Protonation of carboxylate groups

The maxiinmn viscosity of alginate solutions is obtained at pH 3-3.5 because of the protonation of carboxylate groups in the alginate backbone forming hydrogen bonds. 

Modification of hydrophilic proteins with hydrophobic polymers has been also explored as a way to enhance protein transport into a cell. For example, conjugation of a small water-soluble peptide neocarzinostatin, with poly(styrene-co-maleic acid) copolymer (SMANCS) resulted in a 20 to 30 fold enhancement ofthe transport ofthe peptide into a cell. This polymer conjugate also displayed facilitated uptake into cells at acidic pH, which may be due to protonation of carboxylic groups of the polymer chain rendering it even more hydrophobic. Since more acidic environment is... 

From a study of the decompositions of several rhodium(II) carboxylates, Kitchen and Bear [1111] conclude that in alkanoates (e.g. acetates) the a-carbon—H bond is weakest and that, on reaction, this proton is transferred to an oxygen atom of another carboxylate group. Reduction of the metal ion is followed by decomposition of the a-lactone to CO and an aldehyde which, in turn, can further reduce metal ions and also protonate two carboxyl groups. Thus reaction yields the metal and an acid as products. In aromatic carboxylates (e.g. benzoates), the bond between the carboxyl group and the aromatic ring is the weakest. The phenyl radical formed on rupture of this linkage is capable of proton abstraction from water so that no acid product is given and the solid product is an oxide. 

In methylenechloride/trifluoroacetic acid or fluorsulfonic acid trifluoracetoxyl-ation (Eq. 44) [353] or fluorsulfonation[354] of the alkyl chain remote from the proton-ated carboxyl group occurs. 

FIGURE 3.8 Deprotection of carboxyl groups by acid-catalyzed hydrolysis (A) of amides and (B) of esters. Protonation generates a relatively stable carbenium ion that usually requires heat to fragment it. 

Protonation of the amino group usually induces a shielding of all carbon atoms, explained in terms of electric field effects [84] and CH bond polarizations [338], and follows the pattern observed on protonation of carboxylate anions (Section 4.7.4). 

The complexones are weak acids and therefore dissociate one or more protons in aqueous solution, depending on the pH of the solution. For them the general symbol HpL is used, where p represents the number of dissociable protons which is identical with the number of carboxylic groups of the acid. It can be obtained experimentally from the titration of a solution of the neutral acid H,L with a solution of strong base and measuring the pH value. The curve obtained in the case... 

The resistance to cleavage is an indication of the superelectrophilic character of dication 150c. Several aromatic compounds have likewise been shown to produce dicationic species upon the protonation of carboxyl and carbonyl functional groups. Other bis-carboxonium dications have been described involving protonation of carbonyl, amide, and other groups.50 These distonic superelectrophiles (152-153) have been shown to be useful in condensation reactions (eqs 52-53). 

Width of proton-affinity distribution of carboxyl groups. 

---