758
COMMUNICATIONS TO THE EDITOR
trons. However, the investigation of such reactions is feasible with the combined technique in which the triplet states are excited optically while eaq- is generated by the electron pulse.1°
I
I
I
(10) This work was supported by the U. 8. Public Health Service on Grant No. GM-12716 from the National Institute of General Medical Sciences.
DEPARTMENT OF RADIATION THERAPY AND MICHAEL REESEHOSPITAL MEDICAL CENTER CHICAGO, ILLINOIS
L. I. GROSSWEINER A. F. RODDE, JR.
I
RECEIVED OCTOBER26, 1967
I
NoDhfholene sulfonate (mM) I.o
Excited-Molecule Reactions in the Radiolysis of Peptides in Concentrated Aqueous Solution' Sir: The radiation chemistry of simple peptides such as N-acetylglycine and N-acetylalanine in dilute
aqueous solution can be interpreted almost exclusively in terms of the formation and subsequent reaction of the peptide radicals RCONHcRz.2 In neutral solutions such radicals are formed predominantly through OH attack
H20--mrf HzOz, HB,OH, H+, eaq-
+ RCONHCHRz
OH
-+
H20
+ RCONHcR2
(1) (2)
where reaction 1 represents the radiation-induced ~ t e p . a - ~I n the absence of oxygen, the reaction RCONHCRZ
I
2RCONH6Rz --f
(3)
RCONHCRZ leads to the formation of the cr,cu'-diamino succinic acid derivative. A fraction of the RCONHcRz radicals undergo further oxidation through reactions of the type 2RCONHcRz -+ RCON=CRz RCONHCHRZ (4)
+ RCONHcRz
HzOz
+
--+
RCONHC(OH)R,
+ OH
(5)
where the Hz02in reaction 5 is derived from the radiation-induced step l. The products of reactions 4 and 5 are labile and readily decompose on mild hydrolysis, e.g.
HSO
+ RCON=CRz
-+
RCOOH
+ NH3 + RzCO
(6)
to yield ammonia and carbonyl products. In the y radiolysis of evacuated 0.05 M acetylalanine solutions a t pH 7, G(NHa) N G(>CO) N 0.5. We find, however, that there is a very marked increase in the ammonia yield (measured after hydrolysis) The Journal of Physical Chemistry
2 .o Acefylalanine ( M )
3.0
Figure 1. Amide-ammonia yield, G(NH3),as a function of acetylalanine concentration in oxygen-free solution a t pH 7 under y rays. Insert: effect of naphthalene sulfonate on G(NH8) from a 2 M acetylalanine solution.
as the concentration of the acetylalanine is increased above 0.1 M . Data6 obtained in the y radiolysis of Orfree solutions of acetylalanine are given in Figure 1. The ammonia yield tends to level off a t a value of 3 in the concentration range 2-3 M . This G(NH), increase in G(NH3) is not accompanied by a corresponding increase in the yield of carbonyl products; G(>CO) 0.7 over the entire concentration range 0.1-3 M . Hence, the increase in G(NH3) cannot be explained in terms of an enhancement in the yields of reactions 2 and 4.5 In fact, the increase in G(NH3) does not appear to be related in any significant way to the reactivity of the OH radical or its precursor HzO+.' We find, for example, that addition of formate ion, which is an effective OH scavenger
-
-
OH
(IC7
+ HCOO-
--it
HzO
+ COO-
(7)
= 2.5 X lo9M-' sec-1)8 a t concentrations as high
(1) This work was done under the auspices of the U. S. Atomic Energy Commission. (2) (a) W. M. Garrison and B. M. Weeks, Radiation Res. Suppl., 4 , 148 (1964); (b) H. L. Atkins, W. Bennett-Corniea, and W. M. Garrison, J . Phys. Chem., 71, 772 (1967). (3) (a) A. 0. Allen, Radiation Res. Suppl., 4, 54 (1964); (b) E. J . Hart and R. L. Plataman, Mech. Radiobiology, 1, 93 (1961). (4) C. J. Hochanadel and R. Casey, Radiation Res., 2 5 , 198 (1965). (5) The relatively small but apparently very real discrepancies in the reported 100-eV yields of the products of reaction 1 have been discussed by Allen (ref 3). Recent measurements by Hochanadel ~ and Casey (ref 4) give GOH= 2.59, Ge.,- = 2.58, GH = 0.55, G H = 0.45, G H ~ o=~0.72. (6). Experimental methods employed in the present work are described in ref 2a and 2b. The vapor phase chromatography was performed by Mr. H. A. Sokol. (7) The apparent yield of reaction 2 is independent of solute concentration, Le., G(RC0NHCRz) = 2.5 = GOH in both dilute and concentrated solution (ref 2). However, since OH arises from HzO --c HsO + OH (ref 3b), we cannot rule out HzO+ via Ha0 + the possibility that a t the higher solute concentrations a fraction of the HzO+ species reacts directly with RCONHCHRz to give RCONHCRz.
+
+
COMMUNICATIONS TO THE EDITOR
759
as 0.75 M has essentially no effect on G(NH3) from 2 M acetylalanine (IC2 = 2 X 108 M-1 S ~ C - ' ) . ~Negative results were also obtained with phenol as the competing OH scavenger. The evidence is that a quite different reaction mode sets in at acetylalanine concentrations above 0.1 M. Chemical analysess of the irradiated solutions reveal that propionic acid which is produced in negligible yield (G 5 0.1) in 0.05 M acetylalanine solution becomes a major product at the higher solute concentrations; at 2 M acetylalanine the propionic acid yield corresponds to G == 1.6.1° The possibility that amide and propionic acid arise as a consequence of reaction of the type
+ RCONHCHR2 -+-(RCONHCHR2)RCONH- + CHR2 (RCONHCHR2)-
eaq-
4
(8) (9)
must be considered; we have shown elsewhere that = 1 X lo7 for acetylalanine at pH 7." However, addition of chlcracetate ion which is an effective electron scavenger
ks
eaq-
+ RC1+
R
+ C1-
(10)
does not significantly reduce G(NH3) from 2 M acetylalanine even at chloracetate concentrations as high as 0.05 M; under this condition G(NHa) = 2.3 and G(C1-) = 2.5.12 Additional evidence against reaction 9 is the finding that G(NH3) from the 2 M solution at pH 7 remains essentially constant on increasing the acidity to pH 1 (kH++e&s- = 2 X 1010).8 We must conclude then t'hat the removal of OH, eaq- (and also of H 2 0 + and e-) is not involved in the radiolytic degradation of the peptide bond in these concentrated s o htions. Now, certain compounds such as benzophenone and naphthalene, for example, react rapidly with eaq- and OHs and also hiave the additional property of being efficient quenchers of excited states.'$ We find that addition of naphthalene sulfonic acid in millimolar concentration effects a sharp decrease in G(NH8) from 2 M acetylalanine as shown in Figure 1 (insert); a reciprocal yield plot of these data extrapolates to give G = 1.6 as the limiting yield for production of species which the present evidence suggests are excited states of acetylalanine. The mechanisin for formation of RCONHCHR2* has not been conclusively established. However, we have recently found that the propionic acid yield which approaches G = 1.6 in 2 M acetylalanine does not increase further with increasing solute concentration to 10 M . I n fact, this same yield is obtained in the y radiolysis of acetylalanine in the polycrystalline form. l 4 These results indicate that the production of RCONHCHR2 does not involve excitation transfer from species such as H2O* which have been considered15 as possible intermediates in water radiolysis. Our present con-
*
clusion is (a) that preferential excitation by low-energy electrons16is involved, e.g. e-
+ RCONHCHR2 +RCONHCHR2* + e-
(11)
and (b) that the species RCONHCHR2* areremoved through reaction of the type RCONHCHR2*
+ RCONHCHRz +
RCONHcR2
+ RCONH2 + cHR2
(12)
Current work is expected to provide detailed information on the physical and chemical properties of the species RCONHCHR2*. (8) For a recent compilation of rate data, see M. Anbar and P. Neta, Intern. J. Appl. Radiation Isotopes, 17, 493 (1967). (9) Determined through measurements of competition kinetics in solutions of pnitrosodimethylanaline after the method of I. Kralic and C. N. Trumbore, J. Am. Chem. SOC.,87,2547 (1965). (10) The propionic acid yield is strongly dependent on dose. The value G(propionic) i: 1.6 represents the initial yield a t a dose of 2 X 1018eV/g. (11) R. L. S. Willix and W. M. Garrison, UCRL-17285 (1966); Radiation Res., 32, 452 (1967). (12) The fact that the electron can be quantitatively measured as chloride ion in this system also rules out the possibility that reactions 8 and 9 occur prior to the hydration of e-. (13) For a recent review, see F. Wilkinson, Advan. Photochem., 3, 241 (1964). (14) W. M. Garrison, M. E. Jayko, H. E. Sokol, W. BennettCorniea, Abstracts of Papers, 154th National Meeting of the American Chemical Society, Chicago, I l l , Sept 1967. (15) (a) F. 8. Dainton and D. B. Peterson, Proc. Roy. SOC.(London), A267, 443 (1962); (b) T.J. Sworski, Advances in Chemistry Series, No. 60, American Chemical Society, Washington, D. C., 1965, p 263. (16) R. L. Platzman, Radiation Res., 2 , 1 (1955).
LAWRENCE RADIATION LABORATORYMICHAELA. J. RODGERS OF CALIFORNIA UNIVERSITY WARRENM. GARRISON BERKELEY, CALIFORNIA94720 RECEIVEDNOVEMBER 2, 1967
The Reaction of Nitrous Oxide with Excited
Molecules in t h e Radiolysis and Photolysis of Liquid Alkanes'
Sir: Nitrous oxide is used extensively in radiation chemical s t u d i e P 8 to assess the importance of ionic (1) This work was supported by the Research Division of the U. S. Atomic Energy Commission. (2) G.Scholes and M. Simic, Nature, 202, 895 (1964) (3) (a) W. V. Sherman, J. Chem. Soc., A , 599 (1966); (b) W.V. Sherman, J. A m . Chem. Soc., 88, 1567 (1966); (c) W. V. Sherman, J. Phys. Chem., 70, 2872 (1966); (d) W. V. Sherman, ibid., 7 0 , 667 (1966). (4) S. Sato, R.Yugeta, K. Shinsaka, and T. Terao, Bull. Chem. SOC. Japan, 39, 156 (1966). (5) N. H.Sagert and A. S. Blair, Can. J. Chem., 45, 1351 (1967). (6) H. Seki and M. Imamura, Bull. Chem. SOC.Japan, 38, 1229 (1965). (7) R. Blackburn and A. Charlesby, Nature, 210, 1036 (1966). (8) J. M.Warman, Nature, 213, 381 (1967).
Volume 72, Number 2 February 1068
