Proton transfer between bases

Fig. 23.11. VBCMDs, like Fig. 23.1b, for proton transfer between bases (a) The X base is not very stable (strong base), (b) The X base is F (adapted from Ref. [11] with permission of Wiley-VCH STM-Copyright and Licenses).

The mechanism of enolization involves two separate proton transfer steps rather than a one step process m which a proton jumps from carbon to oxygen It is relatively slow m neutral media The rate of enolization is catalyzed by acids as shown by the mechanism m Figure 18 1 In aqueous acid a hydronium ion transfers a proton to the carbonyl oxygen m step 1 and a water molecule acts as a Brpnsted base to remove a proton from the a car bon atom m step 2 The second step is slower than the first The first step involves proton transfer between oxygens and the second is a proton transfer from carbon to oxygen... 

Quantitative Calculations In acid-base titrimetry the quantitative relationship between the analyte and the titrant is determined by the stoichiometry of the relevant reactions. As outlined in Section 2C, stoichiometric calculations may be simplified by focusing on appropriate conservation principles. In an acid-base reaction the number of protons transferred between the acid and base is conserved thus... 

This study is particularly noteworthy in the evolution of QM-MM studies of enzyme reactions in that a number of technical features have enhanced the accuracy of the technique. First, the authors explicitly optimized the semiempirical parameters for this specific reaction based on extensive studies of model reactions. This approach had also been used with considerable success in QM-MM simultation of the proton transfer between methanol and imidazole in solution. 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Mechanism of Base-Catalyzed Hydration The base-catalyzed mechanism (Figure 17.5) is a two-step process in which the first step is rate-detennining. In step 1, the nucleophilic hydroxide ion attacks the carbonyl group, forming a bond to carbon. An alkoxide ion is the product of step 1. This alkoxide ion abstracts a proton from water in step 2, yielding the geminal diol. The second step, like all other proton transfers between oxygen that we have seen, is fast. 

Because water is amphiprotic—because it is both a Bronsted acid and a Bronsted base—proton transfer between water molecules occurs even in pure water, with one molecule acting as a proton donor and a neighboring molecule acting as a base ... 

Because conjugate acids and bases are in equilibrium in solution, we use the equilibrium constant for proton transfer between the solute and the solvent as an indicator of the strength of an acid or a base. For example, for acetic acid in water,... 

Proton transfers between oxygen and nitrogen acids and bases are usually extremely fast. In the thermodynamically favored direction, they are generally diffusion controlled. In fact, a normal acid is defined as one whose proton-transfer reactions are completely diffusion controlled, except when the conjugate acid of the base to which the proton is transferred has a pA value very close (differs by g2 pA units) to that of the acid. The normal acid-base reaction mechanism consists of three steps ... 

Many reactions are catalyzed by acids, bases, or both. In such cases, the catalyst is involved in a fundamental way in the mechanism. Nearly always, the first step of such a reaction is a proton transfer between the catalyst and the substrate. 

Several base pairs of adenine-thymine, including the WC pair, have also been studied [238], It is found that a charge transfer state exists about 1.5 eV higher than the local nn states. Proton transfer between the bases stabilizes a charge transfer state which then crosses with the ground state facilitating radiationless relaxation. This mechanism is not energetically favorable for non WC pairs. 

In acid-base catalysis there is at least one step in the reaction mechanism that consists of a generalized acid-base reaction (a proton transfer between the catalyst and the substrate). The protonated or deprotonated reactant species or intermediate then reacts further, either with... 

Seminal studies on the dynamics of proton transfer in the triplet manifold have been performed on HBO [109]. It was found that in the triplet states of HBO, the proton transfer between the enol and keto tautomers is reversible because the two (enol and keto) triplet states are accidentally isoenergetic. In addition, the rate constant is as slow as milliseconds at 100 K. The results of much slower proton transfer dynamics in the triplet manifold are consistent with the earlier summarization of ESIPT molecules. Based on the steady-state absorption and emission spectroscopy, the changes of pKa between the ground and excited states, and hence the thermodynamics of ESIPT, can be deduced by a Forster cycle [65]. Accordingly, compared to the pKa in the ground state, the decrease of pKa in the... 

Acids and bases, oxygen and nitrogen in aqueous solution, mechanisms of proton transfer between, 22, 113... 

Mechanisms of Proton Transfer between Oxygen and Nitrogen Acids and Bases in Aqueous Solution... 

Acid and base catalysis of a chemical reaction involves the assistance by acid or base of a particular proton-transfer step in the reaction. Many enzyme catalysed reactions involve proton transfer from an oxygen or nitrogen centre at some stage in the mechanism, and often the role of the enzyme is to facilitate a proton transfer by acid or base catalysis. Proton transfer at one site in the substrate assists formation and/or rupture of chemical bonds at another site in the substrate. To understand these complex processes, it is necessary to understand the individual proton-transfer steps. The fundamental theory of simple proton transfers between oxygen and nitrogen acids and... 

In this region, the equilibrium constant for the proton-transfer step in Scheme 7 has a value K2> 1 and the proton transfer step is strongly favourable thermodynamically in the forward direction. This reaction step is a normal proton transfer between an oxygen acid which does not possess an intramolecular hydrogen bond and a base (B) and will therefore be diffusion-limited with a rate coefficient k2 in the range 1 x 109 to 1 x 1010dm3mol-1 s 1. It follows from (65) that kB will have a value which is... 

For proton transfer between a monoprotic acid HA and a base B,... 

Earlier pioneering work by Eigen (56) showed that the exchange process of a proton in aqueous acidic/basic medium between species MOH and MO - can in general be represented by Scheme 4 (which can also be adapted to include the proton transfer between a [MOH2] and a [MOH] species). This mechanism, involving protolysis and hydrolysis (acid and base catalysis) and direct proton transfer, can be... 

---