WYNNA. VOLKERTAND ROBERT R. KUNTZ
3394
The Reactions of Hydrogen Atoms in Aqueous Solutions.
Some Amino Acids1
by Wynn A. Volkert and Robert R. Kuntz2 Department of Chemistry, University of Miasouri, Columbia, Missouri 66801
(Received February 7, 1068)
The 6OCo y radiolysis of acidic aqueous solutions of amino acids and related compounds was carried out in order to determine relative rates of H atom reactions with these compounds. These rates were measured by competition of H atom donor and scavenger species for the radiolytic H atoms. Rate constant ratios were evaluated by a graphical technique utilizing the variation of G(H2) with solute concentration. The H atom donor species were in competition with allyl alcohol for radiolytic H atoms while the aromatic and sulfur-containing acids which serve as H atom scavengers wera allowed to compete with formic acid. Absolute rate constants were derived using known rates for various competitors. Rate constants derived were in the range of 106 to 106 l./mol sec for abstractions and ca. lo9 l./mol sec for additions. The amino acids were selected so the effect of substituents on the side chain could be studied. Decompositions were kept below 0.1% to avoid scavenging of H atoms by reaction products which result from the H atom and OH radical attack on the species. Electrons were scavenged by HsO+ and did not contribute significantly to the over-all decomposition.
I. Introduction I n the past few years rate constants have been determined for reactions of the primary reactive intermediates (H atoms, OH radicals, and eaq-) formed during the radiolysis of aqueous solutions with many organic and inorganic solute^.^ Reaction rates of the OH radical and eaq- with biologically important molecules have been reported, but little information is available concerning the reactivity of H atoms with these compounds. A general parallelism between the reactivities of compounds toward both the H atom and OH radical is ~ b s e r v e d . ~Therefore, it should be advantageous to compare the reaction rates of both radicals toward the same compounds under similar conditions. One of the most widely used methods for obtaining information on the reaction rates of a radiation-induced reaction is to follow the effect of a known scavenger on a selected product yield. If a satisfactory competition can be established, the relative rates of competing reactions can be determined. In this study, deaerated aqueous solutions of some amino acids and other selected compounds were in competition with each other for the radiolytically produced H atoms. From the results of these competitions, the rate constants for H atom reactions with the amino acids were determined.
11. Experimental Section The amino acids were obtained from three sources. Only L isomers were used if they were available. The L isomers of valine, alanine, isoleucine, proline, hydroxyproline, threonine, phenylalanine, tyrosine, cystine, and glutamic acid were the purest grade produced by the Nutritional Biochemicals Corp. The DL isomers of norvaline and a-aminobutyric acid were from Eastman (White Label) Organic Chemicals. DLSerine, L-methionine, and L-lysine monohydrochloride were research grade from Mann Research Laboratories. Allyl alcohol, sodium formate, and sodium nitrate were The Journal of Physical Chemistry
Certified reagent grade from Fisher. All chemicals were used without further purification unless apparent impurities were detected. Recrystallization of the amino acids found to be impure did not alter their reactivity to any noticable degree. Distilled water redistilled from basic permanganate was used as the solvent. Degassing was accomplished by gentle cavitation of the solution. The gaseous hydrogen was evolved after irradiation in a similar manner. The quantity of hydrogen produced from the irradiation was analyzed on a Barber-Coleman thermal conductivity gas chromatograph. All samples were maintained at a hydrogen ion concentration of 0.1 M by the addition of measured quantities of HzS04. The resulting pH of the solution was subsequently verified on a pH meter to A0.20 pH unit. Concentrations of solutes present in solution were adjusted so all the hydrated electrons produced during the irradiation were converted to H atoms in the acidic solution according to the equation
+ H + +H
(1)
y rays from a ‘j0Cosource were used to generate the H atoms. The irradiated samples were maintained at a reproducible sample position in relation to the source. A Fricke dosimeter using G(Fe3+) = 15.5 molecules/ 100 eV was used to determine the dose rate of 2.65 X 1OI6 eV/ml min at the sample position. All irradiations were done at pH 1 and low solute concentrations in order to permit the hydrated elec(1) Work supported by the National Institutes of Health, Grant RH-00321; based on a Ph.D. thesis by W. A. Volkert, submitted to the University of Missouri, Columbia, Mo., 1968. (2) Research Associate of the Space Science Research Center, University of Missouri, Columbia, Mo. (3) M. Anbar and P. Neta, I n t . J . A p p l . Radiat. Isotopea, 18, 493 (1967). (4) M. Anbar, D. Meyerstein, and P. Neta, J. Chem. Sac., B , 742 (1966).
THEREACTIONS OF HYDROGEN ATOMSIN AQUEOUSSOLUTIONS trons to be converted to H atoms (reaction 1). Rate constants of the radiolytically produced H atoms were determined by a competition method first employed by Baxendale and Smithies6 and subsequently used by several other workers.6 This method involves the competition of a hydrogen atom donor (DH) with a hydrogen atom scavenger (S) for the H atoms
H
H
+ DH Ld,H2 + D
+ S -% SH
(2)
(3)
+
Where Gred = GH G,,,- and G,,,-, GH, and G H ~are the primary radiation-produced yields of eaq-, H, and Hz, respectivelg. The GH%Waf; determined as 0.43 rt 0.03 in an acidic 10-3 M KBr solution which is in good agreement with the literature values of 0.40-0.45.7 The total hydrogen yield was 3.96 0.04 when only formic acid at pH 1 was present in solution. The G values used in the calculations were G H ~= 0.43 and Gred =z 3.53.
*
111. Results Competition of HCOOH with NOS- and Allyl Alcohol for H Atoms. The method for determination of the absolute rate constants for the reactions of H atoms with amino acids depends upon the knowledge of the rate constant of the reaction of formic acid with H atoms. Two independent determinations of this value have given identical values of 1.1 X lo6 l./mol sec at pH l.3 The nitrate ion and allyl alcohol, which do not form molecular hydrogen in their reactions with H atoms, were allowed to compete with formic acid for the radiolytic H atoms in 0.1 M H+. The values obtained from this competition were plotted in Figures 1 and 2 according to eq I, and the slopes were determined by the least-mean-squares line which passed through the points. The slopes are equal to the expression 11 Gred(lCs/kd). Thus Using k d = 1.1 x lo6 l./mOl SeC and Gred = 3.53, the values for k , were calculated. k, for H NO3- was found to equal 1.21 X lo7 l./mol sec which agrees well with literature values at the same pH (1.20 X lo7and 1.30 X 10' l./mol ~ e c ) . The ~ rate constant for H[ allyl alcohol was calculated to equal (2.80 f 0.10) X lo9 l./mol sec a t 0.10 A4 hydrogen ion concentration, while the literature reports (2.3 rt 0.5) .X lo9 l./mol sec6b,8measured in neutral media. These two scavengers were used in competition with the H atom donor amino acids and their rate constants were used for calculation of k d for amino acids.
+
:::I 1.0
'0.9 h
r"0,S
" 9 0.6 0.5 0.4
(no Hz produced)
The amount of' molecular hydrogen produced, denoted as G(Hz),is reduced to an extent which depends on the ratio of the concentrations of scavenger to donor in accord with the equation
+
3395
0.3
0.2
. I
0.00
0.0)
0.06
0.09
0.12
0.15
INOjl
/
0.18 IHCOQHI
0.21
0.24
0.27
Figure 1. Competition data for the standardization of the nitrate ion rate constant with formic acid plotted in accordance with eq I: at pH 1.0, [HCOOH] 5 0.4 M , and M . At pH 1.0, formic acid and the nitrate [NO,-] 5 ion scavenge less than 1%of the hydrated electrons.
Effects of Products Formation on G(H2). The G yield of H2 decreases significantly after irradiation of the pure donor compounds beyond a few minutes at this dose rate. This decrease is attributed to the formation of products, which are the results of H atom abstraction from the amino acids and which scavenge the H atoms at a very fast rate. Olefinic compounds have been found to be major radiolysis products apparently resulting from the abstraction of an H atom from alanine.g The presence of a double bond in a product would cause rapid scavenging of H atoms at relatively low conversions (presumably at a rate comparable with allyl alcohol). Thus, the amino acids were never irradiated beyond a point where product formation had any measurable effect on G(H2). It was necessary to keep the decomposition of the aliphatic amino acids below ea. 0.01% to ensure that no scavenging by the radiation products occurred. Reaction of H A t o m with Scavenge7 Ainino Acids. The rates of reaction of five amino acids which acted exclusively as H atom scavengers were determined relative to formic acid. Methionine, cystine, tryptophan, phenylalanine, and tryosine competed with formic acid for H atoms at a 0.10 0.02 M hydrogen ion concentration. The resulting data appear in the Table I. Plotting these data in accord with eq I gives the rate constants summarized in Table 11. These rate con-
*
(5) J. H. Baxendale and 0. H. Smithies, 2. Phys. Chem. (Frankfurt am Main), 7, 242 (1956). (6) (a) J. Rabani and G. Stein, J . Chem. Phys., 37, 1865 (1962); (b) G. Soholes and M. Simio, J . Phys. Chem., 68, 1738 (1964); (e) J. Halpern and J. Rabani, J . Amer. Chem. Soc., 88, 699 (1966). (7) (a) H. A. Mahlman and J. W. Boyle, ibid., 80, 773 (1958); (b) E. Hayon, J . Phys. Chem., 65, 1502 (1961); (0) C. H. Cheek, V. J. Linnenbom, and J. W. Swinnerton, Radiat. Res., 19, 636 (1963). (8) A. Appleby, G. Scholes, and M. Simio, J . Amer. Chem. Sac., 85, 3891 (1963). (9) B. M. Weeks, 5. A. Cole, and W. M. Garrison, J . Phys. Chem., 69, 4131 (1965).
Volume W vNumber 10 October 1968
3396
WYNNA. VOLKERT AND ROBERT R. KUNTZ
stants were calculated on the basis that ~ H + H C ~ O at H pH 1 is 1.1 X lo6l./mol. sec. Rate constants for H atoms with scavenger amino acids are large (in the neighborhood of 109 l./mol sec). I n fact, these rate constants with H atoms are nearly as high as those of OH radicals. lo The presence of an aroTable I : Competition Data for Scavenger Amino Acids Run in Accord with E q I 103 [So]"
[HCOOH]*
10aRC
l/AG
Methionine 200 0.00 1.60 0.32 0.64 0.64 0.32 0.32
0.00 0.10 0.40 0.30 0.30 0.20 0.20 0.40
m
0.00 4.00 1.04 2.13 3.20 1.60 0.80
0.285 1.180 0.518 0.780 1.020 0.645 0.483
5.00 0.00 1.00 1.00 0.50 0.50 0.60 0.40
Phenylalanine 0.00 m 0.10 0.00 0.40 2.50 0.30 3.33 0.40 1.25 0.30 1.67 0.20 3.00 0.20 2.00
0.285 0.680 0.785 0.470 0,580 0.770 0.580
5.00 0.00 1.00 1.00 0.50 0.50 0.20 0.20 0.40
0.00 0.40 0.40 0.20 0.40 0.30 0.30 0.20 0.20
Table I1 : Hydrogen Atom Scavenging Amino Acid Rate Constants
Amino acid
Slopeal x 10-2
Methionine Phenylalanine Cystine Tyrosine Tryptophan
2.26 =k 0.06 1.56 i0.06 8.15 Z!Z 0.30 3.06 f 0.13 8.57 f 0.34
Rate oonstantec
x
10-8
8.79 f 0.54 6.05 i 0.43 31.6 rt 2 6 12.0 f 0.85 33.3 f 2.7
a Slopes were obtained from a plot of the data in Table I according to eq I (the slope equals l/G(ks/kd)). b The slopes were calculated by a least-mean-squares program on an IBM 7040 computer. Rate constants were calculated on the basis that the rate constant for the hydrogen atom reaction with formic acid is 1.1 x 106 l./mol sec.
Tyrosine m
0.00 2.50 5.00 1.25 1.67 0.67 1.oo 2.00
0.285 1I090 1.820 0.690 0.760 0.550 0.615 0,900
Cystine
6.00 0.00 0.12 0.12 0.12 0.20 0.20 0.20
0.00 0.40 0.40 0.20 0.30 0.20 0.30 0.40
m
0.00 0.30 0.60 0.40 1.00 0.67 0.50
0.285 0,550 0.760 0.670 1.090 0.870 0.680
Tryptophan 2.00 0.00 0.20 0.20 0.20 0.08 0.10 0.08
0.00
m
0.40
0.00 0.67 0.50 0.50 0.40 0.33 0.20
0.30 0.40 0.40 0.20 0.30 0.40
0,285 0.860 0.705 0.715 0.590 0.520 0.485
a The concentrations of the scavenger amino acids, [Sc], are The concentrations of formic acid are in in moles per liter. moles per liter. c The ratio R is the ratio of the scavenger concentration over the concentration of formic acid.
The Journal of Physical Chemistry
Figure 2. Competition data for standardization of the allyl alcohol rate constant with formic acid plotted in accordance with eq I at, p H 1.0, to 10-4 M [HCOOH] 5 0.4 M , [allyl alcohol]
-
matic group or sulfur atom in a compound is responsible for the fast reaction rate. Since the hydrated electron also reacts rapidly with these scavenger^,^ the concentrations of these compounds were kept low enough so that no appreciable reaction with the electrons occurred. The hydrogen peroxide concentration never exceeded 3 X M in these experiments and could not, therefore, compete with H + for the hydrated electrons. Donor Amino Acids. The rates of reaction of several amino acids which reacted entirely by donation of H atoms were determined relative to allyl alcohol at a hydrogen ion concentration of 0.10 =t 0.01 M . The resulting data were tabulated (Table 111), arid some representative compounds were plotted according to eq I (Figure 3). Least-mean-squares treatment of these data resulted in the rate constants reported in Table IV. The rate constants were calculated on the (10) G. Scholes, P. Shaw, and R. L. Willson, "Pulse Radiolysis," Academic Press Inc., London, 1965, p 151.
THER E A C T I O N S
OF
HYDROGEN ATOMSI N AQUEOUSSOLUTIONS
3397
Table I11 : Competition Data for Hydrogen Donor Amino Acids Run in Accord with Eq I PHla
RE
WClb
]/A&'
IDHI"
lfiolb
RG
l/AU
Serine 8.00 x 104 0.475 104 20.00 x 104 0.738 5.00 x 103 104 1.00 x 103 104 33.30 X lo4 1.010 5.00 x 103 1.00 x 103 1.210 104 40.00 X l o 4 0.50 x 103 2.50 x 103 0.805 3.33 x 1 0 3 25.00 x 104 104 0.50 x 103 0.00 x 108 104 13.00 X lo4 0.605 08.00x 103 0.875 25.00 X lo4 1.25 x 103 104 01.25 x 103 01.00 x 103 0.00 x 103 Threonine 0.00 x 103 01.00 x 103 0.10 0.00 0.00 0.288 2.50 x 103 01.50 x 103 0.290 0.00 0.00 0.10 Alanine 0.625 2.00 x 104 3.33 x 103 0.06 0.530 0.09 2.00 x 104 2.23 x 103 0,318 0.00 0.00 0.40 0.980 4.00 x 104 6.67 x 103 0.06 0.00 x 104 0.315 0.08 01.00 x 104 0.755 4.45 x 103 0.09 4.00 x 104 0.00 x 104 0.321 01.00 x 104 0.40 0.440 0.12 2.00 x 104 1.67 X loa 2.50 x 104 1.140 0.80 x 104 0.32 0.650 0.06 2.00 x 104 3.33 x 103 1.11 x 104 0.705 0.27 x 104 0.24 0.575 2.23 x 103 0.09 2.00 x 104 0.840 X lo4 0.27 x 104 0.610 0.32 0.820 5.55 x 1 0 3 0.09 5.00 x 104 0.700 1.25 x 104 0.40 x 104 0.32 0.840 0.40 x 104 0.24 1.67 x 104 Lysine 1.080 2.22 x 104 0.24 0.55 x 104 0.00 0.00 0.284 0.05 0.290 0.00 0.00 0.05 a-ABAa 2.280 5.00 x 104 25.00 X lo4 0.20 0.00 0.00 0.287 0.20 1.180 2.00 x 104 10.00 x 104 0.20 0 , 00 0.00 0.290 0.20 0.672 5.33 x 104 0.15 0.80 x 104 0.00 0.00 0,287 0.08 4.00 x 104 0.595 0.20 0.80 x 104 5.00 x 104 o BO x 104 0.990 0.16 0.27 x 104 1.78 x 104 0.15 0.500 1.67 x 104 0.530 0.27 x 104 0.16 1.480 2.00 x 104 13.30 X lo4 0.15 3.33 x 104 0.730 0.12 0.53 x 104 0.915 0.10 0.80 x 104 8.00 x 104 0.40 x 104 2.50 x 104 0.600 0.16 Glutamic Acid 3.33 x 104 0.720 0.40 x 104 0.12 1.67 x 104 0,500 0.00 0.00 o .27 x 104 0.14 0.300 0.16 0.40 x 104 0.635 2.50 x 104 0.14 0.16 0.00 0.00 0 305 0.28 0.80 x 104 2.90 x 104 0.550 Isoleucine 0.28 7.25 x 104 2.00 x 104 0.870 0.00 0.00 0.281 0.25 0.21 0.590 0.80 x 104 3.87 x 104 0.278 0.00 0.00 0.05 0.14 0.80 x 104 5 . 8 0 x 104 0.770 0.00 0.00 0.05 0 283 0.21 0.40 x 104 0.440 1 . 9 3 x 104 5.00 x 103 0.925 0.20 11.00 x 108 0.21 0.710 o i o x 104 0.483 X lo4 2.67 x 103 0.665 0.40 X I O a 0.15 Proline 1.160 0.15 II .oo x 108 6.67 x 103 0.10 0.00 0.00 0.290 0.40 x 103 0.10 4 . 0 0 x 103 0.830 0.20 0.00 0.00 0.291 2.00 x 103 0.560 0.20 x 108 0.10 0.10 0.00 0.00 0.291 0.50 x 103 3.33 x 103 0.715 0.15 0.20 0.40 x 104 2.00 x 104 0.600 0.20 0.50 x 103 2.50 x 103 0.625 0.15 o 40 x 104 0.725 2.67 x 104 0.10 0.40 X lo3 4.00 x 103 0.800 0.20 0.40 x 104 0.590 2.00 x 104 Norvaline 0.10 2.67 x 104 o ,267 x 104 0.685 0.00 0.00 0.300 0.05 0.15 0.535 x 104 3.56 x 104 0.840 0.05 0.00 0.00 0.300 0.15 0.27 x 104 1.80 x 104 0.590 0.20 0.40 x 104 2.00 x 104 0.400 0.20 o 27 x 104 1.34 x 104 0.520 0.470 o .40 x 104 0.15 2.67 x 104 Hydroxyproline 0.80 x 104 4.00 x 104 0.20 0.495 0.10 0.00 0.00 0 288 1 . 3 3 x 104 0.27 x 104 0.20 0.362 0.10 0.00 0.00 0.280 0.15 2.00 x 104 13.33 X lo4 0,910 0.20 2.00 x 104 10.00 x 104 0.800 0.20 :!.oo x 104 10.00 x 104 0,783 0.20 0.80 x 104 4.00 x 104 0.475 0.80 x 104 0.10 8.00 x 104 0.680 0.15 2.00 x 104 13.30 X lo4 0.925 Serine 0.15 0.80 x 104 5.30 x 104 0.540 0.15 0.00 0.00 0.287 0.10 0.80 x 104 8.00 x 104 0.690 0.05 0.00 0.00 0,285 0.10 1.BO x 104 16.00 X lo4 1.080 [DH] repreeents the concentration of amino acids (in moles per liter) which are H atom donors. The amino acids are then listed in their respective positions in the table. * [Sc] represents the concentration (in moles per liter) of the H atom scavenger. In this table the only scavenger used was allyl alcohol. R represents the ratio of the scavenger concentration over the donor concentration. d wABA represents a-aminobutyric acid. 0.25 0.20 0.20 0.20 0.15 0.25 0.20 0.25 0.05 0.20
0.00
Valine 0.00
0.288 1.515 1.490 0,945 1.060 0.289 0.645 0.288 0.290 1.000
0.10 0.10 0.15 0.10 0.20 0.15 0.20
0.80 2.00 5.00 4.00 5.00 2.00 5.00
x x x x x x x
I
I
Q
Volume 78, Number 10 October 1968
3398
WYNNA, VOLKERTAND ROBERT R. KUNTZ squares lines passing through the data points were extrapolated to the expected intercept (approximately 0.28) within experimental error in the absence of scavenger.
1.2 1.1 I,0 0.9
-'
IV. Discussion Egects of INpurities. Amino acids which act as scavengers react rapidly with H atoms. Because of
0.7
-
0.6 0.5
D,4 0.3 I
0
1.0
?,a
3.0
4.0
5.0
I 6.0
[ALLYL ALCQHOLI/[AMINO
I
I
I
I
7.0
8.0
9.0
10.0
A C l 3 1 X lo4
Figure 3. Competition data for donor amino acids with allyl alcohol plotted in accordance with eq I: V, alanine; A, isoleucine; 0 , lysine; B, glutamic acid; 0 , serine. All determinations at p H 1.0. Most serine data points fell beyond the limits of the figure. The graph presents least-mean-squares lines for all points.
basis that k~ alcohol = 2.80 X lo9 l./mol sec at pH 1. By the use of deuterium-labeled solutes, it has been shown that D atoms do not abstract hydrogen atoms to any significant extent from either the COOH Table IV : Hydrogen Donor Amino Acid Rate Constants Rate constantb
x Amino acid
Valine Alanine
a-ABAC Isoleucine Norvaline Serine Threonine Lysine Glutamic acid Proline Hydroxyproline a
Slopea
(0.241 f 0.009) X (0.331 i 0.019) X (1.330 i 0.040) X (0.130 k 0.007) X (0.461 i 0.014) X (0.224 f 0.007) X (1.010 k 0.043) X (0.808 i 0.030) X (0.800 f 0.032) X (0.154 rt 0.005) X (0.497 k 0.019) X
10-0,
l./mol aec
lo3
lo4 loa lo3 lo3 lo3 10% loa los lo4 loa
3.311 0.210 0.241 & 0.021 0.600 & 0.034 6.138 k 0 . 5 0 0 1.731 i 0.100 3.562 & 0.220 7.900 -I: 0.560 0.987 i 0.060 0.997 i 0.068 0.518 f 0.030 1.605 f.0,100
The slopes were obtained from plots of the data in Table
I11 in accord with eq I and were calculated by a least-meanRate constants squares program on an IBM 7040 computer. are calculated on the basis that the hydrogen atom rate constant with allyl alcohol is 2.80 X IOQl./mol sec. a-Aminobutyric acid.
or the NH3+ group and only slightly from the OH group.12 Irradiation of donor amino acid solutions in the absence of scavenger produced hydrogen with G yields within a few per cent of G H ~ Gred (Table 111). Therefore, it can be concluded that the H atoms are not appreciably scavenged by the functional groups in the donor amino acids and that all of the reaction takes place on the hydrogen CY to the amine and acid group or at some point on the side chain. The least-mean-
+
T h e Journal of Physical Chemietry
this, an impurity would need to be present in a considerable quantity (>2%) to affect the rate constant measurement to any great extent. Therefore, the best grades of these compounds available from the suppliers were used without further purification. Decompositions were kept low enough so that the HzOz product could never compete with solutes for radiolytic H atoms. Amino acids which serve as H atom donors have relatively slow reaction rates, and the presence of impurities could be important. Even slight amounts of scavenger (ca. 0.1%) could react with a significant percentage of available radiolytically produced H atoms. Fortunately, the presence of such a compound can be easily detected by measuring the G yield of total Hz when the compound is irradiated in the absence of any added scavenger. As mentioned previously, the donor amino acids reported here all gave hydrogen yields corresponding to complete conversion of H atoms and eaq- to molecular hydrogen via reactions 1 and 2. (Glycine and leucine gave low hydrogeri yields in the absence of scavenger. Further investigation of the reason for this phenomenon is presently underway.) A possible alternative for reaction 2 might be the removal of hydrated electrons by the relatively high concentration of amino acid cations required for the donor studies (reactions 4). Willix and Garrison13 have recently determined rate constants approaching lo9 M-l sec-l
+ NH3+R---+NH3 + R eaq- + NH3+R +H + NHzR eaq-
(4a ) (4b)
for scavenging of electrons by amino acid cations in acid solution. Thus using the highest alanine concentration used in these experiments (0.4 M and kc = 8 X 108 M-' sec-l l 3 and 0.1 M H3+0 and kl = 2.3 X 1O1O M - l sec-' 3), one might expect that) about 12% of the hydrated electrons would be scavenged by alanine. This expectation is clearly inconsistent with the observations that hydrogen yields in the absence of scavenger are, within experimental error, unaffected by a fivefold concentration change for alanine. All H atom donor amino acids were screened for electron scavenging by measurement of G(HJ over a large range of concentrations in the absence of added scavengers. (11) W. M. Garrison, W. Bennett, S. Cole, I€. R. Haymond, and B. M. Weeks, J . Amer. Chem. Soc., 77, 2720 (1955). (12) P. Reisz and B. E, Burr, Radiat. Res., 16, 661 (1962). (13) R. L. 5. Willix and W. M. Garrison, ibid., 32, 452 (1967).
THEREACTIONS OF HYDROGEN ATOMSIN AQUEOUS SOLUTIONS No change was observed in the hydrogen yield. One way in which these observations may be consistent with the high rate constant of scavenging of H atoms by amino acid cations is to invoke the scavenging mechanism which simply produces H atoms (reaction 4b) as the major reaction in this process. This reaction in competition with reaction 1 would have no effect on the kinetic scheme used in this work. It is unfortunate that the presence of impurities (probably other amino acids) which act as H atom donors is not detectable. Small amounts of these compounds (depending upon their nature) might raise the measured irate constants to some degree if the compounds (e.g., alanine) have slow reaction rates. Thus the reported constants for the donor amino acids with low rates must be considered as upper limits. Structural E:$ects. Relatively little information is available on the parameters which determine rates of abstraction by either H atoms or OH radicals from aliphatic compounds. Adams and Boag14 suggested that the only major site of H atom abstraction from aliphatic compounds occurred at the position a to the functional group or groups. The inductive effect was postulated to account for the observed increase in reaction rate with the increase in the length of the alkyl side chain. Anbar, et aZ.,4 calculated, from rate constants obtained by a systematic study of the effect of length and structure of the side chains, that the reactivity of aliphatic compounds toward OH radicals is not determined by the inductive effect only, but they suggested resonance stabilization of the free radical produced as an additional effect. From their rate constant determinations they also calculated that there is significant H atom abstraction from carbons other than those a to the functional group. Trends in the rate constant data (Tables I1 and IV) may be used to draw some conclusions concerning the factors governing these rates. The resonance stabilization effect can be observed in proline. Even though the alkyl group attached to the amino acid group is a four-carbon group (cyclobutyl), the reaction rate of H atom with proline is very low. This decrease of reactivity when small rings are present‘in molecules has also been observed with hydroxyl radicals and was attributed to a limitation in the stabilization in the free radical produced. The rate constants of H atoms with amino acids containing alkyl side chains with no tertiary carbons (alanine, a-aininobutyric acid, and norvaline) were found to increase appreciably with increasing chain length. However, the introduction of tertiary carbon in the side chain (valine and isoleucine) resulted in a much more dramatic increase. Even valine, which has no more carbon atoms in the side chain than norvaline, had a rate constant more than twice as large, and the rate constant for isoleucine is even higher. On the basis of the relative reaction rates measured,
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apparently H atom abstraction from tertiary position in the alkyl side chain is important. No quantitative treatment of the data can be made, however, nor can any conclusions concerning the role of secondary and primary H atoms remote from the a-carbon on the amino acid moiety be reached from these data. Functional Groups. The presence of certain functional groups in the alkyl side chains affected the reaction rates of amino acids with H atoms. The most pronounced effect was observed with the substitution of the OH group on a side chain for an H atom. Substitution of the hydroxyl group for an H atom on both alanine and a-aminobutyric acid on the P-carbon resulted in compounds (serine and threonine, respectively) which reacted nearly 15 times faster than the unsubstituted compounds. The same effect was found when adding an OH group to proline (hydroxyproline), but the increase of rate was not nearly as dramatic. Alcohols, in general, have rapid reaction rates with H atomsa compared with the aliphatic amino acids. Since alcohols are known to have higher reactivities with H atoms and OH radicals than either protonated amines or organic acids,3 the H atom on the carbon a to the hydroxyl group presumably is abstracted faster than the hydrogen atom on the carbon a to the amino acid moiety. The carboxylic acid and the protonated amino group were also added to the alkyl side chain. One of the hydrogen atoms on the terminal carbon in a-aminobutyric acid was replaced by the COOH group (glutamic acid). The resulting reaction rate was nearly doubled. This introduced an a-carbon to the carboxylic acid group in addition to the one already present. The effect of this group was less than the increase in rate observed on addition of an OH group. A terminal H atom on a-aminopentanoic acid was replaced by an NH3+ group (lysine). The resulting reaction rate was significantly lower than expected for a-aminopentanoic acid. Thus the protonated amine group decreases the reaction rate compared with the rate when the pure alkyl group is present. This may also be expected, since the amino acids are of lower reactivity toward OH radicals and H atoms compared with the analogous carboxylic acids. The effect of substituents on the alkyl side chains on the reactivity of the amino acids with H atoms was found to be NH3+< H < COOH
