Covalent proton transfer reaction

A proton transfer reaction involves breaking a covalent bond. For an acid, an H — X bond breaks as the acid transfers a proton to the base, and the bonding electrons are converted to a lone pair on X. Breaking the H — X bond becomes easier to accomplish as the bond energy becomes weaker and as the bonding electrons become more polarized toward X. Bond strengths and bond polarities help explain trends in acidity among neutral molecules. 

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... 

Coupled electron-proton transfer, reactions of oxo-molybdenum centers, 40 57-59 Coupled-ring nitrides molecular structure of, 15 401-6 preparation of, 15 400 properties and reactions of, 15 402-403 purification of, 15 400-401 Covalent bonding... 

Although Bronsted proton transfer reactions appear to belong to a unique category not described by Scheme 14, they are examples of polar-group transfer reactions and are not different in principle from nucleophilic displacement reactions. Deprotonation by hydroxide ion can be regarded as the shift of an electron from HO to the Bronsted acid synchronously with the transfer of a hydrogen atom from the Bronsted acid to the incipient HO- radical, with the reaction driven by covalent bond formation between the HO- radical and the H- atom to form water (equation 161). 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

Metal enolate solutions consist of molecular aggregates (6) such as dimers, trimers and tetramers in equilibrium with monomeric covalently bonded species (7), contact ion pairs (8) and solvent-separated ion pairs (9), as shown in Scheme 1. The nature of the metal cation, the solvent and, to a degree, the structure of the enolate anion itself may significantly influence the extent of association between the anion and the metal cation. In general, the factors that favor loose association, e.g. solvent-separated ion pairs, lead to an increase in the nucleophilicity of the enolate toward alkylating agents and also its ability to function as a base, i.e. to participate in proton transfer reactions. 

Additionally the corresponding tautomeric forms with double O-H N H-bonds were also considered as well as the transition states corresponding to the C N H- - -0=C C=N- - -H 3 proton transfer reaction. Finally, all N- - -H (N H) and H- - -O (O-H) interactions of N-H O and O-H N systems were collected and analyzed. The findings are as follows. For N-H O hydrogen bonds for N H BCPs, the Laplacian values are negative, while for H- - -O contacts, they are positive indicating that these interactions are covalent and noncovalent, respectively. For O-H- - -N hydrogen bonds, the situation is... 

In these reactions, the C2-atom of ThDP must be deprotonated to allo v this atom to attack the carbonyl carbon of the different substrates. In all ThDP-dependent enzymes this nucleophilic attack of the deprotonated C2-atom of the coenzyme on the substrates results in the formation of a covalent adduct at the C2-atom of the thiazolium ring of the cofactor (Ila and Ilb in Scheme 16.1). This reaction requires protonation of the carbonyl oxygen of the substrate and sterical orientation of the substituents. In the next step during catalysis either CO2, as in the case of decarboxylating enzymes, or an aldo sugar, as in the case of transketo-lase, is eliminated, accompanied by the formation of an a-carbanion/enamine intermediate (Ilia and Illb in Scheme 16.1). Dependent on the enzyme this intermediate reacts either by elimination of an aldehyde, such as in pyruvate decarboxylase, or with a second substrate, such as in transketolase and acetohydroxyacid synthase. In these reaction steps proton transfer reactions are involved. Furthermore, the a-carbanion/enamine intermediate (Ilia in Scheme 16.1) can be oxidized in enzymes containing a second cofactor, such as in the a-ketoacid dehydrogenases and pyruvate oxidases. In principal, this oxidation reaction corresponds to a hydride transfer reaction. 

Pang, E.S. Loo, R.O. Yin, S. Boontheung, P Teplow, D.B. Loo, l.A. Ion mobibty mass spectrometry and proton transfer reactions of non-covalent amyloid P-protein ob-gomers. Proc. 56th ASMS Corference on Mass Spectrometry and Allied Topics, Denver, CO, lune 1-5, 2008, TP 231. 

Like ammonia, all amines are weak bases, and aqueous solutions of amines are basic. The following acid-base reaction between an amine and water is written using curved arrows to emphasize that, in this proton-transfer reaction, the unshared pair of electrons on nitrogen forms a new covalent bond with hydrogen and displaces hydroxide ion ... 

This process has been eanied further for electron-transfer than for proton-transfer reactions. This has been possible because in electron-transfer reactions covalent bonds are neither broken nor made, so the calculation of AG is easier and because the charges are more localised, which facilitates calculations of ACqs-... 

The case when hcj kT is very frequent, and some of the bonds exhibit also a purely quantum-mechanical behavior. The latter is especially typical of covalent bonds of hydrogen atoms which usually have frequencies of the order of 15 kT/h. Thus, a proton transfer reaction is virtually always quantum-mechanical. 

In both cases the nitrogen atom uses its pair of nonbonding electrons to make a new covalent bond. This similarity led G. N. Lewis to classify ammonia as a base in its reaction with B (CH3)3 as well as in its reaction with H3 O . Whereas the Br< )nsted definition focuses on proton transfer, the Lewis definition of acids and bases focuses on electron pairs. 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

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