Reactivity of the Carbonate Radical
2099
Reactivity of the Carbonate Radical in Aqueous Solution. Tryptophan and Its choen-nan Chen and Morton 2. Hoffman* ~ e ~ a i f nof? Chemistry, e~~ Boston University, Boston, Massachusetts 02215
(Received June 17, 7974)
The rate constants for the reaction of COB- and C03H radicals, generated in the flash photolysis of aqueous solutions of Co(NH3)&03+, with tryptophan and its derivatives have been measured as a function of pH. At 0.1 M solution ionic strength, k for indole-3-propionic acid, N-acetyltryptophan, and N-methyltryptophan follows a “titration” curve that represents the acid-base equilibrium of the radical (pK, = 9.6 i 0.3). A t 0.003 M ionic strength, the rate constants are the same for the three compounds, independent of pH, and equal to that for indole (4.2 X lo8 M-l sec-l) indicating that the mode of reaction is interaction of the radical with the aromatic system and that the intrinsic reactivity of the acidic and basic forms of the radical are the same. Amino-containing compounds show variation of the rate constant according to the pK, of‘the NH3+ group (tryptamine, tryptophanamide, tryptophan methyl ester, glycyltryptophan, giycylglycyltryptophan, tryptophylglycine, and tryptophan) with the rate constant being greater by almost a factor of 2 for NH3+ than for “2. A small but significant enhancement of the rate is noted for amino compounds lacking the carboxylate group. A model is proposed wherein the reaction of the radical with the aromatic system is enhanced by interaction with the amino group which results in a transitory cooperative mechanism causing the radical to be in closer proximity to the reactive site.
Introduction The carbonate radical (COS-) can be generated in aqueous solution upon the reaction of OH radicals with C032(k = 4.2 X los 1K-l sec-1)2 or C03H- ( h = 1.5 X lo7 M-l sec-1)2 and thus may be an important component in biological systems which suffer radiolytic damage. Whereas the reactivity patterns of the primary radicals generated in the radiolysis of water (eaq-, OH, and H) are rather well k n ~ w n , the ~ - ~behavior of the C03- radical has not been investigated in much de1;ail. Despite the convenient absorp600 nm; €600 1.83 X lo3 tion spectrum of the radical (A, 1M-l crn-l),6 the use of pulse radiolysis is limited to a rather narrow pH range in alkaline solution because of the relatively low reactivity of OH with C03H-, the pK, of C03H(10.36), the pK, of OH (ll.9),2 and the low reactivity of its conjugate base.2 Thus, pulse radiolysis does not permit an examination of the radical’s nature in biologically important neutral solution or as a general function of pH. We have reported’7~~ that the flash photolysis of Co(NH3)&03f generates the COS- radical permitting an evaluation of its acid-base properties (pK, = 9.6 f 0.3 for C03H)9,10and the rate constants for its reaction with biologically important molecules in neutral solution.ll Among the compounds with which the radical reacts fairly rapidly ( h > lo* M-l sec-l) are tryptophan and its derivatives.ll In this paper we examine the reactivity of the radical as a function of pH with this group of compounds and attempt to identify the molecular parameters that give rise to the observed patterns. Experimental Seetian The flash plhotolysis apparatus used and the techniques employed have been described in detail as has been the J~J~ were prepreparation of C O ( N H ~ ) , C O ~ + . ~Solutions pared from triply distilled water containing 5 X 10-5 M complex and up to 2 X M organic scavenger. The pH of the solution was controlled by combinations of KH2P04, K2HPQ4, and KOH solutions which gave a constant solution ionic strength of 0.003 M at pII c11.5. Above that pH,
only KOH was used to establish the acidity and ionic strength of the solution. For an ionic strength of 0.1 M , NaC104 was used with a correction for the Na+ effect on the pH measurement. The pH of the solutions was m?asured before and after the flash with a Beckman Expandomatic SS-2 pH meter and was not changed by the flash. All solutions were freshly prepared from the solid complex and were discarded after one flash. Inasmuch as COB- radicals do not react with 02, the solutions were flashed without deoxygenation. Independently we demonstrated that the presence of air had no effect on the values of k. The following compounds were used without further purification: tryptophan, tryptophan methyl ester, glycyltryptophan, tryptophylglycine, indole (Calbiochern); glycylglycyltryptophan, N-methyltryptophan, N-aeetyltryptophan, tryptamine, and tryptophanamide (Cyclo Chemicals); indole-3-propionic acid (Matheson, Coleman, and Bell). The structures and known pK, values for these compounds are given in Table I. The pseudo-first-order rate constant for the decay of the transient absorption a t 600 nm was determined at a single scavenger concentration; the linear dependence of k on [scavenger] had already been established.l*,ll The value of the second-order rate constant for the reaction of the radical with the scavenger was evaluated from at least four individual decays and is known to be f10% at pH 9 where the effect of ionic strength on the reactivity of the basic form of the radical becomes evident. Tryptophylglycine was especially chosen to test this effect because the pK, of its amino group was expected to be substantially lower than that for tryptophan; amino groups adjacent to the amide linkage are more acidic than ordinary amino groups (e.g.,pK, of glycine is 9.6 but that of glycylglycine is 8.1).18 The three remaining compounds in Figure 3 show the same behavior as the other amino-containing derivatives except that the limiting value of k in basic solution is larger than that of indole. The pH dependence of k at low solution ionic strength, which mirrors the pK, of the amino group, cannot arise from simple electrostatic effects. In addition to the identical behavior of the two forms of the radical seen at low ionic strength (Figure l), the value of h for 1- charged species is the same as for neutral species. Similarly, although it is known that the electron density on the aromatic rings is affected by substitution,lg the possibility of transmission of an inductive effect through a multiatom chain, as in the case of the peptides, is rather remote. In addition, there does not appear to be any evidence favoring a static association of the protonated amino groups with The Journalof Physical Chemistry. Vol. 78, No. 27, 1974
Schoen-nan Chen and M o r t o n 2. Hoffman
21 02
the aromatic system that could serve as an activating influence on the reactivity of the radical. We conclude that the attack by the radical involves a transitory cooperative mechanism involving the interaction of the radical with the protonated amino group. Thus, when NH3+ is present, even in the cases of the flexible peptides, interaction of the radical with NH3+, perhaps via hydrogen bonding and enhanced by the attraction of the electronegative oxygen atom of the radical to the positive charge on the amino group, allows the radical to be held in proximity to the aromatic system for a period of time longer than is the case in random collision (see structure I). The resuli, is a sterically more effective collision
group is present, as in the case of tryptophan, the basis for radical-amino group interaction is removed in basic solution. While we recognize that the pH dependence of the rate constants for these reactions is not a large effect (no more than a factor of 2), the effect is outside of experimental error. The results do demonstrate that in the case of the COS- radical, the rate constants for these less-than-encounter-controlled reactions are affected by small changes in the structure of the scavenger. Such structure-reactivity patterns would be extremely difficult to detect in the case of very reactive radicals, such as OH, which react on virtually every collision.
References and Notes
I
with a greater probability of ultimate reaction and a higher observed rate. Bififerences in the structures of the various derivatives that affect the charge distribution at the amino group would cause subtle changes in the radical reaction rats. Removal of the adjacent carboxylate group, as in tryptamine and tryptophanamide, would result in an increase of the effective charge of NH3+ and of the template effect. The intermediate behavior of the tryptophan methyl ester could be clue to the nature of the ester linkage or hydrolysis during the reaction. The higher rate for the noncarboxylate-containing amino derivatives (as compared to indole) in basic soliution suggests that some interaction occurs between the radical and the amino group even when the latter is deprotonated. Apparently when a carboxylate
The Journal of iDhysical Chemistry, Vol. 78, No. 21, 1974
(1) The support of this research by the National Science Foundation through Grant No. GP-40623X is gratefully acknowledged. Aspects of this research were reported at the 167th National Meeting of the American Chemical Society, Los Angeles, Callf., April 1974: No. PHYS 22. (2) J. L. Weeks and J. Rabani, J. fhys. Chem., 70, 2100 (1966). (3) E. J. Hart and M. Anbar, “The Hydrated Electron,” Wiley-lnterscience, New York, N. Y., 1970. (4) L. M. Oorfman and G. E. Adams, Nat. Stand. Ref. Data Ser., Nat. Bur. Stand., No. 46 (1973). ( 5 ) P. Neta, Chem. Rev., 72, 533 (1972). (6) E. Hayon and J. J. McGarvey, J. fhys. Chem., 71, 1472 (1967). (7) V. W. Cope and M. 2. Hoffman, J. Chem. Soc., Chem. Commun., 227 (1972). ( 8 ) V. W. Cope, S.-N. Chen, and M. 2. Hoffman, J. Amer. Chem. Soc., 95, 3116(1973). (9) S.-N. Chen and M. Z . Hoffman, J. Chem. Sac., Chem. Commun., 991 (1972). (IO) S.-N. Chen, V. W. Cope, and M. Z . Hoffman, J. fhys. Chem., 77, 1111 (1973). 11) S.-N. Chen and M. 2. Hoffman, Radiat. Res., 58, 40 (1973). 12) The symbol COS’ will be used in this paper to designate the radical irrespective of its acidic or basic form unless the specific nature of the radical is essential for the discussion. 13) G. E. Adams, J. E. Aldrich, R . H. Bisby, R. 6.Cundall. J. L. Redpath, and R . L. Willson. Radiat. Res., 49, 278 (1972). 14) R. C. Armstrong and A. J. Swallow, Radiat. Res., 40, 563 (1969). 15) M. Simic, M. 2. Hoffman, and M. Ebert, d fhys. Chem., 77, 1117 (1973). (16) W. Latimer, “Oxidation Potentials,” 2nd ed, Prentice-Hall, Englewood Cliffs, N. J., 1952. (17) M. Simic andE. Hayon, J. Amer. Chem. SOC.,93, 5982 (1971). (18) W. P. Jencks and J. Regenstein, “Handbook of Biochemistry,” Chemical Rubber Co., Cleveland, Ohio, 1970, p J-150. (19) R. J. Sundberg, “The Chemistry of Indoles,” Academic Press, New York, N. Y., 1970.
