1246
M. ANBARAND I. PECHT
The Sonolytic Decomposition of Organic Solutes in Dilute Aqueous Solutions.
111. Oxidative Deamination of Ethylenediamine by OH Radicals’
by M. Anbar and I. Pecht The Weimann Institute of Science, Rehouoth, Israel
(Received August 6 , 1966)
The formation of OH radicals under sonolysis has been investigated by following the oxidative deamination of ethylenediamine. The behavior of ethylenediamine in sonolyzed solutions was compared with that under radiolysis. The yield of OH radicals exceeded that of H atoms by a factor of 50. This is explained by the reaction H HzO --c Hz OH feasible at the high temperatures prevailing in the cavities.
+
The formation of OH radicals in sonolyzed water has been suggested in previous s t ~ d i e s . ~ -It ~ has been assumed that hydrogen peroxide is a. product of the recombination of OH radicals in sonolyzed solutions.3 Subsequently it was shown that the hydrogen peroxide is being produced primarily in the cavities and that OH radicals in solutions, if formed, cannot be considered as the major precursors of hydrogen per~xide.~ On the other hand, experiments using the T1*-Ce4+ system14the hydroxylation of aromatic compounds,0 and the study of other systems’~*strongly suggest OH radicals as intermediates in the sonolysis of water. Oxidations by OH radicals are known to be the predominant processes in the radiolytic damage to molecules of biological importance. The oxidative deamination of amino acids, polypeptides, or proteins, which takes place almost exclusively by OH radicals, is one of the characteristic, detrimental reactions in radiobiology. The expanding application of ultrasound to the fields of biochemistry and biology9 makes it of importance to assess the role of OH radicals in such sonolyzed systems. The sonolytic yields of OH radicals have been inferred in previous studies as the difference between oxidative and reductive proce~ses;~still an unambiguous identification of the oxidizing species in sonolyzed water and the direct determination of its yield were desired. The extent of deamination of amino compounds by OH radicals has been shown to be a direct measure of the yield of these species.1° It was therefore decided to investigate an oxidative deamination reaction under sonolytic conditions using ethylenediamine as a simple model compound. EthyleneThe Journal of Physic& Chemistry
+
diamine has been investigated under radiolytic conditions’O and the information in hand was used for comparison.
Experimental Section Solutions for sonolysis were prepared in triply distilled water saturated with argon or the proper gas mixture and sonolyzed under conditions identical with those previously described.” Ethylenediamine of “Puriss” grade supplied by Fluka was used without further purification. Solutions of this compound were prepared by neutralization with the proper amount of HC1. Methanol (Fluka Puriss grade, >99.9% pure), %propanol (BDH Analar grade), sodium formate (Baker Analyzed reagent), glucose, and T12S04 (BDH Analar grade) were used without further purification. Gas mixtures of oxygen in argon were prepared by compressing argon into a sampling (1) Part 11: M. Anbar and I. Pecht, J.Phys. Chem., 68,1462 (1964). (2) P. Grabar and R. 0. Prudhomme, J. Chim. Phys., 44, 145 (1947). (3) A. Weissler, J. Am. Chem. SOC.,81, 1077 (1959). (4) P. Rivayrand and M. Haissinsky, J . Chim. Phys., 59, 623 (1962). (5) M. Anbar and I. Pecht, J. Phys. Chem., 68, 352 (1964). (6) A. Weissler, Nature, 193, 1070 (1962). (7) N.Miller, Trans. Faraday SOC.,46, 546 (1950). (8) A. Cier, C . Nofre, and L. Welin, Bull. SOC.Chim. Biol., 43, 1229 (1961). (9) E. Kelly, Ed., “Ultrasound in Biology and Medicine,” American Institute of Biological Science, Washington, D. C., 1957. (IO) M. Anbar, R. A. Munos, and P. Rona, J. Phys. Chem., 67,2708 (1963). (11) M. Anbar and I. Pecht, ibid., 68, 1460 (1964).
SONOLYTIC DECOMPOSITION OF ORGANIC SOLUTES
tank previously filled with oxygen a t a calculated pressure. The analytical procedure for the determination of the produced ammonia by a microdiff usion method was as previously described.1° The yield of H202was determined spectrophotometrically by the triiodide m e t h ~ d ;hydrogen ~ was determined by gas chromatography; hydrogen atoms were assayed by deuterium abstraction from formate-d ions.“ The yield of molecular oxygen was determined by isotope dilution, adding 0% to the examined H2018 solution after sonolysis and analyzing the gas for 0 1 8 2 by mass spectrometry.
Results and Discussion The sonolytic deamination of ethylenediamine (en) has been examined a t different concentrations of the substrate a t pH 5.0. The yield increased linearly with time of sonolysis in the range studied (45-200 min). The yield increased with concentration of en (Table I) and reached a plateau above 3 X M . This indicated that the en was deaminated by “indirect action,” i.e., by a product of the sonolysis of water. The slight increase in YNH*observed a t higher en concentrations was most probably due to “direct action,” namely, the decomposition of en inside the cavities. Ethylenediamine, like other nonvolatile solutes at high concentrations, may be introduced into the cavities by sputtering. l 2 Table I: Sonolysis of Ethylenediamine under Argon, pH 5.0-5.5 en, moles/l. x 10’
5 10 20 30 50 100 200
“1,
moles/l. min x 10‘
3.4 4.5
6.5 10.4
10.6 10.7 11.3
It has been previously shown that the deamination of en is a result of the action of OH radicals.10 This conclusion is corroborated by studying the effect Of nonvolatile scavengers on the sonolyzed system (Table 11)* The effects Of formate ions and Of glucose On YNH’have been studied (2.2, 2.3, 2.9, 2.10). The relative rate obtained, ken+OH/kHCOO-+OH= 20, is in agreement with reported in the literature for en and formate ions.la
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The addition of thalous in excess of formate ions counteracts the effect of the latter (2.5); however, when the concentration of formate ions exceeds that of T1+ (2.8), some scavenging action of the former is observed. An analogous behavior is observed in the englucose-T1+ system (2.11, 2.12, 2.13). The specific effect of T1+ in the oxidative deamination of en has been previously discussed.1° It has been shown thak T12+,which is formed by the very fast TI+ OH OH reaction, efficiently oxidizes en. As the Tl+ reaction is about an order of magnitude faster than most reactions of OH radicals with organic solutes, it competes for these radicals and induces oxidative deamination in the presence of organic OH scavengers. The oxidation of cupric ions by OH radicals, which proceeds at a specific rate of 3.5 X lo8 M-’ sec-’,14 i.e., over 5 times faster than the OH en reaction,1° presents another example of ligand oxidation. Copper ions do not inhibit en oxidation (2.14); moreover they counteract the scavenging action of glucose (2.16) by competition for the OH radicals. A rather interesting effect is observed in the presence of both thalous and cupric ions (2.15). The yield of ammonia in this case is significantly diminished. Under the experimental conditions virtually all OH radicals react with T1+ to give T12+. T12+ ions, unlike OH radicals, are incapable of oxidizing Cu2+ to Cu3+ and since practically all en is complexed with copper ions, the T12+ ions have no chance of interaction with free en before undergoing disproportionation to TI+ and T13+.15J6 The effect of volatile organic scavengers (2.192.22), methanol and 2-propanol, on the yield of ammonia is completely different from that of nonvolatile OH scavengers. Methanol diminished Y N H ~(2.19) to an extent over and above that expected from its rate of reaction with OH radicals.” Moreover, unlike the behavior observed under radiolysis, lo the effect of these volatile scavengers is not reversed by the addition of Tl+ ions (2.20, 2.22). It has been previously demon&rated5 that volatile solutes affect the primary yields of the sonolytic products which are formed in the cavities. Thus the decrease in the yield of ammonia
+ +
+
(12)E. ~ ~2. A i ~phys., ~ ~ 12, , 423 ~ (1960). . (13) J. K.Thomas, Trans. F U ~ ~ ~sot., U U 61, 702 (1965). (14) J. H. Baxendale, E. M. Fielden, and J. P. Keene in “Pulse J. p. Keen% and A. J. Radiolysis,’’ J. H. Baxendale, M.
Swallow, Ed., Academic Press Inc., London, 1965, p 328. (15) B. Cercek, M. Ebert, and A. J. Swallow, J. Chem. Soe., Sect. A , 612 (1966). (16) Compilation of rate constants of eaq-, H, and OH reactions. M. Anbar and P. Neta, AEC Report IA-1079, Israel, 1966. (17) G . E.Adams, J. W. Boag, and B. D. Michael, Trans. Faraday Soc., 61, 1417 (1965).
Volumrr 71, Numbsr 6 April 1$67
M. ANBARAND I. PECHT
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Table 11: The Effect of Additivea on the Sonolytic Yields of Ammonia from Ethylenediamine, pH 5.0-5.5, under Argon (Concentrations in Millimoles per Liter) YN€Ii,
en
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24
30 30 30 30 30 50 50 50 50 50 50 50 50 50 50 50 50 100 50 50 100 100 50 50
HCOO-
T1+
Glucow
CU' +
MeOH
...
...
...
... ... ...
...
... ...
... ...
1 2.5
...
2.5
... *.. 5 5
...
25
25
25 5
... ..
...
...
.. ..
25 50 100
...
...
...
.. ...
... ..
... ...
...
... 25
...
...
... 50
... 100
...
...
... ...
... ...
... 5 20 5 20 20
...
... 5 20
I . .
...
in the presence of the volatile alcohols is not due to a competition for OH radicals in solution but to diminution of the primary yield of OH radicals. The addition of a small amount (2%) of oxygen has no effect on YNH,(2.23). Oxygen at this concentration, which was shown to scavenge all H atoms in solution," does not affect the sonolytic yield of deamination. This result corroborates the conclusion that the deamination is due only to OH radicals and is not affected by H atoms or H02 radicals. O2 (30%) in argon diminishes YNH*to a certain extent (2.24), owing to the effect of this additive at high concentrations on the over-all sonolytic yields.19 It may be concluded that OH radicals produced in the cavities in sonolyzed water find their way into the bulk of the solution. The yield of each of the primary products of sonolysis of water, namely, hydrogen molecules, hydrogen atoms, oxygen, hydrogen peroxide, and hydroxyl radicals, has been determined independently under our experimental conditions and found asf follow^: Y H=~ 12.5, Y E = 0.2, Yo, = 0.25, YHZOa = 6.5, and YO= = 10.5 pmoles 1.-l min-l. The yield of molecular oxygen was determined by isotope dilution adding Ole, to a sonolyzed solution of H201*. If no other products were formed, then 2YH, -I- Y H =
... ... ...
... ...
... ...
...
..I
... ... ... ... ..
25 25 25 50
... ... ...
...
... ... ... ... ...
...
...
... ...
e . .
... ...
...
... ...
Oa
...
...
...
*.. ...
GPrOH
... ... ...
... ...
50 100
...
50
.,.
50
...
...
2% 30%
...
... ..
+
... ...
...
...
...
0.1 10.9 5.4 6.7 11.0
... ...
..I
...
3.8 0.3
... ...
*..
10.4 6.2 4.0 10.6 10.8 10.6 10.8
...
... ... ... ...
... ...
fimolea/l. min
3.0 10.7 0.2 10.7 0.7 0.3 0.5 0.5 10.3 6.1
+
2YH10$ YO= 4Y02. Agreement of the experimental data with this relation indicates that no other products are formed with appreciable yields. The fact that the amount of OH radicals found in the bulk of solution exceeds that of H atoms by over an order of magnitude deserves special attention. A possible explanation for this observation could be the fact that in the extremely hot cavities hydrogen atoms are produced with high translatory energies. When these atoms react with water, the reaction H
+ H20
4
H2
+ OH
(1)
takes place and results in an additional production of OH radicals at the expense of hydrogen atoms. This reaction is endothermic by 26.2 kcal/mole.a It has been estimated that the temperatures inside the cavities of water sonolyzed under argon reach >10,000°K.21 At this temperature about 50% of the molecules (18) J. P. Sweet and J. K. Thomaa, J . Phys. Chenz.,68,1363 (ISM). (19) A. Henglein, N&UrWissenaChqfta, 44, 179 (1957). (20) W. M. Latimer, "The Oxidation States of the Elements and Their Potentials in Aqueous Solutions," 2nd ed, Prentice-Hall, Inc., Englewood Cliffs, N. J., 1952. (21) B. E. Noltingk and E. A. Neppiras, Proc. Phye. Soc. (London), B63, 674 (1960); D.Srinivasan and L. V. Holroyd, J . AppE. Phys., 32, 446 (1961).
SONOLYTICDECOMPOSITION OF ORGANIC SOLUTES
and atoms in the cavity will possess kinetic energy equal to or higher than 1.1 ev.22 Hydrogen atoms with this energy when encountering water molecules will undergo the reaction cited above and produce OH radicals plus hydrogen molecules. The rate of reaction 1 with hot H atoms is most probably diffusion controlled; thus the yield of OH radicals will not be affected by the presence of 2% oxygen. The hydrogen molecules produced under sonolysis of water exhibit a very small H I D isotope effect, Le., close to unity.23 Recently it was shown that the “molecular” hydrogen formed under radiolysis is produced with an H I D isotope effect which is significantly lower than that expected from reactions involving hydrated electron^.^* It was also found that the H I D isotope effect is further decreased under radiolysis of high linear energy transfer.26 The participation of
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“hot” hydrogen atoms, Le., with kinetic energies exceeding 1 ev, which might result from the decomposition of highly excited water molecules, could eventually contribute hydrogen molecules formed with a low H I D isotope effect.
Acknowledgment. The research reported in this article has been sponsored by the Air Force Office of Scientific Research under Grant AF EOAR 65-87 through the European Office of Aerospace Research (OAR), U. S. Air Force. ~~~~
(22) S. W. Benson, “Foundations of Chemical Kinetics,” McGrawHill Book Co., Inc., New York, N. Y., 1960, p 143. (23) M. Anbar and I. Pecht, J. Chem. Phys., 40, 608 (1964). (24) M. Anbar and D. Meyerstein, Trans. Faraday Soc., 62, 2121 (1966). (26) M. Anbar and D. Meyerstein, ibid., 61, 263 (1965).
Voluns 71, Number 6 April 1987
