P. NETA,M. SIMIC, AND E. HAYON
1214
Pulse Radiolysis of Aliphatic Acids in Aqueous Solution. 111. Simple Amino Acids by P. Neta,*M. Simic,l and E. Hayon Pioneering Research Laboratory, U.S . Army Natkk Laboratories, h‘atick, Massachusetts 01 760 (Received October 16, 1969)
The pulse radiolysisof aqueoussolutionsof some amino acids-glycine, alanine, a-amino-butyricacids,valineand some of their derivatives (including imino diacetic and nitrolotriacetic acids)-has been studied. Transient optical absorption spectra of amino carboxyalkyl radicals were produced as a result of dehydrogenation by H +
+
t
.
+
atoms and OH radicals, e.g., NHaCHzCOOH OH -.c NHsCHCOOH HzO. The observed changes in the absorption of the transient with variation in pH is attributed to proton dissociation of the carboxyl and amino -H+ groups of the radical, which occurs at a pH different from that of the parent compound: kHsCHCOOH -€I+
e
hH3CHCO0 NHzCHCOO-. In alkaline solutions,a change in the site of attack by OH radicals is suggested, from the C-H to the N-H bond, with the formation of fiHCHzCO0- radicals from glycine. The a-amino-acarboxyalkyl radicals have absorption maxima in the region 250-260 nm, while the 6 and y radicals have maxima well below 240 nm. The previously postulated deamination of amino acids by hydrated electrons was confirmed by direct observation of carboxyalkylradicals, which in the case of glycine and alanine have absorption maxima at 350 and 340 nm, respectively: fiH3CRHCOOThe y radiolysis of a number of amino acids in aqueous solutions has been studied extensively. The observed radiolytic products and the postulated reaction mechanisms have recently been reviewed by Garrison.2 For instance, the major radiolytic products of 1.0 M glycine solutions were ammonia (G = 4.3), hydrogen (G = 2.0), acetic‘acid (G = l.2), glyoxylic acid (G = 2.1))with smaller yields of formaldehyde (G = 0.5) and carbon dioxide (G = 0.6). Equivalent products were observed from the higher homologs of aliphatic amino acids. The OH radicals produced in the radiolysis of water are assumed to attack aliphatic amino acids predominantly at the a position, by abstraction of a hydrogen atom from the C-H bonds. OH
+ NH3CH(R)COO- + NH,C(R)COO-
+ H20
“a+
eaq-
+ &H~CH(R)COO-
+
*CH(R)COO-
(24
/’
H+ NH,CH(R)COO-
(2b)
Direct spectroscopic observation of the transients originating from the reactions of OH and eaq- have been The Journal of Physical Chemistry
reported for a limited number of amino acids. For 260 nm was instance, transient absorption with A, ill aqueous solureported in the pulse radiolysis of tions of glycine3 at pH 6.5 and was attributed to dehydrogenation of the solute by OH radicals. Transient spectra were also observed in the pulse radiolysis of aqueous solutions of cysteine and cystine4and of phenylalanine and t y r ~ s i n e . ~However, these solutes involve reactions qualitatively different from reactions 1 and 2 due to the comparatively much higher reactivity of the -SH group and the benzene ring toward OH radicals, and of the -S-S- and -SH group toward hydrated electrons. Some of the amino carboxyalkyl radicals formed by reaction 1 have also been produced from amino acids by the rapid mixing technique using Tic13 HzOz and were identified by their esr spectra.s
+
(1)
The reaction of the hydrated electron, e,,-, with amino acids was envisaged2 as a reaction with the protonated amino form leading to deamination as well as to partial formation of H atoms and/or Hz
~
+ eaq--+ 6RHCOO- + NHa.
(1) National Academy of Sciences-National Research Council Research Associates at Natick. (2) W. M . Garrison in “Current Topics in Radiation Research,” M. Ebert and A. Howard, Ed., North-Holland Publishing Co., Amsterdam, 1968, p 43. (3) J. V. Davies, M. Ebert, and A. J. Swallow in “Pulse Radiolysis,” M. Ebert, J. P. Keene, A. J. Swallow, and J. H . Baxendale, Ed., Academic Press, New York, N. Y., 1965, p 165. (4) G. E. Adams, G. S. McNaughton, and B. D. Michael in “Chemistry of Ionization and Excitation,” G. R. A . Johnson and G. Scholes, Ed., Taylor and Francis, London, 1967, p 281; E. Hayon and M. Simic, to be published. (6) J. Chrysochoos, Radiat. Res., 33,465 (1968). (6) H . Taniguchi, K.Fukui, S. Ohnishi, H. Hatano, H. Hasegawa, and T . Maruyama, J . Phys. Chem., 72,1926 (1968) ; R. Poupko, B. L. Silver, and A. Lowenstein, Chem. Commun., 453 (1968); W.A. Armstrong and W. G. Humphreys, Can. J . Chem., 45,2689 (1967).
PULSE RADIOLYSIS OF ALIPHATIC ACIDS Experimental Section This work was carried out using a Febetron 705 pulsed radiation source, and the details of the experimental conditions have been described elsewhere.' Briefly, this source produces an electron beam of 2.3 MeV energy and single pulses of = 30 nsec duration. The monitoring light source was an Osram XBO 450-W xenon lamp, and the light output from the lamp was increased by a factor of 25-30 times by increasing the current to the lamp with pulses of -1.2 msec duration. A double monochromator, consisting of two high-intensity Bausch and Lomb monochromators in series, was used to reduce scattered light and improve wavelength resolution. The amino acids and derivatives used were either Calbiochem A Grade or Cyclochemical Grade I reagents. t-Butyl alcohol was Mallinckrodt AR, nitrilotriacetic and iminodiacetic acids were supplied by Eastman, and the inorganic chemicals were Baker and Adamson reagents. Nitrous oxide and argon were supplied by Matheson. Solutions were prepared using water purified by triple distillation, radiolysis, and photolysis. The solutions were buffered using perchloric acid, potassium hydroxide, sodium tetraborate (1-3 mM), and potassium phosphates (1-3 m u ) . Dosimetry was carried out using a 0.1 M KCNS solution and the (CNS)2- radical formed was observed a t 500 nm (e 7600 M-I cm-l). Doses of 2.4 to 36 krads/pulse were used and were derived on the basis of GB-aq= GOH = 2.8. The choice of solute concentration was made on the basis of known radical-radical and radical-solute rate constants,8 and complete scavenging of radicals was also experimentally established by comparing yields of transient species a t different solute concentrations. The optical densities were usually read a t 1 to 2 psec after the pulse and were extrapolated to zero time. The reproducibility of readings was better than *5%. Results and Discussion As in our previous pulse-radiolysis studies of carboxylic acids,9JO the amino acid transients were produced in aqueous solutions through the reaction of OH radicals with the dissolved amino acids. The hydrated electrons were in general converted (-98%) into OH radicals by saturating the solutions with nitrous oxide. At p H
