Proton acceptor, base

Dzendolet pointed out that the property common to all sweet compounds is that of being a proton acceptor. Based on electrophysiological... 

The water equilibrium illustrates the amphiprotic nature of H2 O. In this reaction, one water molecule acts as a proton donor (acid), and another acts as a proton acceptor (base). 

Dissolve glutaric anhydride (Chapter 1, Section 4.2) in DMF at near saturation (use a fume hood). Add triethylamine to a concentration of at least 1 mg/ml to function as a proton acceptor (base). 

In 1923, Johannes Br nsted and his English counterpart Thomas Lowry independently developed a more general acid-base theory. According to the Br nsted-Lowry model, an acid is a proton (H+) donor and a base is a proton acceptor. Each proton donor (acid) has its pair (conjugate) proton acceptor (base). In an acid-base reaction, H (proton) is transferred from an acid to a base. For example ... 

In the case of water, however, the concentration of either the OH- or H+ ions can be decreased by adding proton donors (acids) or proton acceptors (bases). Thus, a proton donor releases hydrogen ions into the solution (i.e., increases), and the only way the product Kw (i.e., COH-CH+) remains a constant is by a decrease in ( OH ... 

Proton acceptors (bases like amines) H+/pH-glass electrode... 

The four electrostatic interactions which involve the dipole moments and the polarizabilities of the solute and solvent molecules are termed non-specific because they do not imply a fixed geometry or stoichiometry of the molecules. In a specific association the molecules form a loose complex, of which hydrogen bonding is the most important example. Hydrogen bonding takes place between a protic (proton donor or acid) molecule such as water and a proton acceptor (base) such as an amine (Figure 3.48). 

Catalysis of the enolization of 2-propanone by acids involves first, oxonium-salt formation and second, removal of an a proton with water or other proton acceptor (base) ... 

In the reverse reaction, H30+ acts as the proton donor (acid) and A- acts as the proton acceptor (base). Typical examples of Bronsted-Lowry acids include not only electrically neutral molecules, such as HC1, HN03, and HF, but also cations and anions of salts that contain transferable protons, such as NH4+, HS04 , and HC03-. 

To predict the direction of reaction, use the balanced equation to identify the proton donors (acids) and proton acceptors (bases), and then use Table 15.1 to identify the stronger acid and the stronger base. When equal concentrations of reactants and products are present, proton transfer always occurs from the stronger acid to the stronger base. 

Both in the case of sensory rhodopsin in humans and of bacteriorhodopsin (a heptahelical membrane protein in halobacteria which is not coupled to a G protein) translocation of a Schiff-base proton is the essential step in making the protein functional (reviewed in ref 58). In rhodopsin the conversion of the inactive AH state to the AHI state that binds to the G protein is coupled to proton transfer from the Schiff base to the counterion, Glul 13, and proton uptake from the medium to the highly conserved Glul34, which serves as proton acceptor. Based on that similarity, one could consider sensory rhodopsin as an incomplete proton pump. Furthermore, a property shared by all G-protein-coupled receptors is a triplet, formed by residues 134-136 in rhodopsin, consisting of Glu-Arg-Tyr. The consequences of mutational replacement of Glul34 supports the notion that the state of protonation of this amino add is crudal for activity, and that its protonation triggers the conformational transition of the receptor from the inactive to the active state. 

A cationic C- and S-coordinated mercaptocarbene complex undergoes deprotonation when treated with proton-acceptor bases or basic nucleophiles to give a neutral mercaptocarbyne complex ... 

A base does not have to contain the Many types of proton acceptors (bases) do not contain the hydroxide ion. 

The number of moles of hydrogen ion equivalent to the excess of proton acceptors (bases formed from weak acids with a dissociation constant K < 10 at 25 °C and zero ionic strength) over the proton donors (acids with K > 10 ) in one kilogram of sample. 

Br0nsted acidity comes into play in condensed phases, where proton dissociation is enhanced by the solvent or by other solute molecules which act as proton acceptors (bases) and stabilize the charge of the bare proton. The acid dissociation of phenol in solution may be written as in equation 10,... 

A) carbodiimides (B) aminium salts (C) phosphonium salts (D) proton acceptor-based aminium salts. 

Amides are not proton acceptors (bases) because the highly electronegative carbonyl oxygen has a strong attraction for the nitrogen lone pair of electrons. As a result they cannot "hold" a proton. 

The acid and base of a half-reaction are called conjugate pairs. Free protons do not exist in solution, and there must be a proton acceptor (base) before a proton donor (acid) will release its proton. That is, there must be a combination of two half-reactions. Some acid-base reactions in different solvents are illustrated in Table 7.1. In the first example, acetate ion is the conjugate base of acetic acid and ammonium ion is the conjugate acid of ammonia. The first four examples represent ionization of an acid or a base in a solvent, while the others represent a neutralization reaction between an acid and a base in the solvent. 

Many types of proton acceptors (bases) do not contain the hydroxide ion. However, when dissolved in water, these substances increase the concentration of hydroxide ion because of their reaction with water. For example, ammonia reacts with water as follows ... 

Ka for HP04 is 4.8 x 10 Note that HP04 is the conjngate base of H2PO4, so Kb is 1.6 x 10 Comparing the two A s, we conclnde that the monohydrogen phosphate ion is a much stronger proton acceptor (base) than a proton donor (acid). The solntion will be basic. 

The most widely used Friedel-Crafts catalyst systems are HCkAlCT and HBr AlBr3. These systems are indeed superacids by Gillespie s deflnition. However, experiments directed toward preparation from aluminium halides and hydrogen halides of the composition HAIX4 were unsuccessful in providing evidence that such conjugate acids are formed in the absence of proton acceptor bases. 

The regulation of the acid-base balance involves the transfer of ions to yield either new base or new acid. When the pH of the blood drops below normal values, the total concentration of free proton donors (acids) increases whereas that of free proton acceptors (bases) decreases. In the body fluids Na, , Ca, ... 

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