Proton rate constant

Gutman M 1986 Application of the laser-induced proton pulse for measuring the protonation rate constants of specific sites on proteins and membranes Methods Enzymol. 127 522-38... 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

The protonation rate constant of the triplet state of thionine has been measured as a function of temperature and hydrogen ion concentration using flash photolysis with a frequency-doubled neodynium laser 1614)... 

Deprotonation rate constant (sec-1) kp (Protonation rate constant) (M-1/sec) Tp tdp ... 

The rate constants for protonation of the excited singlet states of several compounds were determined by Weller (1961). Although the measurement of excited state equilibrium constants has become more common, there have been relatively few determinations of the rate constants involved. Trieff and Sundheim (1965) investigated the effects of solvent changes on the rates of protonation and deprotonation of 2-naphthol in the S) state. The dissociation rate constant decreased progressively with the addition of methanol or glycerol to the aqueous solution but the protonation rate constant varied in a more complex manner. As mentioned above, Stryer (1966) found both rate constants smaller in D20 than in H20. 

The thermodynamics and kinetics of H+ binding to cobalt(I) and nickel(I) macrocycles have been determined. The pAia of Ni(cyclam)(H), / / 5 5 -NiHTIM(H) + and A-rac-CoHMD(H) + are 1.8, 1.9 and 11.7, respectively [14, 24, 27]. As seen from Table 3, protonation rate constants for A-rac-CoHMD depend on acid strength. The results are consistent with an associative reaction of the square-planar complex with an acid, HA. Whereas the spectrum of 7V-rac-CoHMD(H) + suggests the formation of a [Co (H )] + species with an absorption band at 440 nm (520 M cm ), Ni(cyclam)(H) + shows no significant absorbance in the 300-700 nm region [14, 24]. 

Assuming that the protonated form of FITC is non-fluorescent, i.e., that Qhfi = 0, P corresponds to the fraction of protonated FITC, i.e., P = [HFr]/([Fl2-] + [HFr]). prot is the protonation rate constant given by ... 

Figure 10. Plot of AV for the reaction of CoL with HA versus AV° <> of HA. The size of each marker is correlated to the magnitude of the protonation rate constants. The slope of the plotted line is 0.44.

Absolute rate constants (4) for this protonation reaction (in units of M"1 sec.-1) are given in Table II for four aliphatic alcohols. These protonation rate constants show a good correlation (4) with the relative acidity of the alcohols (18, 26), which is to be expected if the protonation involves the hydroxyl proton, and which may therefore be taken as an indication of the nature of Reaction 2. It is, however, clear that in changing the alcohol the solvent as well as the "reactant is being changed and that the correlation is therefore not completely straightforward. 

Table III. Protonation Rate Constants for Some Aromatic Radical Anions in Isopropyl Alcohol at 25°C.

The protonic character of the alcohols is not the only important parameter which determines the magnitude of k2. There is a variation of at least two orders of magnitude in k2, for a given alcohol, for the radical anions investigated. Table III shows the values of the protonation rate constants in isopropyl alcohol for seven different aromatic radical anions. 

Based on the depicted equilibrium and the observed lifetime a rate constant for the forward reaction of 10 NT s" was estimated. The slow protonation rate of the one-electron reduced fullerene n-radical anions can be understood in terms of the charge delocalization and also the hybridization of the generated carbanion. Furthermore, the heterogeneous and hydrophobic environments of the host s interior can be assumed to be beneficial for the slow-down of the protonation dynamics. In homogeneous aqueous solutions the protonation rate should be faster, a hypothesis that was substantiated by recent radiolytic experiments with bisfunctionalized fullerene derivatives. The latter compounds are soluble in aqueous solutions without employing a solubiiizer (host) and give rise to protonation rate constants of 3 x 10 M s" (38). 

Figure 1. Correlation of protonation rate constants ffcg) with singlet-singlet separation The compounds are identified by the numbers as given in Table L...

With increase in the ionic strength of the solution the first kinetic wave (pH 6.0) increases in height and the second wave (pH 3.5 ) decreases. Evidently both waves have surface character. The protonation rate constant of the benzophenonetetracarboxylate anions was evaluated formally on the assumption that the principal proton donor is the hydrogen ion and with allowance for the change in concentration of charged particles at the electrode surface under the influence of its field. In these calculations account was not taken of the variation in the acid dissociation constant during its adsorption on the electrode surface (see Section 1, Point f). 

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