Proton transfer in water

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

Bimodal intermolecular proton transfer in water photoacid-base pairs studied with ultrafast infrared spectroscopy... 

Scheme 5.1. Approximate relative rates of proton transfer in water at thermoneutrality (ApKa = 0) [35, 39, 51].

Figure 2.18 Schematic of the Grotthus mechanism of long-range proton transfer in water molecules. In this, hydrogen bonds and covalent bonds interchange, releasing a proton from one end of the chain as a new proton is introduced at the start of the chain...

Examples of catalysis involving concerted cyclic proton transfer in water have recently been reported in the hydrolysis of N-phenyl-iminotetrahydrofuran (Cunningham and Schmir, 1966, 1967). There is no effect of H2PO, HCO3, and CH3COOH on the rate of hydrolysis, but the type of product obtained, butyrolactone or 7-hydroxybutyranilide is dependent on their concentration. The following scheme was proposed [equation (8)]. 

A SCF CNDO calculation [108] for the HsOJ species similarly suggests that proton transfer in water occurs by tunnelling, the estimated rate coefficient being of the order of 1014 sec-1. 

Fig. 2. (a) Proton transfer in water, (b) Proton transfer from strong acid to strong base, (c) Proton transfer from weak acid to weak base. 

There are no data available on the rate of formation of dialkyloxonium ions like protonated 1,3-dioxolane in Eq. (16). It is remarkable that the rate constants of formation of secondary onium ions from linear ethers, acetals, sulfides etc. are also utdcnovra. These should however be lower than the rate constants of proton transfer in water (an upper limit) being close to 10 mole 1 s but certainly higher than the rate constantsof protonation of olefins... 

R. Contreras and J. S. Goraez-Jeria, /. Phys. Chem., 88, 1905 (1984). Proton Transfer in Water Polymers as a Model for In-Time- and Solvent-Separated Ion Pairs. 

Wraight CA. Chance and design—Proton transfer in water, channels and bioenergetic proteins. Biochim. Biophys. Acta 2006 1757 ... 

D.E. Sagnella and M.E. Tuckerman, An empirical valence bond model for proton transfer in water, J. Chem. Phys., 108(1998), 2073-2083. 

Glycine (intramolecular proton transfer in water) Okuyama-Yoshida 1998 Guanosine triphosphate and imido/ methylene analogs Cannon 1994... 

An important question regarding proton transfer in water is whether it occurs by a classical mechanism or by quantum-mechanical tunneling. This problem can be elucidated by comparing the rates of proton and deuteron transfer. In this way it was concluded that transfer occurs by both mechanisms [15]. 

Rini, M., Mohammed, O. F., Magnes, B.-Z., Pines, E., Nibbering, E. T. J, Bimodal intermolecular proton transfer in water photoacid-base pairs studied with ultrafast infrared spectroscopy, in Ultrafast Molecular Events in Chemistry and Biology , J. T. Hynes, M. Martin (Eds.), Elsevier, Amsterdam, 2004,... 

This Section addresses, in two sets of two chapters, two recent developments in proton transfer the ultrafast vibrational spectroscopic probing of the microscopic details of proton transfer in water and elsewhere, and the emergence of proton-coupled electron transfer reactions as a major reaction dass. 

The treatment of exchange amongst more than two sites is possible by means of the McConnell equations and it can be seen that for the slow-passage solutions Bn algebraic equations result, where n is the number of distinct Larmor frequencies or magnetic sites available. The McConnell equations have been used by Meiboom (1961) in the problem of proton transfer in water amongst seven possible Larmor frequencies. In this case, the population of one Larmor frequency in H20 greatly exceeds the other six in The result obtained by Meiboom by... 

The calculated free energy changes from neutral to zwitterionic glycine are obtained from the calculation of the potential of mean force of the proton transfer in water, with the contribution of the water degrees of freedom being ensemble-averaged out. 

Leung K, Rempe SB (2005) Ab initio molecular dynamics study of glycine intramolecular proton transfer in water. J Chem Phys 122 184506... 

The difficulties in implementing such a coherent approach stem from the fact that one has to deal with complex interactions in a heterogeneous many particle system and that proton transfer in water is a genuinely complicated many-particle phenomenon itself. In the beginning of everything lies the fundamental phenomenon of PT in aqueous networks. 

Notably, the outlined defect mechanism of proton transfer resembles the protocol of proton transfer in water, viz. transformation between localized and delocalized proton states (water H904 -0-11502TAM solid stable crystal configuration -o- metastable intermediate state), triggered by hydrogen-bond fluctuations and molecular rotations. Sequences of these transformations, hydrogen-bond rotations and proton transfers generate the net proton motion. 

It was possible to rationalize the family of Arrhenius plots measured for Nafion 117 at different water contents [46]. Under an assumption that the surface conductivity has higher activation energy, supported by microscopic considerations in Refs. 40, 43, the Arrhenius slope should become steeper with the decreasing amount of water in the membrane [39], that is, the smaller the amount of the bulk water that we have in pores. Activation energies obtained from these plots are 0.1 eV for the largest possible water contents (Activation energies of proton transfer in water, estimated from nuclear magnetic resonance relaxation times, are 0.1 eV [47].) and 0.3-0.4eV at small water contents. How to rationalize this variation. ... 

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