Proton transfer rate constant

After some straightforward manipulations of A3.8.22. the PI-QTST estimate of the proton transfer rate constant can be shown to be given by 48... 

Van der Spoel,D., Berendsen, H.J.C. Determination of proton transfer rate constants using ab initio, molecular dynamics and density matrix evolution calculations. Pacific Symposium on Biocomputing, World Scientific, Singapore (1996) 1-14. 

DR. NORTON The answer is, as yet, no. I think the experiment to do would be to take a weak base with all three of those hydrides as acids and look at the line broadening. By varying the concentration of the weak base over an enormous range, one should be able to obtain the proton transfer rate constants of all three of those hydrides to the same weak base. That is the next experiment on our list. 

Fig. 10 Empirical relationship between the logarithm of the proton transfer rate constants (Table 1) and the corresponding free energies of reaction ArG°. Triangles (y) k /(M 1 s 1). Filled circles ( ) k /(M 1 s-1). Empty circles (O) kjf/s-1.

Cui Q, M Karplus (2002) Quantum mechanics/molecular mechanics studies of triosephosphate isomerase-catalyzed reactions Effect of geometry and tunneling on proton-transfer rate constants. J. Am. Chem. Soc. 124 (12) 3093-3124... 

P. M. Kiefer and J. T. Hynes (2004) Adiabatic and nonadiabatic proton transfer rate constants in solution. Solid State Ionics, 168, pp. 219-224... 

A universal correlation (equation 19) was suggested to exist between the excited-state proton-transfer rate constant and photoacidity in aqueous solutions ... 

Table II. Proton Transfer Rate Constants for the Decay of Radical Anions in Aliphatic Alcohols at 25°C. The Rate Constant is in Units of M-1 sec.-1...

Figure 5. BrSnsted Plot of Exchange Catalysis for the N-1 Proton of 2, 3 -cGMP. P mechanism proton transfer rate constants were obtained from catalytic broadening data (360 MHz, 3 + l C). Catalyst pK values were obtained by direct potentlometrlc titration of experimental mixtures and by extraction from the kinetic data (eq. 1). The pK of the HjO H2O + r reaction Is -1.7.

On the other hand, in the reaction of morpholine with 2 = 2.4 X 10 sec" is relatively low here it is the unusually low proton transfer rate constants (entries 6-9 in Table II) which make k2p < reasons for the slow proton... 

A selection of proton transfer rate constants is summarized in Table II. Our results for the 2/morpholine system are particularly striking. All rate constants refer to thermodynamically strongly favored (ApK >> 0) proton transfers between normal acids and bases and thus would be expected to have values in the order of 10 to 10 s " ( v Iq for T + (3). However the... 

Other possible interpretations for the low proton transfer rate constants such as intramolecular hydrogen bonding or the involvement of the aci-form have been discussed and rejected (5). 

Table 8. Calculated proton transfer rate constants (in s ) for the CH4+ CHj- system (power of 10 in parentheses).

A kinetic study of the acid-catalysed loss of alkoxide and thiolate ions from alkoxide and thiolate ion adducts, respectively, of benzylidene Meldrum s acid, methoxy-benzylidene Meldrum s acid, and thiomethoxybenzylidene Meldrum s acid has been reported. The reactions appear to be subject to general acid catalysis, although the catalytic effect of buffers is weak and the bulk of the reported data refers to H+ catalysis. a-Carbon protonation and, in some cases, protonation of one of the carbonyl oxygens to form an enol compete with alkoxide or thiolate ion expulsion. This scenario rendered the kinetic analysis more complex but allowed the determination of p/fa values and of proton-transfer rate constants at the a-carbon. In conjunction with the previously reported data on the nucleophilic addition of RO and RS ions to the same Meldmm s acid derivatives, rate constants for nucleophilic addition by the respective neutral alcohols and thiols could also be calculated. ... 

The picture of prototropic trjinsformations of the nucleic acid base tautomers will never be completed without a knowledge of inter- and intramolecular proton transfer kinetics. The most general data describing the kinetics of proton transfer are the set of temperature dependent rate constants. These data for nucleic acid bases are not yet available from either experimental or theoretical studies except the very recent paper [ 134] where the authors attempt to estimate the water assisted proton transfer rate constant for adenine. However, the calculated values of proton transfer barrier for both non-water assisted and water assisted pathways are available for the adenine, guanine and eytosine [119, 123, 134]. These data are collected in Tables 12 - 16, where, for convenience, we have defined as forward reaction the proton transfer process from the normal (canonical) to the hydroxo- (imino-) form. 

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

ArO ). This second term cannot be attributed to the presence of some impurities, since the fluorescence decay curves of the same compounds in aprotic solvents are precisely monoexponential. Registration wavelength was carefully selected by monochromator to exclude the fluorescence of the anion. Neither first nor second term cannot be attributed also to the dissociation of unsolubilized ArOH since the proton transfer rate constants in water are much greater [4-6, 15-17]. 

It is seen that ion association occurs with the diffusion rate constant of an order of 10 l/(mol-s). The transfer of a proton from an acid to a water molecule requires an energy (AG > 0) and occurs, therefore, more rapidly or slowly. The following rule can be used for the approximate estimation of the proton transfer rate constant in the transition states O...H—O, N...H—O, and N...H—N in aqueous solutions ... 

---