 1) with no major change. In addition, we have measured the solution viscosity, and it is in good agreement with ref 7. We can only assume that different techniques for the preparation of the solutions must change the distribution of water between the different polymer and free solvent. Our solutions were dissolved using warm water and stirring exclusively. The solutions of reference one were made using an ultrasonic bathsz1 It is conceivable that either preparation technique might have altered the solution characteristics or the polymer. Fragmentation of dextran has been Seen in solutions irradiated with ultrasonics,2* and there is evidence that the undissolved polymer would be more affected by the ~ltrasonics.2~However, the ultrasonic power used to dissolve the dextran is probably much less than that used in the above-cited references. No decrease was seen in the reaction rates of the electron under high-viscosity conditions in ref 11. In those experiments, the concentrations of polymer were much lower than in the present experiments and thus not so much of the solution will be organized. Those experiments also were done primarily with ions, a complication that we wished to avoid. A series of experiments have been done in which the conductivity of the electron has been probed in solutions with materials that form gels.25 These experiments were done at reduced temperatures so that presumably the water that is not bonded to the polymer will be ice. In these experiments, it was found that the conductivity seemed to depend approximately linearly on the water fraction above a threshold. This threshold was approximately 30 f 5% water. This is reminiscent of the results of refs 2 and 3; however, the fraction of water is lower. The measured results depend on both the concentration of the electron and its mobility, and thus the concentrationdependent value is difficult to compare with the present experiments. The results of those experiments suggest that there is water that is strongly associated with the polymers. These water molecules do not act like normal water (or ice for this experiment) in that the conductivity of the electron is not supported.
Conclusions These experiments show that dextran in these solutions does not inhibit the solvation of the electron. The electrostatic force on the water molecules by an electron is stronger than the hydrogen bonding between the dextran OH'Sand the water molecules and will thus disrupt the water-dextran binding. Neutron-scattering experiments in dextran have also suggested that the binding of the water to dextran was weaker than the binding to water. The structure of the water molecules in the first shell around Ni2+was unaffected by a high concentration of dextran. Even the second-shell water molecules were only slightly affected by the presence of the dextran.24 The differences between the experiments described here and those described in refs 2 and 3 are not clear. Several possible
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5183
explanations are given; however, no definitive reason can be found for the differences. We expect that the difference arises from the sample preparation techniques, but with the information available that cannot be definitively shown. The results suggest that the water is in small regions (at high concentrations) which are of suffkient size to allow the solvation of the electron. The reactions of the electron in very high concentrations of dextran are very similar to those occurring in pure water. This suggests that the hindered diffusion is compensated by a much larger reaction radius. We suggest that this larger reaction radius is due to the electron having two diffusive motions-one within a localized water regime that is fast and a second between water regimes that is considerably slower. Acknowledgment. We acknowledge Alan Youngs for his clever and skillful construction of the moving flow cell that made these experiments possible. The work of Donald T. Ficht and George L. Cox in running the linear accelerator was critical to the success of these experiments. Discussions with A. D. Trifunac, D. M. Bartels, and M. C. Sauer, Jr. are gratefully acknowledged. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE, under Contract W-3 1- 109-ENG- 38. Registry No. Dextran, 9004-54-0; water, 7732-18-5; nitromethane, 75-52-5.
References and Notes (1) (a) Fak, M.; Hartman, K. A. Jr.; Lord, R. C. J. Am. Chem. Soc. 1963, 85,387,397. (b) Adams, G. E., as quoted in: Jonah, C. D. Radial. Res. 1985, 104, S47. (c) van Lith, D.; Warman, J. M.; de Haas, M. P.; Hummel, A. J . Chem. Soc., Faraday Tram. 1 1986,82, 2933. (2) Koulkes-Pujo, A. M.; Tran-Thi, T. H. Radiar. Phys. Chem. 1985,26, 201. (3) Tran-Thi, T. H.; Koulkes-Pujo, A. M.; Mialocq, J. C.; Folcher, G. Laser Chem. 1986, 5, 351. (4) Jonah, C. D. Rev. Sci. Insrrum. 1975,46,62. (5) Bronskill, M. J.; Wolff, R. K.; Hunt, J. W. J . Phys. Chem. 1969,73, 1175. ( 6 ) Philips, G. 0.; Griffths, W.; Devriw, J. V. J. Chem. Soc. B 1966,194. (7) Volf, E. These, Universite De Paris XI, 1981. (8) Jonah, C. D.; Miller, J. R.; Matheson, M. S.J . Phys. Chem. 1977,81, 1618. (9) Olea,A. F.; Thomas, J. K. J . Am. Chem. SOC.1988,10,4494. (10) Phillies, G. D.J.; Gong, J.; Li, L.; Rau, A.; Zhang, K.; Yu, Li-P.; Rollings, J. J. Phys. Chem. 1989, 93, 6219. (11) Phillips, G. 0. Wedlock, D. J.; Micic, 0. I.; Milosorljevic, B. H.; Thomas, J. K. Radiar. Phys. Chem. 1980, IS, 187. (12) (a) Taft, R.; Malm, L. E. J . Phys. Chem. 1931, 35, 874. (b) Ibid. 1939, 43, 499. (13) Napier, M. A.; Hadler, N. M.Proc. Narl. Acad. Sci. U.S.A. 1978, 75, 2261. (14) Cercek, B. Inr. J. Radiat. Phys. Chem. 1975, 7, 223. (15) Jonah, C. D.; Miller, J. R.; Hart, E. J.; Matheson, M. S.J . Phys. Chem. 1975, 79, 2705. (16) Smoluchowski, M. V. Z . Phys. Chem. 1917, 92, 129. (17) Morris, E. R.; Norton, I. T. In Aggregation Processes in Solurion;
Wyn-Jones, E., Gormally, J., Eds.;Elsevier Scientific Publishing: Amsterdam, 1983; pp 550-555. (18) Flory, P. J. Proc. R. Soc. London, A 1956, 50, 234. (19) Lewis, M. A.; Jonah, C. D. J. Phys. Chem. 1986, 90, 5367. (20) Booth, G. C.; Gold, V. J . Chem. Soc. 1956, 3380. (21) Tran-Thi, T. H. Private communication. (22) Basedow, A. M.; Ebert, K. H. Makromol. Chem. 1976, 176, 745. (23) Suslik, K. S. Ulrrasound: Irs Chemical, Physical and Biological Effects; VCH Publishers: New York, 1988; pp 126-127. (24) Enderby, J. Private communication. (25) Eden, J.; van Lith, D.; Warman, J. M.; Hummel, A. J . Chem. Soc., Faraday Trans. I 1989,85,99 1.
