Reactivity of the carbonate radical in aqueous solution. Tryptophan and its derivatives

Abstract

The rate constants for the reaction of C03-and CO3H radicals, generated in the flash photolysis of aqueous solutions of Co(NH3)4C03+, 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, IV-acetyltryptophan, and N-methyltryptophan follows a "titration" curve that represents the acid-base equilibrium of the radical (pKa = 9.6 ± 0.3) . At 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 108 M~l sec-1) 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 pKa of the NH3+ group (tryptamine, tryptophanamide, tryptophan methyl ester, glycyltryptophan, glycylglycyltryptophan, tryptophylglycine, and tryptophan) with the rate constant being greater by almost a factor of 2 for NH3+ than for NH2. 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.

Temperature dependence of the rate constants for reactions of the carbonate radical with organic and inorganic reductants

“…The fact that the reactions of C0,'-with I -and with methionine have strongly negative temperature dependencies, however, indicate that these reactions, at least, proceed by a more complex mechanism. For the phenolates and ascorbate, if we accept a near-zero activation energy for the reactions of ascorbate, hydroquinone, and p-methoxyphenolate as calculated above (by correcting for diffusion), we find that the reactions for the substituted phenolates and ascorbate do not correlate with the reduction potentials, suggesting that these reactions also take place via addition, as mentioned before [6]. On the time scales of these experiments, the addition of CO3'-to these reactants does not appear to lead to observable intermediates, suggesting that their steady-state concentrations are very low.…”

“…The reaction with tryptophan is fast and has almost no temperature dependence. This reaction has been proposed to take place by addition of the radical to the 7~ system of the indole rings, rather than by direct electron transfer [6].…”

“…This radical appears to react primarily as an oxidant, although the electron transfer may take place via an addition mechanism [6]. Studies of the reactions of C03'-with substituted phenols [7] and anilines [8] found that the rate constants correlated with the Hammet substituent constant a, with negative slopes of p = -1.0 and -1.1, respectively, suggesting that the radical is strongly electrophilic.…”

Steady‐state concentrations of carbonate radicals in field waters

“…In natural waters, it was assumed that the primary scavengers of •OH were carbonate/bicarbonate ions and DOC. The OH reacts with excess carbonate or bicarbonate with second‐order rate constants of 4.2 × 10 8 /M/s and 1.5 × 10 7 /M/s, respectively [14]. Hydroxyl reactivity toward DOC has been reported to range from 1.8 to 3.3 × 10 4 (L/mg DOC s), with an average of 2.4 × 10 4 (L/mg DOC s) [15], and 2.5 × 10 4 (L/mg DOC s) [6,16]; the 2.5 × 10 4 (L/mg DOC s) value was used for this study.…”

A new method for measuring carbonate radical reactivity toward pesticides

“…Aromatic compounds show variation in the reactivity depending on the nature of the substituent on the aromatic ring. The reaction rate constant is high for indole and its derivatives such as tryptophan ( k > 10 8 /M/s) [11]. The rate constants have been measured for the reaction of carbonate radicals, generated in the flash photolysis of aqueous solution of Co(NH 3 ) 4 CO 3 + , with a series of substituted benzene derivatives.…”