The Carboxyl Group

Sialidases require a negatively charged carboxyl function on the a-linked sialic acid-containing substrate for catalytic activity. V. cholerae sialidase did not cleave the sialic acid methyl esters of bovine submandibular gland mucus glycoproteins (Gottschalk 1962) or IPNeu5AcLac (Kuhn / al 1966). In colominic acid, esters 

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Heating the acids or their salts with soda lime eliminates the carboxyl group, volatile products being often detectable. 

Physical Properties. Glycine is a colourless crystalline solid soluble in water. Owing to the almost equal opposing effects of the amino and the carboxylic groups. its aqueous solution is almost neutral (actually, slightly acidic to phenolphthalein) and glycine is therefore known as a neutral ampholyte. f It exhibits both acidic and basic properties. 

Sigel and co-workers" investigated the interaction between the aromatic rings of phenyl carboxylates (Ph-(CFl2)n-C02) and 1,10-phenanthroline in ternary copper(II) complexes. Variation of the number of methylene units between the aromatic ring and the carboxylate group (n=0-5) revealed that the arene - arene interaction is most pronounced for n=l. This interaction is more efficient in a 60% 1,4-... 

The reduction of the carboxylic group is easier in the 2-position than in the 4- or 5-positions. These differences in reductibility run parallel to chemical reactivity (58). 

Systematic names for carboxylic acids are derived by counting the number of car bons m the longest continuous chain that includes the carboxyl group and replacing the e ending of the corresponding alkane by oic acid The first three acids m Table 19 1 methanoic (1 carbon) ethanoic (2 carbons) and octadecanoic acid (18 carbons) illus trate this point When substituents are present their locations are identified by number... 

The structural features of the carboxyl group are most apparent m formic acid Formic acid IS planar with one of its carbon-oxygen bonds shorter than the other and with bond angles at carbon close to 120°... 

Table 19 3 lists the ionization constants of some substituted benzoic acids The largest effects are observed when strongly electron withdrawing substituents are ortho to the carboxyl group An o nitro substituent for example increases the acidity of benzoic acid 100 fold Substituent effects are small at positions meta and para to the carboxyl group In those cases the values are clustered m the range 3 5-4 5... 

Notice that the carboxyl group that stays behind during the decarboxylation of mal onic acid has a hydroxyl function that is not directly involved m the process Compounds that have substituents other than hydroxyl groups at this position undergo an analogous decarboxylation... 

Glycine is the simplest ammo acid and the only one m Table 27 1 that is achiral The a carbon atom is a chirality center m all the others Configurations m ammo acids are normally specified by the d l notational system All the chiral ammo acids obtained from proteins have the l configuration at their a carbon atom meaning that the amine group IS at the left when a Fischer projection is arranged so the carboxyl group is at the top... 

Ester formation is a typical reaction of the carboxyl group... 

To direct the synthesis so that only Phe Gly is formed the ammo group of phe nylalanme and the carboxyl group of glycine must be protected so that they cannot react under the conditions of peptide bond formation We can represent the peptide bond for matron step by the following equation where X and Y are amine and carboxyl protecting groups respectively... 

Protect the ammo group of the N terminal ammo acid and the carboxyl group of the C terminal ammo acid... 

Deprotect the ammo group at the N terminus and the carboxyl group at the C terminus... 

Section 27 5 Ammo acids undergo reactions characteristic of the ammo group (e g amide formation) and the carboxyl group (e g esterification) Ammo acid side chains undergo reactions characteristic of the functional groups they contain... 

Section 27 18 In the Memfield method the carboxyl group of an ammo acid is anchored to a solid support and the chain extended one ammo acid at a time When all the ammo acid residues have been added the polypeptide is removed from the solid support... 

L Ammo acid (Section 27 2) A descnption of the stereochem istry at the a carbon atom of a chiral ammo acid The Fis cher projection of an a amino acid has the ammo group on the left when the carbon chain is vertical with the carboxyl group at the top... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Dipeptide (Section 27 7) A compound in which two a amino acids are linked by an amide bond between the amino group of one and the carboxyl group of the other... 

Trypsin (Section 27 10) A digestive enzyme that catalyzes the hydrolysis of proteins Trypsin selectively catalyzes the cleavage of the peptide bond between the carboxyl group of lysine or arginine and some other amino acid... 

Zaitsevs rule (Section 5 10) When two or more alkenes are capable of being formed by an elimination reaction the one with the more highly substituted double bond (the more sta ble alkene) is the major product Zwitterion (Section 27 3) The form in which neutral amino acids actually exist The ammo group is in its protonated form and the carboxyl group is present as a carboxylate... 

The same is true for polybasic acids, with the proviso that all the carboxyl groups must be changed to aldehyde then it is not necessary to introduce affixes. Examples are... 

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