Proton Transfer Reaction Pathway

PROTON TRANSFER REACTION PATHWAY 4.1 Free Energy Profile... 

FIGURE 9.1. The potential surface for proton transfer reaction and the effect of constrainir the tiA B distance. The figure demonstrates that the barrier for proton transfer increasi drastically if the A — B distance is kept at a distance larger than 3.5 A. However, in solutic and good enzymes the transfer occurs through pathway a where the A - B distance is arour 2.7 A. 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

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. 

Why is no addition product observed in the gas phase, in contrast to solution This is not a case of no endothermic reactions both the proton transfer reaction (6b) and the alkoxide addition reaction (6a) are exothermic pathways. When an exothermic reaction occurs in solution, the excess energy is passed to the solvent. In the gas phase, with no solvent available, the excess energy remains in the intermediate. This can result in an effective internal temperature for that intermediate of hundreds to thousands of degrees. If there is some other bond that can be broken to yield a product ion plus a neutral in a pathway that is exothermic with respect to the reactants, the intermediate will fragment by that method, and the observed product will be that fragment ion. This internal temperature is the reason for the very short lifetime of the intermediates mentioned above. 

It had been anticipated that the reaction pathway for a proton transfer reaction should be characterized by a double minima in the potential surface.100 The failure of Qementi s calculation to predict a barrier along the reaction pathway for the transfer of H + from HQ to NH3 to form NHJC1-, must be accepted with some caution in view of the lack of polarizing functions on N and Cl in the basis set. This work, however, broke new ground and dementi99 includes in his paper a detailed discussion of the merits and faults to be expected for the Hartree-Fock method in the different regions of the potential surface. 

On a second front, in all of the systems described here, the hydrogen bond interface is required to maintain assembly of the supramolecular complex. This construct makes it difficult to assess the effect of pA a on the coupled electron- and proton-transfer reactions. By placing a network proximal to, yet distinct from, the electron transfer pathway while maintaining independent spectroscopic signatures for electron and proton transfer, the kinetics for the isolated events can be examined as the pATa of the environment is systematically varied. 

Scheme 4.33 Mechanism of the suicide inhibition of tymidylate synthase by 5-fluorouracil. The reaction pathway with the natural substrate (dUMP) is depicted on the left, the analogous sequence with 5-fluoro-dUMP on the right. The key to the irreversible blocking of the enzyme reaction site is the inability of fluorine to functionally replace hydrogen in proton-transfer reactions, for example the -elimination liberating the enzyme thiolate group [10], In addition, the transient positive charge on the methylene group during hydride transfer is destabilized by the jff-fluorine.

The reduction sequence depicted in Figure 1 can be formulated as a series of electron- and proton- transfer reactions. For reduction at heterogeneous surfaces there is strong evidence that the dissolved reactants and intermediates and the corresponding species adsorbed on the metal surface are in dynamie equilibrium. The specific reaction pathway of a transformation depends on many factors,... 

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