J. Phys. Chem. 1984, 88, 2091-2095 Acknowledgment. The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Many helpful discussions with J. J. Margitan, W. B. DeMore, J. H. Goble, M. J. Molina, L. Froi-
2091
devaux, and R. A. Marcus are gratefully acknowledged. We also thank R. Patrick (SRI) for his useful comments on this manuscript. Registry No. H02, 3170-83-0; DO,, 13587-55-8.
Reactivity of OH with Tyrosine in Aqueous Solution Studied by Pulse Radiolysis S . Solar, W. Solar, and N. Getoff* Institut fur Theoretische Chemie und Strahlenchemie der Universitat Wien and Ludwig Boltzmann Institut fur Strahlenchemie, A-1090 Wien, Austria (Received: June 20, 1983)
The specific OH attack on various sites of the tyrosine molecule in neutral aqueous solutions (pH 6-8), saturated with N20, has been investigated. The main process (-50%) is the formation of ortho-directed OH adduct (R,) with k = (7.0 i 0.5) X lo9 dm3 mol-' s-I (A, = 330 nm, €330 = 300 f 30 m2 mol-'), which decays by water elimination according to a first-order reaction (k'= (1.8 f 0.2) X lo4 s-I) under formation of phenoxyl radical, as well as by second order with 2k = (3.0 & 1.0) X lo8 dm3 mol-' 8. The phenoxyl radical is additionally formed as a primary product (-5%) with k = (6.0 f 1.0) X 108 dm3 mol-' s-I. It possesses two absorption maxima, 260 nm (€260 = 600 f 50 m2 mol-') and 405 nm (€405 = 260 f 20 m2 mol-'), and decays with 2k = (4.0 f 1.0) X lo8 dm3 mol-' s-l). The meta isomer of the OH adducts (R2)is formed to -35% with k = (5.0 f 0.4) X lo9 dm3 mol-] s-l, having two absorption maxima at 305 nm (e305 = 280 h 30 m2 mol-') and 540 nm (esdo = 2 3 & 3 m2 mol-'), and disappears with 2k = (2.0 & 0.5) X lo9 dm3 mol-' s-'. The rest of -10% OH radicals attack most probably the para and to a small extent ipso positions of the phenol ring under formation of the corresponding adducts. The H abstraction from the alanine moiety cannot be excluded.
Introduction The aromatic amino acids represent an essential part of proteins and hormones, and the elucidation of their reaction mechanism with the primary water radiolytic products, particularly the O H attack on these molecules, is of special importance for radiobiology and radiation chemistry. Pulse radiolysis of tyrosine and phenylalanine has been already carried out at different pH's.' In the case of tyrosine a simultaneous formation of OH-addition transients (absorption bands a t 310 and 320 nm) and phenoxyl radicals (A, = 405 nm) has been observed. In neutral media the latter results from a direct OH reaction with the hydroxyl group of the substrate as well as from water elimination from the corresponding cyclohexadienyl radicals. The total rate constant ( k , ) for OH attack on tyrosine at p H 4 has been determined to be k,(OH tyrosine) = 4.2 X lo9 dm3 mol-' d,'and k = 1.4 X 1O'O dm3 mol-' s - I . ~ For the uncatalyzed water elimination (pH -7) a first-order rate constant of k = 1 X lo3 s-' has been e~tablished.~The attack of various radical anions such as SO4-., B r p , etc4y5and N3s6on tyrosine, leading to formation of phenoxyl radical, has been also reported. An attempt has now been made to obtain more information about the specific O H reaction on various sites of the tyrosine molecule in neutral aqueous solution. The investigations were focused on the resolution of the distinct absorptions of the transients, obtained by pulse radiolysis, and on determining their individual kinetic and spectroscopic parameters. A semilinear optimization procedure7ss subsequent to the registration of the transient absorption change as a function of time was used to master this problem.
+
(1) J. Chrysochoos, Radiat. Res., 33, 465 (1968). (2) J. Feitelson and E. Hayon, J . Phys. Chem., 77, 10 (1973). (3) E. J. Land and M. Ebert, Trans. Faraday Soc., 1181 (1967). (4) M. L. Posener, G.E. Adams, P. Wardman, and R. B. Cundall, J . Chem. SOC.,Faraday Trans. 1,72, 2231 (1976). ( 5 ) K. M. Bansal and R. W. Fessenden, Radiat. Res., 67, 1 (1976). (6) A. Singh, G. W. Koroll, and R. B. Cundall, Radiat. Phys. Chem., 19, 137 .- ( 1 9x2) \ - - - - I -
(7) S. Solar, W. Solar, and N. Getoff, J . Chem. S O ~ Faraday ., Trans. 2, 79, 123 (1983). ( 8 ) S . Solar, W. Solar, and N. Getoff, Radiat. Phys. Chem., 21, 129 (1983).
0022-3654/84/2088-2091$01.50/0
Experimental Section The chemicals were of purest grades available (E. Merck, Darmstadt, F.R.G.). The solutions of tyrosine (5 X 104-104 mol dm-3) were prepared using four times freshly distilled water, and the pH 6-8 was adjusted by means of aqueous barium hydroxide. In order to convert e,; into OH, the solutions were saturated with N 2 0 (k(e,< N,O) = 9.1 X lo9 dm3 mol-' s - ' ) ~and irradiated in a suprasil quartz cell (3 X 2 cm light path) with 400-11s pulses from a 3-MeV Van de Graaff accelerator (Type K, High Voltage Ing. Corp., Burlington, VT). The applied dose per pulse was -5 J kg-' (-0.5 krd). An XBO 450-W Xenon lamp (Osram) served as the analyzing light source. A double-prism monochromator (Zeiss MM12) combined with a wide-range sensitivity photomultiplier (Hamamatsu, R 955) rendered the kinetic and spectroscopic measurements. The data were digitized with a Biomation 8 100 transient recorder and processed by a minicomputer (PDP-1 l / 10, DEC) for averaging a large number of measurements. The final mean value of the stored traces was transferred to another computer (PDP-10, DEC), where the data collection program was run. Finally, each trace was displayed in 100 points, normalized for dose, and averaged for improving the signal to noise ratio. Details on the pulse radiolysis equipment and dosimetry have been previously reported.lOJ1
+
Computation Method Based on the measurements of the time-dependent change of the optical densities of the irradiated solutions (time range 1-2000 ~ susing , three different solute concentrations), the applied computation method permits one to resolve individually the kinetic and spectroscopic parameters in such a way that the observed course of the chemical processes is reproduced by the proposed reaction model in an optimal way. The Gear integration routine12-14 was used, including some essential modifications for (9) E. Janata and R. H. Schuler, J . Phys. Chem., 86, 2078 (1982). (10) N. Getoff and F. Schworer, Radiat. Res., 41, 1 (1970). (11) N. Getoff and F. Schworer, Int. J . Radiat. Phys. Chem., 5 , 101 (1973). (12) C. W. Gear in "Numerical Initial Value Problems in Ordinary Differential Equations", Prentice-Hall, Englewood Cliffs, NJ, 1971, Chapter 11. (13) C. W. Gear, Commun. ACM, 14, 176 (1971).
0 1984 American Chemical Society
2092 The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 0.025
1
I
OD
cm
0.020
0.015
I
\ , r 250
300
350
LOO
450 nm
Figure 1. Measured total absorption spectra of transients resulting from the OH attack on tyrosine 8 1.1s (A), 80 ps (B), and 150 ps (C) after pulse end, not corrected for substrate consumption. Solution was composed of lo4 mol dm-) tyrosine and 2.8 X mol dm-' NzO,pH 7.7. Applied dose was 4-5 J kg-'/O.Cps pulse. OD/cm values were normalized to 10 J kg-I (1 krd).
speeding the solution of high-order linear algebraic equations required for each integration step. The algorithmic procedure of this optimization model allows partitioning of the unknown model parameters into two groups: linear spectroscopicparameters (molar extinction coefficients) and nonlinear kinetic ones (specific rate constants). The advantage of this technique7p8is that only initial information for the rate constants, but no spectroscopicdata of the transients, is needed in the iterative procedure. The field of iterations is confined to the kinetic parameters, which were varied until the sum of the squared deviations between the calculated (CTA) and measured total absorptions (MTA) reaches a minimum. The squared deviations for every set of data at each investigated wavelength were weighted according to their different absorption intensities. For numerical treatment of the reaction mechanism, the previous reported technique15 for calculation of the rate and Jacobian matrix was introduced in the applied computation p r o c e d ~ r e .The ~ main advantages of this technique are the following: (1) ease in varying the reaction mechanism in the optimization procedure, (2) the elementary reaction are transformed automatically by the program to the system of differential equations, and (3) the Jacobian matrix can be evaluated exactly and quicker instead of repeated calculations of the differential equation system using a finite difference technique. In order to check the reliability of the optimized parameters, their influence on the overall OD/cm value for given wavelengths and time after the pulse is investigated critically by means of a sensitivity matrix.a
Results and Discussion In the presence of NzO (2.8 X
mol dm-3) the reaction of tyrosine with eaq-is completely eliminated, and hence G(OH),,,I = G(0H) G(e,;) = 5.6. With use of 5 X 104-104 mol dm-3 tyrosine, only a fraction of the H atoms (G I0.2) react with the substrate, whereas the rest combine either with OH or with H. Hence, the contribution to the total transient yield resulting from the H attack on tyrosine is less than 5%. Taking [Tyr] I2 X mol dm-3, this share is even below 2%. The total absorption spectra of the various transients produced by reaction of lo4 mol dm-3 tyrosine with OH radicals (pH 7.7) are given in Figure 1. They were normalized for 10 J kg-' (1 krd) and are presented in the range from 240 to 600 nm (see also insert) for 8, 80, and 150 ps after pulse end. Regarding the rate of absorption buildup, the bands can be divided into two groups: one reaching its maxima within 10 ps (A, a t -310, 330, and 540 nm; spectrum A, Figure 1) and a
+
(14) C. W. Gear, Commun. ACM, 14, 185 (1971). (15) R. N. Stabler and J. P. Chesick, Int. J. Chem. Kinet., 10, 461 (1978).
Solar et al.
-
second type needing 150 ps for its complete formation (A, at 260, 300, and 405 nm; spectrum C, Figure 1). The slow production of a tyrosine transient (pH 6.5) absorbing at 405 nm has been reported previously and was assigned to the phenoxyl radical, resulting from water elimination of the OH a d d ~ c t s .It~ was further established' that phenoxyl radicals can also appear by a direct reaction of O H with the substrate. This could be confirmed also by our measurements, where already 8 ps after pulse end a noticeable absorption of the phenoxyl species is observable (spectrum A, Figure 1). The several absorption bands in the range from 300 to 330 nm have been ascertained to belong to the OH-addition products of tyr~sine.',~The very weak absorptions at 540 nm may also belong to these species, as has been shown for OH adducts of phen01.~*~ The overall rate constant for the OH attack on tyrosine was determined at p H 6.5-7.5 by studying the absorption buildup at various wavelengths, delivering a mean value of k(OH tyrosine) = 1.4 X 1O1O dm3 mol-' s-'. It is in good agreement with previously obtained ones: k = 1.05 X 1O'O dm3 mol-' s-l at p H 6.516 and k = 1.4 X 1Olo dm3 mol-' s-I a t p H 5.2.z The transient decay rate showed strong differences at the various absorption maxima and was second order a t 260,405, and 540 nm but mixed order in the range from 300 to 350 nm. On the basis of the obtained experimental results and previous data, the reaction mechanism (eq 1-4) for the OH attack on
+
+ CHZCH-Nq ,
I
X
3
I
OH
OH
(phenoxy1 r a d i c a l 1
(31
0'
OH ITyrosine)
k.Y
o t h e r pathways
(4)
various sites of the tyrosine molecule seems to be operative (the alanine moiety is denoted by X). Reactions 1 and 2 are expected to be the predominant ones, since the following are true: (1) The O H reaction on ortho positions prevails to about 40-48%, independent of the nature of the substituent, as has been shown for phenol,17 a n i ~ o l e , ' * ~and ' ~ toluenez0 as well as for ben~onitrile'~ and nitrobenzene.z1 (2) The ipso position (which is already occupied by a substituent) is attached only to a small extent by O H radicals, e.g. for phenol 8%" and for anisole was found 0.14%19and 6%,18 respectively. (3) The para position (C,) of phenol and anisole react with about 36-40% OH,17-19 but its reactivity decreases to 13-22% for 4-methoxyphenol.22 The OH adduct of the latter decays by splitting of methanol. Reaction 4 includes all other possible processes like OH addition on ipso positions (C, and C,) as well as H abstraction from the side chain. The latter is negligible, since the reactivity of OH with alanine zwitterion is k = (4.6-7.9) X lo7 dm3 mol-' s d 23 ~
~
~~~
~~
(16) T. Masuda, S. Nakano, and M. Kondo, J. Radiat. Res., 14, 339
(1973).
(lf) N. V. Raghavan and S. Steenken, J . Am. Chem. SOC.,102, 3495 (1980). , I (18) S. Steenken and N. V. Raghavan, J . Phys. Chem., 83, 24 (1979). (19) M. K. Eberhardt, J . Phys. Chem., 81, 1051 (1977). (20) M. K. Eberhardt and M. I. Martinez, J. Phys. Chem., 79, 1917
-
(1975) ,- - , ,.
(21) M. K. Eberhardt, J . Phys. Chem., 79, 1913 (1975). (22) S . Steenken and P. ONeill, J . Phys. Chem., 81, 505 (1977). (23) Farhataziz and A. B. Ross, "Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution. 111. Hydroxy Radical and Perhydroxy Radical and Their Radical Ions", National Bureau of Standards, Washington, DC, 1977.
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2093
Reactivity of O H with Tyrosine TABLE I: Reactions and Rate Constants (k) Considered in the Pulse Radiolysis of Tyrosine in Neutral Aqueous Solutions, Saturated with N,O
ODr
~
.. *
295
nm
I
k , dm3 no.
1 2 3
4 5
6 I 8
9 10 11 12 13 14 15
16 17 18 19
20 a
s-l2,23,25,26
reacn tyrosin
+ OH
OH adduct (R, ) - + O H adduct (R,) -+ phenoxyl radical (R,) -+other radicals (R,) tyrosin H -+ H adduct 2H adduct -+ products 2R, -+ dimeris and/or disprop 2R, -+ dimeris and/or disprop R, -+ R, + H,O 2R3 -+ dimeris and/or disprop tyrosin eaq- -+ T (transients) 2T -+ products N,O eaq- + 2H,O -+ N, OH OHOH OH -+ H,O, OH + eaq- OHeaq_ - eaq' 2H,O-+ H, 20" H + H,O-+H, + O H H+H-+H, H OH+H,O eaq- H+ -+ H -+
+
+
+ + + +
+
+
-+
+
+
+
+
k, = ? k, = ? k, = ? k, = ? k, = 8 X 10' 2k, = 6.2 X 10' 2k, = ? 2k8 = ? k,' = ?a 2k,, = ? k , , = 2.8 X 10' 2k,, = 6.8 X 10' k,, = 9.1 X lo9 2k1, = 1.2 X 10" k,, = 2.2 X 10'' 2k,, = 1.1 X 10" k , , = 2.5 X 10'' 2k,, = 2.3 X 10" k,, = 2 X 10" k,, = 2.3 X 10"
k' is given in s-'.
10-3x 6 -
260 nm
" I , ; #,I -5..
............................... ,
-2
0
10
20
30
50 ps
LO
Figure 3. Resolved optical densities (OD/cm) as a function of time (ps) at 295 nm for computed formation of R,, R2, and R, transients as well as for substrate consumption (SC) in comparison to measured (MTA) and computed total absorption (CTA). Insert shows OD, (mean value of five measurements) at 295 nm vs. time (ps). Solution was composed of mol dm-, N20,pH 7.1. Applied mol dm-, tyrosine and 2.8 X dose was 6.3 J kg-'/O.4-pspulse.
m cm L-
I
I
10
20
I
I
30
LO
I
50 ps
Figure 2. Resolved optical densities (OD/cm) as a function of time (ps) a t 260 nm for computed formation of R,, R,, and R, transients as well as for substrate consumption (SC) in comparison to measured (MTA) and computed total absorption (CTA). Insert shows OD, (mean value of five measurements) at 260 nm vs. time (ps). Solution was composed of 2 X mol dm-, tyrosine and 2.8 X mol dm-' N20, pH 7.9. Applied dose was 4.7 J kg-I/0.4-ps pulse.
and that with the side group of methylated benzene is k N 4 X lo8 dm3 mol-' In order to elucidate the simultaneous occurring reactions 1-4 and to determine the distinct absorption and kinetic characteristics of the tyrosine transients, the above-mentioned semilinear optimization procedure was applied.7.8 The main reactions which are taken into consideration for the computations are summarized in Table I. However, when tyrosine concentrations of 1 2 X mol dm-3 are used, the transients resulting from the H attack on the substrate (reaction 5) are less than 2%; hence, they can be neglected. In addition, reactions 11, 12, 15-17, and 20,in which ea< is involved, are completely suppressed under the experimental conditions (2.8 X loM2mol dm-3 N20), because reaction 13 is predominant by several orders of magnitude. Following the (24) K. Sehested, H. Corfitzen, and H.C. Christensen, J . Phys. Chem., 79, 310 (1975). (25) M. Anbar, M. Bambenek, and A. B. Ross, 'Selected Specific Rates
of Reactions of Transients from Water in Aqueous Solution. I. Hydrated Electron", National Bureau of Standards, Washington, DC, 1973. (26) M. Anbar, Farhataziz, and A. B. Ross, "Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution. 11. Hydrogen Atom", National Bureau of Standards, Washington, DC, 1975.
I
0
10
20
I
I
30
LO
I
50 ps
Figure 4. Resolved optical densities (OD/cm) as a function of time (ps) at 330 nm for computed formation of R,, R,, and R, transients in comparison to measured (MTA) and computed total absorption (CTA). Insert shows OD, (mean value of five measurements) at 330 nm vs. time (ps). Solution was composed of mol dm-' tyrosine and 2.8 X mol dm-, N20, p H 7.1. Applied dose was 5.7 J kg-'/0.4-pspulse.
above-described manner of the pulse radiolysis combined with the optimization procedure,7B8the individual resolved optical densities as a function of time at each desired wavelength are obtained. This is shown in Figure 2 for the kinetic courses at 260 nm, illustrating the ability of the applied method. In addition to the substrate consumption (SC),the simultaneous formation of OH adducts on the ortho position (R,) and meta position (R2) as well as of the predominant transient (R3), produced by a consecutive two-stage process (reactions 1 and 9,Table I), is observed. At 295 nm (Figure 3) the measured optical densities (MTA) can be related again to four processes: the appearance of R1 and R2, the slow buildup of R3, and a small substrate consumption (SC). At 330 nm (Figure 4) only a small contribution of the phenoxyl radical (R3) is obvious, and the measured total absorption (MTA) can be attributed mainly to the transient R,, which decays faster than the more stable R2 species. It might be pointed out that at all investigated wavelengths the computed (CTA) and measured total absorptions (MTA) showed a satisfactory consistency. The resolved absorption spectrum for the two isomers R, and R,, as well as for the phenoxyl
2094
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
Solar et al.
TABLE 11: Rate Constants ( k ) for Formation and Decay as well as SpectroscopicCharacteristics of Transients Formed by OH Attack on Tyrosine in Neutral Aqueous Solutions k , dm3 mol-'^‘^ absorpn characteristics transients forma tion decay Amax, nm E, mz mol-' 2k, = (3.0 i 1.0) X l o 8 k , = (7.0 i. 0.5) X l o 9 330 300 i 30 Rl k,' = (1.8 f 0.2) X l o 4 a 305 280 f 30 k, = (5.0 i 0.4) X l o 9 2k, = (2.0 f 0.5) X l o 9 RZ 540 23 i. 3 k, = (6.0 i 1.0) X 10' 0 600 i. 50 26 2k1, = (4.0 f 1.0) X 10' R3 k,' = (1.8 i. 0.2) X l o 4 a 405 260 f 20 a
k' is given in s-l.
TABLE 111: Sensitivity Matrix of the Reaction Model for the OH Attack on Tyrosine in Neutral Aqueous Solution" para-
meter
AODtotal, %
to be
reacn deterno.b mined 1 k, 2 kz 3 k,
4 7
k7
8
k'
k4
9 k', 10 kill
10
30
28 5 1 9 -9
-8
260 nm 60
h = 295
12OC 10 30
22
nm
h= 330 nm
60 120' 10
17 52 31 -9 -9 -17 4 3 1 0 -7 -7 -4 -7 -4 -5 0 -1 -1
0
0
-1
17
29
22
0
0
66 46 49
0 -1
-1
13
-1
-1
0
.T 6
(8)
Products
OX
-1
7 9 18 16 6 -5 -16 -1 -4 0 0 - 1 - 4 0 0 14 14 24 15 8 3 86 71 8 20 14 56 44 35 27 7 20 92 99 24 44 61 14 7 0 0 4 8 0 0 0 0 Prom Table I. ' Microseconds after pulse.
radical R,, is presented in Figure 5 in the range from 250 to about 600 nm. Comparing these individual transient absorptions with the measured total absorption spectrum C in Figure 1, it is obvious that the last one shows an noticeable absorption minimum at 275 nm. This difference can be explained by the absorption of tyrosine in this wavelength range (Figure 5, insert). From the course of the optical densities at -540 nm, a predominant absorption of the meta isomer could be computed. A contribution from the ortho isomer, however, cannot be excluded, since the absorptions in this region are of comparable orders of magnitude as are the standard deviations. Finally, the individual formation and decay rate constants, as well as the corresponding extinction coefficients ( 6 ) of all three transients R, to R,, are compiled in Table 11. The error estimates correspond to 99% confidence limit. Based on the specific formation rate constants (Table 11), the following distribution of the tyrosine transients is obtained: 50% R,, 35% R2, and 5% prompt production of R,. The remaining 10% species, formed with a total rate constant of k4 = (1.4 0.4) X lo9 dm3 mol-, s-l, include the O H adducts on CI- and C4positions. Regarding the O H reactivity with phenol and methoxylated phenol^,'^,^^ it can be assumed that the C4 adduct prevails. As given in Table 11, the main transient R1 (ortho-directed product) exhibits two decay pathways: dimerization and/or disproportionation (reaction 8), as well as water elimination, leading to R3 (reaction 9). The latter process is the predominant
(R,)
30
21 20 9 10 -13 -15 30 11 1 0 6 3 8 6 -10 -10 -10 -5 -6 1 2 2 3 -1 -3
34 30 76 €(dimer) 1 3 mol dm-3 tyrosine. c(RI) e(R,) e(R3)
a
h=
+
H20
(9)
5 3
0' (Rj)
one, but at higher dose rates (>6 J kg-'/pulse) reaction 8 gains
h= 405 nm
h=540nm
60 120' 26 24
10 30 60 120' 10 30 60 120' 19 26 27 35 --23 -27 -26 -26 2 -14 -20 -22 63 49 43 39 .-18 -18 7 4 -3 -3 -3 -3 2 35 0 15 -6 -6 -7 -13 -7 -4 -5 -5 -3 -7 -1 -3 -5 -9 -16 -1 -2 0 0 -1 -2 0 -1 0 0 0 0 0 - 1 - 2 -20 -15 40 46 2 9 1 0 0 0 0 0 -1 -3 0 0 0 0 - 1 - 3 0 0 5 3 0 0 0 0 55 26 19 9 0 0 97 97 97 94 10 11 0 0 35 64 84 90 0 0 0 0 9 5 9 6 0
0
200
0
0
300
0
0
LOO
0
0
0
500
h n m
0
Figure 5. Resolved absorption spectra of transients produced by the OH attack on tyrosine in neutral aqueous solutions. Insert shows the absorption spectrum of mol dm-3 aqueous tyrosine at pH 7.
more importance. Regarding the formation of phenoxy1 radical (R,), it should be stated that although the share of reaction 3 to the total OH attack on tyrosine is very small ( - 5 % ) , its existence as a primary-produced species could be derived from the simulation computations. A computer fit with satisfactory consistency of the measured optical densities within 20 ps after pulse end could only be achieved including the consecutive two-stage reactions 1 and 9 and the direct O H attack on the substrate with k3 = (6 f 1) X lo8. This is especially obvious in the wavelength range about 405 nm, where the total absorption can be attributed exclusively to R 3 species. Their prompt formation has also been observed previously,l but no rate constant has been given. The decay of these transients under our experimental conditions (3-5 J kg-'/pulse, pH 7 ) was determined to be 2k = 0.4 X lo9 dm3 mol-' s-l. A previous value, 2k = 1 X lo9 dm3 mol-' s-l (41 J kg-'/pulse, p H 9),2 has been reported. Further, a decay rate constant of R,, 2k = 0.6 X lo9 dm3 mol-' s-I, could be also derived from the 2k/c = 2.2 X lo5 cm s-l I by using our e405 = 260 m2 mol-'.
J. Phys. Chem. 1984, 88, 2095-2104
2095
R2(meta OH adduct) decays by pure second order; no water splitting from this transient isomer could be observed. This is not in unison with the previous conclusions3 that all OH-adduct isomers are capable of eliminating water. In order to examine the reliability of the optimized parameters, a sensitivity matrix has been evaluated and is presented in Table 111. The elements of the matrix represent the change of the overall O D values in percent, which is produced by increasing k and E values by a factor of 2. It is evident that the parameters kl-k3, k9/, and all t values are very sensitive at least at some wavelengths. On the other hand k4, k7, k8, and k l o are relatively insensitive, demonstrating their small influence on the total absorption in the time range up to 200 p s after pulse end. The sensitivity of the parameters k7 and k , , increases considerably after 200 p s . From Table I11 is also evident that the prompt formation of R3(phenoxyl radical; parameter k 3 ) is very sensitive in the time up to 30 p s at the wavelengths 260 and 405 nm. Hence, its small contribution can be determined with sufficient accuracy.
molecule could be elucidated. More than 90% of the OH radicals form adducts on the phenol ring. It was further stated that ortho-directed transient (R,) decays predominantly by water elimination, resulting in phenoxyl radical, whereas for the meta isomer exclusively second-order decay could be established. A small share of the O H radicals are involved in an H-abstraction reaction from the OH group of the phenol ring, resulting in prompt formation of phenoxyl radical. The O H attack on both ipso sites (Cl- and C,-positions) is in total about 10%; on the alanine moiety it is less probable.
Conclusion The kinetics of the specific-site O H attack on the tyrosine
16-8;
+
Kinetics of the Reaction OH SO2 4- M Dependence in the Falloff Region
-
Acknowledgment. S.S. and N.G. thank Prof. Dr. D. Schulte-Frohlinde, director of Max Planck Institut fur Strahlenchemie, Mulheim/Ruhr, F.R.G., for use of the pulse radiolysis equipment. The financial supp~rtwarded by Bundesministerium fur Wissenschaft und Forschung, Austria, is gratefully appreciated. Thanks are expressed to Dipl. Phys. F. Schworer, F. Reikowski, and K. H. Toepfer for valuable help. Registry No. Tyrosine, 60- 18-4; tyrosine OH-adduct (R]), 89462tyrosine OH-adduct (R2), 89462-15-7; tyrosine phenoxy radical
(R3),16978-66-8; hydroxyl, 3352-57-6.
HOS02 4- M. Temperature and Pressure
P. H. Wine,* R. J. Thompson, A. R. Ravishankara, D. H. Semmes,+ C. A. Gump, A. Torabi, and J. M. Nicovich Molecular Sciences Group, Engineering Experiment Station, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received: July 11, 1983; In Final Form: February 16, 1984)
-
The flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the combination reaction OH + SO2 + M HOSO, + M. A total of 58 bimolecular rate coefficients are reported for varying conditions of temperature (260-420 K), pressure (13-696 torr), and buffer gas identity (He, Ar, N2,SF6). Complicating side reactions involving SO2 photofragments have been eliminated by filtering the photolysis flash with SOz. The reaction is found to be in the falloff region between third and second order over the entire range of conditions investigated. Falloff parameters are derived from the data by the method of Troe. Under conditions of atmospheric pressure and gas composition, rate coefficients derived from our data are about 30% lower than currently recommended values.
Introduction The kinetics of the reaction OH
+ SO2 + M
-
HOSO2
+M
(1)
have been the subject of intensive investigation in recent years. This interest has been stimulated by the importance of reaction 1 as a n initiation step in the atmospheric oxidation of SO?.to sulfate, and its resultant role in the chemistry of acid rain, visibility reduction, and climate modification. Also, because reaction 1 is in the "falloff" region between third and second order over the range of total pressures typically accessible to laboratory studies, it is of interest as a test case for theories of unimolecular decomposition-recombination reaction rates. Reaction 1 has been studied a t pressures below 10 torr in discharge flow systemsl-2and at pressures in the range 10-1000 torr by using flash p h o t o l y ~ i ssteady-state ,~~ photoly~is,~-'~ and pulsed radiolysis' techniques. Despite this large data base, the dependence of kl on pressure and temperature is not well defined. The uncertainty in k , is about f50% at T = 298 K, P = 760 torr of N, and is much larger at lower temperatures and pressures. The poor agreement between the numerous laboratory investiPresent address: Department of Chemistry, California Institute of Technology, Pasadena, CA 91 125.
0022-3654/84/2088-2095$01.50/0
gations of reaction 1 can be largely attributed to experimental difficulties associated with such studies. These include heterogeneous reactions in flow tube and steady-state photolysis experiments and side reactions involving SO2photofragments in flash photolysis studies. In this paper we report the results of a flash photolysis-resonance fluorescence study of reaction 1. Bimolecular rate coefficients have been measured over a wide range of temperature (1) G. W. Harris and R. F. Wayne, J. Chem. Soc., Faraday Trans. J,71, 610 (1975). (2) M. T. Leu, J . Phys. Chem., 86, 4558 (1982). (3) R. Atkinson, R. A. Perry, and J. N. Pitts, Jr., J . Chem. Phys., 65, 306 ( 1976). (4) D. D. Davis, A. R. Ravishankara, and S. Fischer, Geophys. Res. Lett., 6, 113 (1979). (5) G. W. Harris, R. Atkinson, and J. N. Pitts, Jr., Chem. Phys. Lett., 69, 378 (1980). ( 6 ) G. Paraskevopoulos, D. L. Singleton, and R. S. Irwin, Chem. Phys. Lett., 100, 83 (1983). (7) R. A. Cox, Int. J . Chem. Kinet. Symp., 1, 379 (1975). (8) A. W. Castleman, R. E. Davis, H. R. Munkelwitz, I. N. Tang, and W. P. Wood, Int. J . Chem. Kine?. Symp., 1, 629 (1975). (9) A. W. Castleman and I. N. Tang, J . Photochem., 6, 349 (1976). (10) R. A. Cox and D. Sheppard, Nature (London), 284 330 (1980). (11) S . Gordon and W. A. Mulac, In?. J . Chem. Kinet. Symp., 1, 289 (1975).
0 1984 American Chemical Society
