Bronsted acid-base catalysis

Bronsted acid/base catalysis (equation, a, p) 113, 210, 355ff.,360ff., 392 Buckminsterfullerene, reaction with ArN 188... 

Once the substrate is bound, other residues at the active site carry out the catalytic reaction. The elementary steps involved are similar to those we covered in Chapters 3 and 4. Broadly speaking, enzyme catalysis is divided into two common mechanisms Bronsted acid/base catalysis and nucleophilic catalysis [26]. 

Bronsted acid/base catalysis is the most common enzymatic mechanism, since nearly all enzymatic reactions involve a proton transfer. This means that nearly all enzymes have acidic and/or basic groups in their active site. In add catalysis, the substrate is protonated by one of the amino add residues at the active site (typically aspartic acid, glutamic acid, histidine, cysteine, lysine, or tyrosine). This residue itself must therefore be protonated at the readion pH (typically between pH 5 and 9), with a pKa just above this value. Conversely, in base catalysis, the pJCa of the deprotonating residue must be just below the physiological pH. Some enzymes can even carry out bifunctional catalysis, by protonating and deprotonating two different sites on the same substrate molecule simultaneously. 

Proposed intermediate Bronsted acid/base catalysis... 

Gattermann-Koch reaction 230 Gattermann reaction 230 General acid/base catalysis, see Bronsted catalysis... 

General acid/base catalysis is less significant in natural fresh waters, although probably of some importance in special situations. This phenomenon can be described fairly well via the Bronsted law (relating rate constants to pKa and/or pKb of general acids and bases). Maximum rates of general acid/base catalysis can be deduced from a compound s specific acid/base hydrolysis behavior, and actual rates can be determined from relatively simple laboratory experiments (34). 

If we use the Bronsted concept of an acid as a proton donor and a base as a proton acceptor, consideration of acid-base catalysis may be extended to solvents other than water (e g., NH CH3COOH, and SO,). An acid, on donating its proton, becomes its conjugate base, and a base, on accepting a proton, becomes its conjugate acid ... 

CHIRAL BRONSTED BASE-BRONSTED ACID BIFUNCTIONAL CATALYSIS... 

When a reaction is susceptible to catalysis by Bronsted acids and bases, the phenomenon is characterized as generalized acid or basic catalysis. One of the earliest reactions discovered to be subject to such generalized acid-base catalysis was the mutarotation of optically active glucose ... 

The view of generalized acid-base catalysis as a prototropic shift assisted by acids and bases raises, quite naturally, the question of the relationship between the catalytic power of the acid or base and its own ionization constant. It had early been recognized that there is a correlation between the two constants. Taylor proposed the first quantitative relation, that the acid-catalytic constant of an acid knA was proportional to XjaA, the square root of its ionization constant. For generalized acid-base catalysis, the Bronsted equation, proposed later,"" has gained wide empirical use ... 

Fia. XVI. 1. Bronsted plot for the base-catalyzed decomposition of nitramide. (Data from compilation by R. P. Bell, Acid-Base Catalysis, Oxford University Press, New York, 1941.) + Doubly charged bases, B++, mostly aquo-iona, [M(H20) (0I1)]++. o Doubly charged bases, B". o- Uncharged bases, B° all substituted anilines. 

In acid-base catalysis,proton addition to or abstraction from reactant molecules (with Bronsted acids or bases), or formation of coordination bonds (with Lewis acids), and subsequent bond breaking and rearrangement are the key reaction processes. Most cases involve ionic reaction intermediates bound to the surface by electrostatic interactions. 

There is extensive evidence from site-directed mutagenesis and other studies of enzymes that catalyze proton transfer that acidic and basic amino side chains and, in some cases, metal cations, are required for the observation of efficient catalysis. However, catalysis of the deprotonation of a-carbonyl by small molecule analogs of these side chains, and by metal cations is generally weak. Relatively little attention has been directed towards understanding the mechanism for the enhancement of Bronsted acid/base and electrophilic catalysis for enzyme-catalyzed reactions... 

In addition to possible general acid-base catalysis where a buffer can act as either a proton donor or acceptor (Bronsted acid or base), buffer species can also act as a Lewis acid or base through nucleophilic or electrophilic mechanisms. 

If it can be assumed that the Bronsted relation remains approximately valid for the ions and molecules of the solvent, it is possible to make some interesting deductions about the possibility of detecting general acid-base catalysis. The point is best illustrated by taking a particular example, e.g., acid catalysis in an aqueous solution 0.1 A with respect to both acetic acid and acetate ions. We assume that the catalytic effect of acids (without reference to charge or structure) is given approximately by... 

Bronsted studied chemistry at the University of Copenhagen. He received a master of science degree in 1902 and a doctorate degree in 1908. After receiving temporary appointments at the University of Copenhagen, he became an assistant professor there in 1905, and full professor in 1908. Bronsted studied chemical affinity, electrolytes, isotope separation, reaction kinetics, thermodynamics, and acid-base catalysis. 

The paper of Bronsted and Guggenheim (loc. cit.) gives results for several other acids and bases. Kilde and Wynne-Jones (Trans. Faraday Soc. 1953,49,243) describe experimental work to measure the catalytic constants for ammonia, the hydroxyl ion, and the glucosate ion. BeU ( Acid-base catalysis . Clarendon Press, 1941, p. 82) discusses the interrelation between strength and catal3dic effect of adds and bases in this and similar reactions, and reference may be made to papers by Swain (J. Amer. Chem. Soc. 1950, 72, 4578) and Bell and Jones (J. Chem. Soc. 1953, 88) for interesting discussions of the detailed role of the catalysts. 

The solid/gas interface was traditionally studied with respect to adsorption and catalysis. Here the assertion that the Bronsted definition of acidity is a particular case of the Lewis definition is neither obvious nor even helpful. It suffices to say that many reactions in heterogeneous catalysis require specifically the presence of either Bronsted or Lewis acidic (or basic) sites, and the reaction mechanisms depend on the nature of the surface site. A long-term goal of surface studies for the characterization of solid catalysts was to distinguish and quantify the number of Bronsted or Lewis sites with potential catalytic activity for gas-phase reactants. For that reason, when discussing the acid-base behavior of solid surfaces, it is no longer possible, nor desirable, to adopt the viewpoint that subsumes Bronsted acid-base properties in the more general Lewis definition. 

Lewis Base/Bronsted Acid Bifunctional Catalysis. As we have seen with some detail in Section 2.2.1.2 above, proline (and a-amino acids in... 

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