Partial proton transfer reactions

Hydrogen-bonding is essentially a partial proton-transfer reaction. Thus, the ionic-resonance mnemonic (5.29a), which expresses the partial covalency of H-bonding, suggests an immediate relationship to the degree of completion of the actual proton-transfer reaction... 

FIGURE 1.6 Gas-phase concentration of bovine serum albumin. Mass spectra derived (a) from native ESI (b) after partial proton transfer reactions and in the absence of ion parking mode and (c) after partial proton transfer reactions with ion parking mode. (Reproduced from Reid, G.E. Wells, J.M. Badman, E.R. McLuckey, S.A., Int. J. Mass Spectrom. 2003, 222, 243-258. With permission from Elsevier.)... 

The occurrence of proton transfer reactions between Z)3+ ions and CHa, C2H, and NDZ, between methanium ions and NH, C2HG, CzD , and partially deuterated methanes, and between ammonium ions and ND has been demonstrated in irradiated mixtures of D2 and various reactants near 1 atm. pressure. The methanium ion-methane sequence proceeds without thermal activation between —78° and 25°C. The rate constants for the methanium ion-methane and ammonium ion-ammonia proton transfer reactions are 3.3 X 10 11 cc./molecule-sec. and 1.8 X 70 10 cc./molecule-sec., respectively, assuming equal neutralization rate constants for methanium and ammonium ions (7.6 X 10 4 cc./molecule-sec.). The methanium ion-methane and ammonium ion-ammonia sequences exhibit chain character. Ethanium ions do not undergo proton transfer with ethane. Propanium ions appear to dissociate even at total pressures near 1 atm. 

A formalism similar to that used for partially adiabatic proton transfer reactions was applied in the calculation of the transition probability. This model of the diffusion jump is similar to the model of the diffusion of light defects in solids which was first considered in Ref. 62. 

The above discussion shows the great variety of basic processes which the photoexcited molecules undergo. It is probably safe to assume that each of these processes has some finite probability in most photochemical reactions and at this point, specific predictions of any nature for real molecules might appear hopeless. Yet quite a few calculations have been reported and virtually all of them claimed at least partial success. For the purpose of our discussion they will be divided into two classes according to the nature of the questions asked. The relatively simple case of certain proton-transfer reactions will be mentioned separately afterwards. 

In summary, we may say that the NBO/NRT description of partial proton transfer in the equilibrium H-bonded complex(es) is fully consistent with the observed behavior along the entire proton-transfer coordinate, including the transition state. At the transition state the importance of partial co valency and bond shifts can hardly be doubted. Yet the isomeric H-bonded complexes may approach the TS limit quite closely (within 0.2 kcal mol-1 in the present example) or even merge to form a single barrierless reaction profile (as in FHF- or H502+). Hence, the adiabatic continuity that connects isomeric H-bond complexes to the proton-transfer transition state suggests once more the essential futility of attempting to describe such deeply chemical events in terms of classical electrostatics. 

These species, for oxygen-containing compounds, have a large stability (in excess of 20 kcal mol - ) and require a very loose association. This stability allows for partial proton transfer to the alcohol, even when the ester has a larger proton affinity than the alcohol. The scheme is also consistent with the picture that reactions will take place only at centres prone to form a carbenium ion, namely secondary or tertiary carbon centres. 

In the activated complex, the original bonds have lengthened and weakened and the new bonds are only partially formed. For example, in the proton transfer reaction between the weak acid HCN and water, the activated complex can be pictured as consisting of an HCN molecule with its hydrogen atom involved in a hydrogen bond to the oxygen atom of a water molecule and poised midway between the two molecules. At this point, the hydrogen atom could reform HCN or leave as the product,... 

We consider the same reaction model used in previous studies as a simple model for a proton transfer reaction. [31,57,79] The subsystem consists of a two-level quantum system bilinearly coupled to a quartic oscillator and the bath consists of v — 1 = 300 harmonic oscillators bilinearly coupled to the non-linear oscillator but not directly to the two-level quantum system. In the subsystem representation, the partially Wigner transformed Hamiltonian for this system is,... 

In all proton transfer reactions, the electron-rich base donates an electron pair to the acid, which usually has a polar H—A bond. Thus, the H of the acid bears a partial positive charge, making it electron deficient. This is the first example of a general pattern of reactivity. 

Although we have only discussed the partial-proton-transfer model for one of the B 12-dependent carbon-skeleton mutases [69], we can expect that there also exists a continuum between no protonation and full protonation of the substrate in the other reactions. Analogously, partial hydride removal from the substrate of glutamate mutase may serve to facilitate this rearrangement. 

Compounds that have PA significantly lower than ammonia (i.e. carboxylic acids) do not undergo proton transfer reactions, partially accounting for the diversity of sensitivities observed in thermospray HS. Derivatization to increase the PA of a sample is one way to Increase Cl sensitivity. Diethylamlnoethyl (DEAE) derivatives (reaction shown below) can change the PA of carboxylic acids, greatly enhancing the signal level obtained by thermospray HPLC/MS [15]. 

The pK found in this way may be directly compared with Forster-cycle calculations. However, straightforward utilization of the fluorescence titration method is usually limited to moderately strong photoacids due to partial deactivation processes of the photoacid occurring in very concentrated mineral acid solutions. The most accurate method of finding the pK of a photoacid is by direct kinetic measurements of the excited-state proton dissociation and recombination rates °. However, these measurements are not trivial and are limited to a relatively small number of photoacids where accurate measurement of the excited-state reversible dynamics of the proton-transfer reaction is possible. 

Scheme 2 Partial cancellation of microscopic medium effects in proton transfer reactions...

Natural substrate (guanidinium) and inhibitors (carbamoyl) differ by the presence or absence of a positive charge on this part of the molecule. Thus, addition of water (as OH ) to the carbamoyl moiety and hydrolysis of the inhibitor would result in the development of charge rather than in its dispersal (as for the natural substrate ) in the partially hydrophobic environment of the active site. This explains why carbamoyl sarcosine acts as inhibitor for creatinase. In succinamic acid, the central NCH3 is replaced by CH2 and the proton transfer reaction (step 3) cannot occur. 

When a substance dissolves in water, it will often partially or completely dissociate or ionize. Partially dissociated electrolytes are called weak electrolytes, and completely dissociated ones are strong electrolytes. For example, acetic acid only partially ionizes in water and is therefore a weak electrolyte. But hydrochloric acid is completely ionized and thus is a strong electrolyte. (Acid dissociations in water are really proton transfer reactions HOAc + H2O HsO + OAc ). Some substances completely ionize in water but have limited solubility we call these slightly soluble substances. Substances may combine in solution to form a dissociable product, for example, a complex. An example is the reaction of copper(II) with ammonia to form the CulNHs)/ species. 

The aldose-ketose isomerases constitute the best studied class of enzymes catalyzing 1,2-proton transfer reactions (Tables IV and V). Isomerization generally involves significant intramolecular hydrogen transfer with variable amounts of solvent proton exchange (16). This argues for the formation of an enediol(ate) intermediate facilitated by a single active site base partially shielded from solvent [Eq. (16)]. 

Enzyme catalyzed mechanisms represent fundamentally familiar reactions from organic chemistry (Figure 2.17). Acid-base catalysis is associated with the donation or subtraction of protons. Acid catalysis is a process in which partial proton transfer from an acid lowers the free energy of the reaction transition state, while base catalysis is a process in which partial proton subtraction by a base lowers the free energy of the reaction transition state. Concerted acid-base catalysis, where both processes occur simultaneously, is a common enzymatic mechanism. 

Recently much attention has been devoted to the detailed mechanism by which the class of enzymes called serine proteases work. These enzymes catalyze the ubiquitous and paramount cleavage of peptide bonds and all have the so-called catalytic triad (His-Asp-Ser) in common. A number of studies have suggested that a low barrier hydrogen bond (LBHB) is involved in the reaction mechanism as a partial proton transfer between His and Asp (N-H---0). In... 

That P should be less than one is reasonable, since ionization involves complete removal of the proton, whereas the activation step involves only partial removal of the proton. A molecular basis can be established in a qualitative way [21, 28] if we consider the potential-energy curves of a proton-transfer reaction as a function of the distance of the proton from the two centers X and Y in the reaction... 

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