2691
Prehydration Scavenging of eaqbuffers were placed in 3.4-cm quartz cells which were then enclosed in a high-pressure optical bomb. A spectrum from 650 to 350 mp was recorded a t atmospheric pressure and then a t a series of higher pressures to 6500 kg/cm2. After release of pressure, another spectrum was recorded. The adjusted optical density at A,, typically differed by no more than 1%between the initial and final 1-atm spectra. Some baseline shift was observed with increasing pressure and this was taken into account in calculating the optical density a t Amax. Shifts of less than 3 mp were observed in thi3 positions of the absorption bands over the 6500-kg/cm2 pressure range. pH Measurements. For the purpose of checking the pH values a t 1 atm, measurements were made on the various buffers and other solutions using a Radiometer p H meter and Radiometer electrodes. pK1 Measurements for Indicators a t 1 atm. MeasurementslO in buffer of pK1 for p-nitrophenol give a value of 7.05 and our data are in agreement with this number. Several pK1 values in the range 5.1-5.2 have been reported fcJr 2,5-dinitrophenol.12 Using acetate and cacodylate bufflers the data shown in Table I11 were obtained a t atmospjieric pressure. Reliable data could not be obtained a t p H values less than 5"1to 5.2 because of the development of an interfering absorption band at 360 mp from the neutral phenol. Absorption bands of p-nitrophenoxide ion and 2,5-dinitrophenoxide ion used in these studies were a t 400 and 440 mp, respectively.
Acknowledgments. Financial support for this work was provided by NSF grants to W.K. and R. C. N., and an N%Hfellowship to R. C. N. The pressure apparatus used in this work was obtained through an NIH grant to W.K.
Supplementary Material Available. Tables containing the data shown in Figures 1 and 2 will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 20X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-73-2687.
References and Notes (1) (a) National Institutes of Health Special Research Fellow, 19711972. (b) Permanent address and address for correspondence: Department of Chemistry, University of California, Riverside, Calif. 92502. (c) Presented at the 165th National Meeting of the American Chemical Society, Dallas, Tex., April 8-13, 1973, INDE 038. For a aeneral review see S. D. Hamann. "Hiah Pressure Phvsics and CGemistry," Voi. 2. R. S. Bradley, Ed., Academic Press,.New York, N. Y., Chapter 7ii. (a) A preliminary report has been published: A. Zipp, G. Ogunmola, R. C. Neuman, Jr., and W. Kauzmann, J. Amer. Chem. SOC., 94, 2541 (1972); (b) A. Zipp and W. Kauzmann, Biochemistry, 12, 4217 (1973). (a) The values of log KI,/Ko for acetic are molal quantities obtained in dilute solution at 25". (b) D. A. Lown, H. R . Thirsk, and Lord Wynne-Jones, Trans. Faraday SOC., 64. 2073 (1968). A. Disteche, Symp. Soc. Exp. Biol., 26, 27 (1972). (a) The use of optical indicators to measure effects of pressure on pH has been previously attempted, however, no quantitative results were reported.6b (b) R. E. Gibson and 0. H. Loeffler, Trans. Amer. Geophys. Un., 503 (1941). P. W. Bridgman, Proc. Amer. Acad. ArfsSci., 48, 309 (1912). See paragraph at end of paper regarding supplementary material. (a) S. D. Hamann, Division of Applied Chemistry, Technical Paper No. 3, CSIRO, Australia, 1972. (b) Unpublished experiments by Dr. Frank Gasparro in this laboratory. F. J. Kezdy and M. L. Bender, Biochemistry, 1, 1097 (1962). G. Gomori, "Methods of Enzymology," Vol. I. Academic Press, New York, N. Y., 1955, pp 138-146. C. M. Judson and M. Kilpatrick, J. Amer. Chem. SOC., 71, 3110, 3113 (1949).
Prehydration Scavenging of eaq- and the Yields of Primary Reducing Products in Water y-Radiolysis 2. D. Draganic and I . G. Draganit* Boris K/drii: lnstffufe of Nuclear Sciences, Beograd, Yugoslavfa
(Received May 14. 7973)
The formation of primary reducing yields of water radiolysis was examined in neutral aqueous solutions of alanine and glycine. Increase in concentration of the amino acids was found to lead to decrease in G(eaq-), G(H), and G(H2). LOWreactivities toward eaq- in the solutions studied suggest that the prehydration scavenging of eaq- is the cause of decrease in G(eaq-) and therefore in G(H) and G(H2). The consequences of prehydration scavenging for the water decomposition yield are considered.
Introduction Experimental testing of the free-radical model of water radiolysis has confirmed the importance of eaq- reactions for the formation of primary yields of H and H2.1,2 I t might be expected, therefore, that a possible prehydration scavenging of eaq- should influence not only G(eag-) but
also G(H) and G(H2). It should also affect the values of G(-H20) calculated from the data on the yields of primary reducing species by the equation of material balance. The accumulating evidence on the importance of eaq- precursor reaction^^-^ and some inconsistency concerning G( -HzO) dependence on the reactivities toward The Journal of Physical Chemistry. Vol. 77.
No, 22, 7973
2692
Z. D. Draganic and I . G . Draganic
eaq- and OH7 pointed out that the prehydration scavenging could be important for the formation of all primary yields in irradiated water. The present work provides some clear evidence for the reducing products. The solutes used were alanine and glycine, both known as fairly inefficient in scavenging eaq-, H: and OH, but assumed to react effectively with the precursor of the hydrated elect r ~ n .The ~ . ~irradiated solutions (natural pH) consisted of the amino acids studied at various concentrations and of appropriate amounts of nitrite, or formate and nitrate, to react with the primary free radicals in the bulk only. The amount of chemical change and the steady-state kinetics were used in the calculations of the primary yields. Experimental Section The chemicals used were BDH or Merck of the highest purity available. Irradiations were carried out in a radioactive cobalt source giving 2.1 x l.019 eV g--1 hr-1; the absorbed doses ranged from 2 X 1017 to 7 x 1017 eV 8-1. Molecular hydrogen was determined by gas chromatography and the nitrite concentration by spectrophotomet)ry. Sample preparation, irradiations, and chemical analyses were described in full detail in the preceding paper.7 Results and Discussion Table 1 summarizes the yields of stable products determined in irradiated solutions and used in the calculations of primary yields. G ( H 2 ) . Primary yield of molecular hydrogen was directly measured in aqueous solutions of amino acids (AA) containing the nitrite ion (0.01 or 0.05 M). Xitrite is effective in scavenging OH, H, and eaq- without producing Hz. Its concentration was sufficient to prevent some additional formation of hydrogen by reaction H AA Hz + P, and
+
G(H,)
=
G(HJmeai
-
(1)
The measured yields are lower than 0.45 because of interference with the Hz formation in the spurs, caused by somewhat higher reactivities of nitrite toward eaq-. G(H). In aqueous solutions of amino acids we had nitrate ion to scavenge eaq- and HCOO- to remove OH and H species from the bulk. Because of the reaction H + HCOO- + H z COO-, we had
+
=
G(HJ
+ cxE-r)
2)
The nitrate ion present (5 x 10--3 or 2.5 x M) removes the hydrated electron from the bulk. However, a t higher concentrations of amino acid one has also to take into consideration the reaction eaq- + NHs +CH(R)COOH iNHzCH(R)COO- and the values of G(H) have to be calculated from the following relation -+
G(HL),,c,, = G(HJ
+ G(H) 4-
fG(eaq-)
X
The value of f amounts to 0.09 and 0.37 for alanine and glycine, respectively. It takes into account the fraction of eaq- yield leading to the formation of H atoms in a solution of the given amino acid; we have derived it from the following values of Hz yields reported for neutral aqueous solutions of 1 M alanine and 1 i\/l glycine: 1 . 2 P and 2.02.9 respectively. As G(H2) in eq 2 and 3 we have used the experimental values of Hz measured at corresponding amino The Journal of Physicai Chemistry, Vol. 77. N o . 22, 7973
___-
AMIYO A C I D , M
Figure 1. Dependence of experimentally derived Fwimary yields of eaq-. H, and H2 on the concentration of amino acid: A , alanine; 0 , glycine. The open symbols refer to a solution containing 5 X M NaNQ and 0.1 M HCOONa. The solid symbols M NaN03 and 0.1 M refer to a solution containing 2.5 X
HCOONa. acid concentration (Table I ) . The reactivities of nitrite ion in these solutions corresponded to the reactivities of nitrate (5.25 X l o 7 and 2.6 X IO8 sec- ') toward the hydrated electron. G(eaq-). The yield of the hydrated electron was measured in aqueous solutions of amino acid containing n small amount of nitrate, to remove eaq- from the bulk, and the formate ion to react with the H and OH radicals and to convert the intermediate of nitrate-hydrated electron reaction to the nitrite ion. Under these conditions the vield of nitrite was used as a measure of eaq- yield. Sixice some hydrated electrons are lost in the reaction with amino acid we had to use for G(e,,-) calculation the following relation
Since the pH of solutions was about 6.4, the values of h(e,,- i- AA) used in eq 4 refer only to the reactions of the hydrated electron with the dipolar ions of amino acids.1° The eventual contribution of undissociated molecules, formed within the spurs due to the H30- from the water radiolysis, could not be taken into account because of the complex pH situation within the places of localized energy deposit in the systems studied. As can be seen in Table I, the presence of sodium hydroxide in irradiated samples does not affect the nitrite ion yields, which suggests that this contribution can be neglected. Dependence of the Primary Yields on A m i n o Acid Concentration. Relations 1-4 and the data given in Table I were used to calculate the primary yield us. amino acid concentration curves shown in Figure 1. As was to be expected, the increasing concentrations of amino acids lead to depression of all the primary reducing yields. Solid and open symbols are used to distinguish between the solutions with various reactivities toward the hydrated electron. A t increased reactivities, represented by solid symbols, the absolute values are different because of some scavenging from the spurs but the trends are practically the same. Alanine is somewhat more effective in reducing the yield of eaq- than glycine; the difference is not significant. In solutions containing simultaneously up to 1 M of alanine and of glycine, the decrease in the yield is very
Prehydration Scavenging
2693
Of eaq-
TABLE I: Yields of Stable Radiolytic Products Measured in Deaerated, Neutral, Aqueous Solutions of Alanine and Glycine [Amino acid],
Alanine
-
M
G ( Hzlmeas
G(H2)measa
Glycine G( N O Z - ) ~
0.1 0.25 0.5 0.5 1 .o 2.0 2.0 2.5
G (H2)measa
G( N O Z - ) ~
0.94 0.94
0.96 1 .oo 1.11
2.73 2.66 2.54 2.37 2.15 1.68 1.33
0.88 0.88 0.83 0.80
3.19 3.17 3.04 2.85
0.78 0.78
2.87 2.35 2.34;' 2.29;g 2.33h 2.06
M NaN03
5 X
0.1 0.25 0.5 1.0 2.0 2.5
G (Hz) meas
0.40*
0.94
2.73
0.37b 0.36b 0.34*
0.90 0.85 0.83
2.50 2.35 2.12
0.34c
0.88
0.32c O.3lc
0.82 0.83
O.3Oc
0.75
0.40b 0.40b 0.37O 0.35O Q.34h 0.306 0.2gb
2.5 X M NaN03 3.19 0. 34c 3.15 0.33c 2.92 0.32c 2.75 O.3Oc 2.82;d 2.65e 2.57 O.2gc 0.25c 0.25c
0.74
M Solution contains 0.01 M NaN02; N a N 0 3 absent. Solution contains 0.05 M NaN02; N a N 0 3 absent. 2 X a Solution contains 0.1 M HCOO-. M NaOH added (pH 7 . 6 ) . g 5 X l o - ' M NaOH added (pH 8 . 4 ) . 0.1 M NaOH added NaOH added (pH 8 . 6 ) . e 0.1 M NaOH added (pH 9 . 4 ) . 1 X (PH 8.7)
TABLE I I : Yields of e R q -in Aqueous Solutions of Alanine and Glycine Mixturesa [Amino acid], M
a
ples
Alanine
Glycine
0.5 1 .o 0.5 I .o
0.5 1 .o 2.0 2.0
G(eaq-) G(N02-)
2.70 2.35 2.40 2.14
M N a N 0 3 and 0 1 Calculated from nitrite yield
25 X
M e
b
2.77 2.45 2.58 2.30
C
2.52 2.30 1.97 1.73
HCOONa present in irradiated samDerived from Figure 1
nearly equal to the sum of their combind effects (Table
rI). The prehydration scavenging of eaq- should also influence the formation of G ( 0 H ) and G(H202). According to the free-radical model, an efficient removal of the hydrated electron leads to a n increase in OH yield because of depression of water re-formation in reaction eaqOH. This increase was experimentally proved with various systems? and points out that, a t the amino acid concentrations used in this work, it should be about 10% because of prehydration scavenging.
+
T h e Yield of W a t e r Decomposition and the Prehydration Scavenging. In calculating the radiation chemical yield of water decomposition, use is widely made of the assumption of material balance. G(-H20) = G(e,,-) G(H) 2G(Hz). When the data from Figure 1 are used, the calculated G( -H20) values decrease considerably with increasing amino acid concentration. At 2.5 M glycine, for example. this decrease is about 1.3G units. It shows that the water decomposition yield cannot be derived only from the measurements in the bulk of the solutions and that one has to take into account the water molecules decomposed due to the prehydration scavenging. This contribution is for the present not well defined. One can only estimate the yield of the precursor scavenged from the difference between G(-HzO), calculated from
+
+
the primary reducing yields, obtained in the absence and in the presence of amino acid. It is worth noticing that previously reported correlation of primary yields with reactivities turned out t o be less successful in the case of the reducing products of water radiolysis than for the oxidizing one^.^,^,^ Also, significant deviations from the yield-reactivity curves have still remained after the dependence of the eaq- rate constant on ionic strength has been taken into account.illl The present work suggests t h a t a better understanding of the reactions between the eaq- precursor and the solutes used in the examination might be helpful for a satisfactory explanation of these inconsistencies. The experimental results obtained in this work do not allow any conclusion about the nature of the precursor species or the scavenging mechanism. The prehydration scavenging takes place already a t low amino acid concentrations and the results do not contradict the assumption that we are dealing with a highly mobile electron which can traverse rather long distances before it has fully developed its ionic atmosphere.
References and Notes Z . D. Draganic and I . G. Draganic, J . Phys. Chem., 76, 2733 (1972). Z. D. Draganic and I . G. Draganic, J . Phys. Chem., 75, 3950 (19 7 1 ) . W. H. Hamili, J. Phys. Chem., 73. 1341 ( 1 9 6 9 ) . R. K . Wolff, M . J. Bronskill, and J. W. Hunt, J . Chem. Phys.. 53, 4211 ( 1 9 7 1 ) . J. E. Aldrich, M .J. Bronskill, R. K . Wolff, and J . W. Hunt, J. Chem. Phys., 55. 530 (',971) 0. Micic, V. Markovic, atid D. Nikolic, J . Phys. Chem., 77, 2527
(J973). Z . D. Draganic and I . G . Draganic, J Phys. Chem.. 77, 765 (19731. 9. M . Weeks, S. A. Cole. and W.M , Garrison, J. Phys. Chem.. 69, 413: ( 1 9 6 5 ) . C.R. Maxwell. D . C. Patterson, and N. E. Sharpless, Radiat. Res.. 1, 530 (1954) J. V. Davies, M . Ebert, and A . J. Swallow, in "Pulse Radiolysis," M . Ebert, J. P. Keen. A. J. Swaliow. and J . H. Baxendale, Ed.. Academic Press, New York, N . Y., 1965, p 165; R. Brams, Radiat. Res.. 27, 319 ( 1 9 6 6 ) . E. Peled and G. Czapski, J. Phys. Chem., 7 4 , 2903 ( 1 9 7 0 ) .
The Journal of Physical Chemistry, Vol. 77.
No. 22, 7973
