4188
J. CHRYSOCHOOS
Pulse-Radiolysis Studies of p-hydroxy phenylpropionic Acid1 by J. Chrysochoos Department of Chemistry, The University of Toledo, Toledo, Ohw ~8606 (Received April 28, 1960)
The pulse radiolysis technique has been used to investigate the initial radiolysisproducts of aqueous p-hydroxyphenylpropionic acid (8). The transient absorption spectra show that OH and H adducts are formed initially in acidic and near-neutral solutions while in alkaline solutionsboth addition products and phenoxyl radicals are formed simultaneously. The decay of the addition products in acidic and near-neutral solutions is accompanied by a parallel formation of phenoxyl radicals by a process obeying first-order kinetics with k = (2.7 j=0.3) x 108 sec-I. Both the OH and H adducts have overlapping intense uv absorptions. Their molar extinction coefficients at 320 and 330 mp are 5.0 X loa, 3.5 X lo8and 3.4 X lo8,4.1 X l o a 1W-l cm-l, respectively. The reaction rate constants of H atoms with p-hydoxyphenylpropionicacid is (1.5 f 0.3) x log M-' sec-'. Evidence was obtained for reactions of 02 with the phenoxyl type radicals in alkaline solutions of p-hydroxyphenylpropionic acid and L( -)-tyrosine, and with the OH adduct of Dbphenylalanine, Introduction A variety of pulse-radiolysis studies have been made on the reactivity of aromatic compounds with the initial water radiolysis products. Addition of OH radicals and H atoms to the benzene ring lead to cyclohexadienyl-type radicals which have similar intense Addition of OH radicals to uv absorption aromatic rings carrying hydroxyl groups is particularly interesting because of the possibility of elimination of water from the OH adduct to form phenoxyl-type radic a l ~ . Such ~ a reaction was not observed in the pulse radiolysis of I,(-)-tyrosine in either acidic or basic solutiom8 However, some evidence for water elimination was obtained in near-neutral solutions of L( -)tyrosinea8 The purpose of this investigation is to obtain additional information on the water elimination reaction in p-hydroxyphenylpropionic acid, to study the reactions of this molecule with H atoms, and to determine the molar extinction coefficients of the OH and H adducts. Experimental Procedure The irradiation source was a linear accelerator providing l-psec pulses of 30-34eV electrons a t a dose of 1560 rads/pulse as determined with the modified Fricke d ~ s i m e t e r . ~The samples were contained in a 5 cm long cylindrical fused silica cell which could be evacuated to 2 X 10-8 Torr. The transient spectra were taken by monitoring the transmission changes using an Osram XBO-450 xenon arc and a Hilger E498 spectrograph with the E720 photoelectric scanning unit. Details of the electronic and optical arrangements are given elsewhere.10~1l Transient changes were monitored to a Tektronix 535 oscilloscope using three time scales, namely 10, 60, and 200 psec/cm. I n this way, the buildup and decay a t various wavelengths were readily studied. Triply distilled water was used. The other chemicals were of reagent grade and they were used without The Journal of Physical Chemistry
further purification. To study reactions with 02, air-saturated solutions were used, containing 2.5 X 10-4 M o2at 200. Results Transient Spectra. The transient absorption spectra obtained under various experimental conditions in the pulse radiolysis of p-hydroxyphenylpropionic acid are given in Figures 1A and 1B. In almost neutral deaerated solutions (pH 6.0) a broad absorption band was observed 5 psec after the pulse with an absorption maximum below 320 mp and a broad shoulder a t about 345 mp (Figure 1A-a). Similar results were observed at pH 4.3 with a possible absorption maximum located at about 300 mp (Figure 1B-a). However, absorption spectra measured 45 psec after the pulse show a slight decrease in the intensity of the broad band and the appearance of another weak band with a possible peak maximum at about 415 m p (Figure 1A-b). This observation was verified in all cases in which the pH was below 7. On the other hand, transient absorption spectra taken 5 psec after the pulse in alkaline solution (pH 11.4) show intense and broad bands a t both 415 mp and below 360 mp (Figure 1A-e). In the presence (1) The experimental work was carried out at the Department of Radiation Therapy, Michael Reese Hospital and Medical Center, Chicago, Ill. (2) (a) L.M.Dorfman, I. A. Taub, and R. E. Btlhler, J. Chew. Phys,, 36,3061 (1962); (b) D.F.Sangster, J . Phys. Chem., 70, 1712 (1966). (3) R.Wander, P.Neta, and L. M. Dorfman, ibid., 72, 2946 (1968). (4) M. C. Sauer, Jr., and B. Ward, ibid., 71,3971 (1967). ( 6 ) K.D . Asmus, B. Cercek, M. Ebert, A. Henglein, and A. Wigger, Trans. Faraday Soc., 63,2435(1967). (6) P.Neta and L. M.Dorfman, J. Phys. Chem., 73, 413 (1969). (7) E.J. Land and M. Ebert, Trans. Faraday Soc., 63, 1181 (1967). (8) J. Chrysochoos, Radiat. Res., 33,466 (1968). (9) L. M.Dorfman and M. S. Matheson, Progr. Reaction Kinetics, 3, 237 (1966). (10) J. Chrysochoos, Chim. Chronica, 3lA, 94 (1966). (11) J. Chryaochoos, J. Ovadia, and L. I. Grosaweiner, J . Phys. Chew., 71, 1629 (1967).
PULSE-RADIOLYSIS STUDIES OF
4189
HYDROXYP PHENYL PROPIONIC ACID
0.4
0.32--
0.2 O*I6t
I
0 320
360
I
8b
400
240
160
3 60
t
F-
$ .32 W
n J
Q
2
k
n 0 O.l€
I1
0 280
3;O
360 WAVELENGTH,
0
mp
Figure 1. Transient spectra from the pulse radiolysis of p-hydroxyphenylpropionic acid. (A) 5.0 mM deaerated p-hydroxyphenylpropionic acid buffered with OHNa, optical length 10.4 cm: 0, 5 psec after the pulse; 0, 45 psec after the pulse (both a t pH 6.0); 0, 5 psec after the pulse; (pH 11.4); A, 5 psec after the pulse; pH 11.4, 0.1 M HCOO-. (B) 0.1 mM phydroxyphenylpropionic acid buffered with OHNa; p H 4.3, optical length 10.4 cm: 0, 5 psec after the pulse, deaerated; 0, 5 psec after the pulse; 0.25 mM 02 (OH adduct); A, difference of curves (a) and (b) (H adduct).
of 0.1 M formate, the absorption bands were almost completely quenched (Figure 1A-d) in either acidic or basic solutions. Therefore, the transient species formed are attributed to the reactions of OH radicals and H atoms with the substrate. The absorption band below 360 mp is attributed to OH and H adducts, while the band a t 415 mp is attributed to a phenoxyl-type radical. The latter was not observed in the pulse radiolysis of DL-phenylalanine,* a molecule which does not have a hydroxyl group in the aromatic ring. The transient spectra obtained in aerated solutions at pH 4.3 (0.25 mM 0,) are attributed to the OH adducts (Figure 1B-b) since under these conditions about 98% of the H atoms are scavenged by 0 2 (see text). The reaction of eaq- with the substrate is very slow8 (IC 5 (0.9 f 0.2) X lo7 M-' sec-l) and therefore, the conversion of eaq- to H atoms in deaerated solutions at pH 4.3 is complete (ti/,(eaq-) 2 psec). In aerated
IBo
ab TIME,
2ko
psec
Figure 2. Variation of transient optical densities with time. 5.0 mM p-hydroxyphenylpropionicacid, pH 6.0, optical length 10.4 cm. (A) Decay of the addition products a t 320 mp (a) and 340 mp (b) and formation of the phenoxyl radicals a t 415 mp (c) and 405 mp (d). (B) First-order plot of the decay of the addition product at 320 (a), 340 (b), and 360 mp (c).
solutions eaq- are partly converted to H atoms and 1 psec and tal2partly scavenged by 02(tl/,(eaq-) (H) II 2 psec). The difference between spectra a and b in Figure 1B represents the absorption spectrum of the H adducts (Figure 1B-c) with two peak maxima, one a t almost 350 mp and another one below 300 mp. The decay of the OH adducts and the simultaneous buildup of the phenoxyl radicals at p H 6 during the early stages are depicted in Figure 2A. I n the later stages, both the decay at 320-360 mp and buildup at 400-425 mp become more complex due to the slower formation of the H adduct (t1I2(eaq-) 50 psec) and the decay of the phenoxyl radicals. However, during the first 200 psec the OH adducts decay by a first-order process, studied a t 320, 340, and 360 mp, with rate constants 2.6 X loa, 2.6 X lo3, and 2.8 X lo3sec-', respectively (Figure 2B). Since the phenoxyl radicals are produced via the OH adducts, a plot of log (D, - Dt), vs. t, where D, is the maximum optical density due to phenoxyl radicals, should yield rate constants similar to those mentioned above. However, the behavior of the phenoxyl is more complex due to its own decay, and D, cannot be determined with accuracy. Volume 78, Number 12 December 1069
J. CHRYSOCHOOS
4190
,
4~
2:
D
6
IC? x $1
=
+
(DO)SOH. (Do)sH.-
+
[HO]LBSH.[JCZ[~~]/(~C~[~] kz[O]2)] (11)
(A)
Combining (I) and (11) we obtain
+
(Do - D)-' = [[H]oLes~.]-l
[ [HIoL~sH. ]-l(h/h L
By plotting (Do - D)-' vs. [SIwe can determine the values of both k1 and ESH.. The results of the competition are plotted according to eq I11 in Figure 3B at 320 and 330 mp leading to the values
II
0
[SI (111)
16
32 104x
48
~4
Figure 3. (A) Dependence of the transient optical density upon the initial concentration of the substrate a t 320 (a), 330 (b), and 340 mp (c). Deaerated solutions, pH 2.0-2.3, 20 psec after the pulse, L = 10.4 cm. (B) Dependence of {DO- D ) on the initial concentration of the substrate a t 320 (a) and 330 m p (b); 0.25 mM 02,pH 2.0-2.3, 20 psec after the pulse, L = 10.4 cm.
Reactions with H Atoms. The transient absorptions a t 320, 330, and 340 mp in deaerated solutions and a t p H 2.0-2.3 (OH and H adducts) increase with increasing concentration of the substrate and a t concentrations larger than 1 mM, they attain constant values (Figure 3A). These spectra, taken 20 psec after the pulse, indicate that 1 mM substrate completely scavenges all the OH radicals and the H atoms produced during the electron pulse. If DO represents the maximum optical density a t a wavelength X, we will have
Do = (DO)SOH. d- (Do)sH.
[OH]OLESOH. -k [H IoLEsH. (1)
where [OHIO and [HIo represent the total amount of OH radicals and H atoms formed during the pulse, L is the optical path and EDOH. and BSH. represent the molar extinction coefficients of the OH and H adducts, respectively, at the given wavelength. The values of [OHIOand [HI0 are calculated from the dose and are based on the G values of OH, H, and eaq-, namelys G,,,- = 3.2. Under these condiGOH = 2.6; G H tions we obtain [OHIO = 4.3 p M and [HI0 = 5.2 p M . I n the presence of 0 2 there will be a competition for H atoms between the substrate, S, and 0 2
+
The Journal of Physical Chehernistry
kl = (1.50 (EgH.)320 =
f
0 . 3 ) X lo9 M-'sec-' based on k2 = 1 . 9 X 1O'O M-' sec-'
3 . 4 X lo3M-l cm-1 and (EgH.)330 = 4 . 1 X lo3M-' cm-'
Introducing the values of
BSH. into
eq I we obtain
( B ~ ~ =~ 5.0 . ) X~ lo3 ~ M-l ~ cm-l and
( B S ~ ~ = . ) ~ ~ ~
3 . 5 X lo3M-l cm-I
Reactions with 02. The presence of O2 in the pulse radiolysis of p-hydroxyphenylpropionic acid not only suppresses the formation of the H adducts due to the scavenging of H atoms and eaq- but also alters the rate of formation and decay of the transient species formed. The decay of the phenoxyl radical is much faster in air-saturated than in deaerated solutions a t pH 11.4. This fast decay is accompanied by an almost parallel buildup of a transient absorption at 300340 mp which is completed a t about 400 psec. The initial formation of both the phenoxyl and the cyclohexadienyl radicals, due to the reactions of the initial radiolysis products of water, is completed within 20 psec. The slow buildup of the transient absorption at 300-340 mp is given in terms of D , - DZO, where D , is the absorption a t time t at any wavelength X and DZO is the initial absorption a t 20 psec (Le., OH and partly H adducts). The presence of formate ion does not alter this behavior but only reduces the maximum absorptions a t 20 psec of both the phenoxyl and the OH adducts. Figure 4A depicts both the fast decay of the phenoxyl in the absence (d) and presence of formate
4191
PULSE-RADIOLYSIS STUDIES OF HY HYDROXY PHENYL PROPIONIC ACID 9L
--
8 7.-
6-54
20
Id0
100 TIME,
--
260
p e C
Figure 4. (A) Variation of transient optical densities with time: 0.2 mM p-hydroxyphenylpropionic acid, 0.25 mM 02, pH 10.6, L = 10.4 cm. Slow buildup of the optical density a t 320 m p , given in terms of (Dt - D 2 0 ) 3 2 0 in the absence of formate ion (a) and in the presence of 0.2 mM formate ion (b); decay of the phenoxy radical in the absence of formate a t 405 mp (d), and in the presence of 0.2 mM formate ion at 400 mp (e); (c) decay of the phenoxyl a t 405 mp in the pulse radiolysis of 0.1 mM deaerated substrate a t pH 10.6. (B) First-order kinetics for the decay of the phenoxyl and the appearance of the transient absorption a t 320 mp; 0.2 mM substrate, 0.25 mM 0 2 , p H 10.6, L = 10.4 cm. Decay of the phenoxyl a t 405 mp in the absence of formate (c) and a t 400 mp in the presence of 0.2 m M formate ion (d); appearance of the transient absorption a t 320 mp in terms of (D,- &)a20 in the absence of formate (e) and in the presence of 0.2 m M formate ion (f); decay of the phenoxyl a t 405 mp in 0.1 mM deaerated substrate at pH 10.6 (b); decay of the addition product a t 320 m p in 5 mM deaerated substrate a t pH 11.4 (a).
ion (b) whereas curves (a) and (b) represent the corresponding behavior of D , - Dzo with time a t 320 mp. Curve (c) shows the decay of the phenoxyl in deaerated solutions. Both the decay of the phenoxyl and the slow buildup at 320 mp obey first-order kinetics (Figure 4B). The latter is given in terms of D , - D,, where D, represents the maximum optical density attained at 320 mp. The decay of the cyclohexadienyl radicals a t 320 mp in deaerated solutions is very slow (Figure 4B-a) and therefore it does not interfere with the slow buildup at this wavelength. The half-times measured a t 405
presence of 0.2 mM formate ion; (e) 320 mp in the absence of formate ion; (f) 416 mp in the presence of 0.2 mM formate ion; curves (a) and (b) depict the decay of the addition products and phenoxyl radical in deaerated solutions plotted as simple, first-order processes a t 330 and 415 mp, 5.0 mM L ( -)-tyrosine, p H 10.0; L = 10.4 cm.
and 320 mp in air-saturated solutions are 120 and 110 psec, respectively, while the half-times at 400 and 320 mp in air-saturated solutions containing 0.2 mM formate ion have the values of 105 and 100 psec, respectively (Figure 4B-c-f). Curve b shows the decay of the phenoxyl radicals in deaerated solutions. Similar results were observed with L( -)-tyrosine a t p H 10.6 (Figure 5). However, the decay of the cyclohexa'dienyl radicals in phenylalanine is faster in the presence of 02. Since the latter amino acid does not have a hydroxyl group (no phenoxyl radical formed), the slow buildup a t 300-340 mp observed in both p-hydroxyphenylpropionic acid and L( )-tyrosine is attributed to the reactions of 0 2 with the phenoxyl radicals
-
phenoxyl
+
0 2
+oxidation product
+ 02- (4)
The rate constants for (4) are determined from the decay of the phenoxyl radicals (Figure 4B). The values of (2.6 f 0.4) X lo7 and (2.4 i 0.4) X lo7 M-1 sec-1 are obtained for p-hydroxyphenylpropionic acid and L( -)-tyrosine, respectively. Volume 73,Number 18 December 1969
4192
J. CHRYSOCHOOS
6T
absorb to some extent at 320 mp, the value of lc5 is taken as a lower limit; k~ 2 4.6 X lo7A4-l sec-l. Discussion. The transient spectra indicate that in acidic and near-neutral solutions, OH and H addition to the benzene ring leading to cyclohexadienyl radicals are the predominant processes between the substrate and the initial radiolysis products of water. The cyclohexadienyl radicals are transformed, a t least during the early stages, to phenoxyl-type radicals by elimination of water. r
It 20
IO0
180
TIME,
7-60
Possible reactions of 0 2 with the OH adducts in p-hydroxyphenylpropionic acid and L(- )-tyrosine cannot be studied in these systems, because if such reactions do occur, they will be masked by reaction 4. However, evidence for such reactions was obtained in the ca6e of DL-phenylalanine in which case reaction 4 does not participate. The decay of the cyclohexadienyl radicals at 320 mp in aerated solutions of DL-phenylalanine (pH 2.0 and 10.0) is faster than in deaerated solutions and it obeys first-order kinetics (Figure 6). The first-order decay at 320 mp is accompanied by a parallel increase in the optical density at 300 mp. I n deaerated solutions, the decay at 320 mp is slower than in aerated solution and it does not obey first-order kinetics. The fast decay at 320 mp in the presence of O2is attributed to the reaction
+
The value of k6, (2.7 f 0.3) X lo3 sec-l, is comparable to the corresponding values of 5.8 X lo3 and 4.7 X 10* sec-l measured from the decay of the OH adduct of phenol a t 330 mp at pH 6.3 and 7.15 (buffered), respectively.' In alkaline solution, in which the hydroxyl group is ionized, OH radicals both add to the benzene ring and react with the OH group simultaneously.
pec
Figure 6. Kinetics of the decay of various transient species in the pulse radiolysis of Dbphenylalanine; 5 mM substrate, L = 10.4 cm. (a) 320 m p , pH 10.0, deaerated solution; (b) 320 m p , pH 2.0, deaerated solution; (c) 300 m p , pH 10.0, deaerated solution (e) 300 m p , pH 2.0, deaerated solution; (d) 300 mp, pH 1.0.0, 0.25 mM 02;(f) 300 mp, pH 2.0, 0.25 mM 02;(g) 320 mp, pH 2.0, 0.25 m M 02;(h) 320 mp, pH 10.0,0.25 mhl 02.
cyclohexadienyl radical
r
0 2 ----f
hydroperoxy radical (5) with a rate constant k6 = 4.6 X lo7 M-l sec-'. The increase in optical absorption a t 300 mp is attributed to the hydroperoxy radical. Since the latter might also The Journal of Physical C h m k t r y
The decay of the phenoxyl radical produced in (7) is very fast while the decay of the OH adducts is extremely slow. It is not clear whether the phenoxyl radicals produced in (6) are exactly identical with those produced in (7) because elimination of water can remove either hydroxyl group from the cyclohexadienyl radicals (reaction 6). The reactivity of H atoms toward p-hydroxyphenylpropionic acid, i.e., (1.5 f 0.3) X 109 M-l sec-l, is a little lower than the reactivity of the OH radicals under similar conditions, i.e., (3.7 f 0.6) X lo9 2M-l sec-l,* and it is comparable to the reactivity of H atoms toward phenylacetic acid and p-OH- C6H$OOH, namely 1.01 X l o 9 and 1.45 X lo9 M-' sec-', respectively.6 The OH and H adducts not only have overlapping uv absorptions but they also have cqmparable molar extinction coefficients. The corresponding values a t 320 and 330 mp, namely = 5.0 X lo3, (eSOH.)330 = 3.5 x IO3, ( E S H . ) ~ = ~ ~ 3.4 X lo3, and ( E S H . ) ~=~ ~4.1 X 103 M-' cm-l compare well with the molar extinction coefficients of OH and H adducts of phenol measured a t 330 mp,' namely 4.4 X loa and 3.8 x 103 M-l cm-l. Molar extinction coefficients cannot be determined in alkaline solutions since both the phenoxyl and cyclohexadienyl radicals are formed
4193
PULSE-RADIOLYSIS STUDIESOF p-HYDROXYPHENYLPROPIONIC ACID Table I : Summary of Rate Constantsa H Substrate
p-hydroxy phenylpropionic acid L-(-)-Tyrosine
+ substrate;
M - 1 sec -1
sec - 1
1.5 f0.3 X lOQ (2.0-2.3)
2.7 =k 0 . 3 X lo8 (6)
DLPhenylalanine Phenol p-Hydroxyphenylacetic acid Benzene a
Water elimination from OH adducts,
5.8 x 108; 4.7 x 108 (6.3) (7.15)
+
Phenoxy1 01, M -1 sec -1
+
OH adducts 02: M -1 see - 1
Ref
This work
2.6 f 0 . 4 X 10' (11.4) 2.4 =t0 . 4 X lo7 (10.6)
This work 24.6
x
107
This work 7 6
1.45 X log 5.0 f 0.6 X 108
2a
Figures in parentheses indicates pH.
simultaneously. The aforementioned molar extinction coefficients in acidic and near-neutral solutions do not refer to a single species since more than one cyclohexadienyl radical can be formed. These values should be taken as average molar extinction coefficients where et represents the actual molar extinction coefficient of the species, i, a t A, and ct the corresponding concentration.
The reactivity of 02 with the cyclohexadienyl radicals in DL-phenylalanine, Le., kg 1 4.6 X lo7 M-l sec-' is lower than the corresponding value determined in benzene, namely (5.0 i 0.6) X los M-' sec-'. The peroxy radical formed was not studied but it is assumed to be responsible for the absorption buildup observed a t about 300 mp. A summary of the rate constants determined in this work and relevant values from the literature is given in Table I.
Volume 78, Number 12 December 1969
