Protonation proton emitter

Proton decay should be a simple extension of a decay with the same ideas of barrier penetration being involved. A simplification with proton decay relative to a decay is that there should be no preformation factor for the proton. The situation is shown in Figure 7.12 for the case of the known proton emitter 151Lu. One notes certain important features/complications from this case. The proton energies, even for the heavier nuclei, are low (Ep 1 —2 MeV). As a consequence, the barriers to be penetrated are quite thick (Rom = 80 fm), and one is more sensitive to the proton energy, angular momentum changes, and so forth. 

The phenomenal increase in excited-singlet-state acidity of aromatic hydroxy compounds makes them good excited-state proton emitters, with deprotonation... 

Using 2NpOH and 2-naphthol-3,6-disulfonate [121] as excited-state proton emitters, a transient high proton concentration is achieved on the membrane surface. With bromocresol green dye adsorbed on the membrane serving as a pH indicator, it has been found that the protons first react with the acidic ionized moieties on the surface and then reach the strongest base on the surface by rapid exchange. 

Additional examples of 3-delayed two-proton emitters may be sought among the A = 4n+2, Tz = -2 nuclei such as 46Mn and 50Co [CAB 84a]. However, our preliminary searches for 2p emitters among the products of... 

Figure 20.1. Transient protonation of Bromocresol Green adsorbed on mixed micelles. The reaction was carried out at pH 7.3 + 0.1 in a solution containing 500 pM Bromocresol Green, 1 mM of the proton emitter 2-naphthol-3,6-disulfonate, 500 pM micellar concentration of Brij-58 (40 mg mL ...

The reversible protonation of the dye was measured in the time resolved domain using photoexcited pyranine as a proton emitter [65]. The rate constants of the various proton transfer reactions were determined by kinetic analysis [66, 66a] and the results are given in Table 20.2. 

Alkalinization Pulse by the Conjugate Base of the Proton Emitter 62... 

Figure 11 demonstrates the equivalence of salt solutions by the rate of proton dissociation from two proton emitters. Thus, the kinetic method for determination of can be... 

Figure 11. Correlation between water activity coefficient of MgCl2 and NaC104 solutions as estimated from the rate of proton dissociation from two proton emitters, 2-naphthol-6-sulfonate (ordinate) and hydroxypyrene trisulfonate (abcissa). ( ) MgCl2 ( ) NaC104.

IV. DETECTION OF THE PROTON BY ITS REACTION WITH THE EXCITED ANION OF THE PROTON EMITTER... 

The recombination rate in such a microspace can be estimated according to Goselle et al. (1979). When two reactants, one of which is much smaller and more diffusive (H+) than the other (0-), are locked in a space with internal diameter R, we can regard the heavier reactant (0-) as practically immobile target in the center of a reaction sphere. The proton can diffuse through the reaction space, but wherever it penetrates the Coulomb cage of the proton emitter, protonation takes place. The rate constant of the reaction is thus controlled by two radii, the Debye radius (RD), where electrostatic interaction dominates, and the radius of the reaction sphere, out of which the proton cannot escape. 

The rate of proton dissociation was calculated from rapid kinetics measurements (decay time of < >OH, or rise time of 0- ). The rate is comparable with that measured in water, that is, 1010 sec-1, indicating that the water in the vicinity of the trapped proton emitter is out of the extensive hydration layer of the phospholipid (LeNeveu et al., 1977 Rand et al., 1980 Parsegian et al., 1979 Lis et al., 1982). 

In a case where the number of reactants (proton emitters) per site is small (most likely one,) the number of identical observed sites is high, and the event is highly synchronized (the perturbation is short with respect to the relaxation time), the difference between the rate constants calculated according to classical formalism or stochastic approach is less than 15% (Vass, 1980). Thus, in most cases the classical formalism can be employed, but its applicability should always be examined. 

The Detection of Protons by Their Reaction with the Ground-State Anion of the Proton Emitter... 

The ground-state anion of the proton emitter is the straightforward detector for the ejected protons and represents the simplest system for analysis (Forster and Volker, 1975, Gutman etal., 1981). In the absence of other acceptors, the transient increment of H+ concentration (AH+), is identical with the increment of < >0 above its prepulse (equilibrium) concentration (Achemical relaxation, the concentration of the reactants is given as a sum of the equilibrium concentration plus the incremental deviation from equilibrium, which is the time-dependent variable. For the single-component system... 

The time constant for hydrolysis is approximated by Thydroiysis — Ka x 105 (sec). Thus, for proton emitters with pK0 < 10 this reaction will be too slow to affect of overall dynamics. 

The rate constants of other proton emitters are listed in Table IV. The graphical superposition of the simulated curve (Gershon, 1982 Gutman... 

Figure 25. The dependence of the macroscopic parameters on the rate constant of proton recombination with the proton emitter anion. The macroscopic parameters were calculated for simulations describing the experimental conditions defined in Figure 23. The frames represent y, (A), y3 (B), Tmax (C), and Fmax (D) as a function of the rate of protonation of CT. In each figure, there are three curves calculated for k3 with the values of 3.2 x lO10 Af"1 sec-1 ( ), 4.2 x lO10 Af"1 sec-1 (—), and 6.2 x 1010 Af"1 sec 1 (—). The experimentally determined macroscopic parameters are indicated as parallel horizontal lines. The vertical lines denote the range of A, values that will yield macroscopic parameters compatible with the measured ones.

Figure 26 relates the effect of the prepulse pH on the macroscopic parameters (X0 = 4.25 jtM). Within the range where the experimental results are conveniently measured, the computed macroscopic parameters and the measured ones are essentially identical. It is of interest to point out that at low pH both 71 and y2 increase but at the upper limit 72 does not vary much. This is the range where the < )0 concentration increases to such an extent that it competes effectively for any H+ dissociating from HIn, thus 72 approaches the value of A4. The signal size has a clear maximum, determined mostly by the pK values of the proton emitter and the proton detector. At low pH, the depletion of In- limits the formation of HIn, whereas at high pH the absence of undissociated < >OH reduces the size of the pulse and increases the competitivity of <(>0 for the protons. 

In the above sections, we referred only to one product of the photodissociation—the proton. Still, it is produced together with a strong base, the ground-state anion of the proton emitter. The reason that the effect of 4>CT was not observed is due to the experimental conditions that were set to mimimize it. The diffusion of the conjugate base is 10 times slower than that of the proton. Thus, the reactions of the proton with other solutes is the dominating event. Furthermore, the electrostatic repulsion between the emitter and the detector reduces the contribution of <(>0" to the measured dynamics. Once these factors are understood, it is possible to predict and set experimental conditions where the reaction of < >0-with the indicator will dominate the observed reaction. By setting the experimental pH to be lower than the pK of the proton detector (pH <<... 

For each pair of indicator/proton-emitter, there will be a point, pH, where the response of the indicator will switch its direction. At higher pH values, the indicator will be acidified, whereas at lower pH values alkalin-ization will dominate. The value of this transition is given by ... 

The kinetic experiments (Figure 32) were carried out with 50 iAf Neutral Red and 40 mg/ml Brij 58 (equivalent to 500(jlM of micellar concentration). At this concentration of detergent, 98% of the indicator is adsorbed. The initial pH of the experiment (7-7.5) ensured that before perturbation the Neutral Red was mostly deprotonated, whereas the proton emitter (2-naphthol, 3,6-disulfonate, pKo = 9-3) was undissociated. Perturbation of the equilibrium by a laser pulse, dissociates Xo molecules of OH, and the relaxation of the system is described by equations (37) and (38). (Direct proton exchange between < >0 and bound indicator can be ignored in this case.) The experimental curve and the simulated function are given in Figure 32. The rate constants of the reaction are listed in Table V. 

This section will demonstrate how a combination of well adsorbed proton emitter and detector can be used for this purpose. To avoid the complications by the nonhomogeneous surface of a protein, the model system are micelles of the uncharged detergent Brij 58. Studies using phospholipid surfaces and protein are presently under current experimentation. 

A proton emitter in such experiments should be well adsorbed, yet the dissociating proton must be in contact with the aqueous phase of the interface, otherwise no dissociation will take place. The proton emitter of choice, is 3-naphthol. It has an intensive absorption at the wavelength of... 

In the absence of micelles, (3-naphthol is a very efficient proton emitter, and the protonation of Bromo Cresol Green is easily analyzed by simulation technique (Figure 30). 

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