Aqueous solution, protonation carboxylate groups

The dipolar ion has both an acidic group, the ammonium cation, and a basic group, the carboxylate anion, so it can act as either an acid or a base. Such compounds are termed amphoteric. The species that is present in aqueous solution depends on the pH. In the pH range near neutral the amino acid is present in the form of the dipolar ion. In acidic solution the carboxylate group becomes protonated and the amino acid is present as a cation, whereas in basic solution the ammonium group gives up a proton and the molecule exists as an anion ... 

The monomeric building blocks of the biopolymers, called proteins, are the a-amino acids. The simplest amino acid is glycine, which has the molecular structure shown in Figure 23.18. An amino acid, as indicated by the name, must contain an amine group (—NH2) and a carboxylic acid group ( —COOH). In a-amino acids, the two groups are bonded to the same carbon atom. In acidic aqueous solution, the amine group is protonated to form —NHj in basic solution, the... 

We saw in Sections 20.3 and 24.5 that a carboxyl group is deprotonated and exists as the carboxylate anion at a physiological pH of 7.3, while an amino group is protonated and exists as the ammonium cation. Thus, amino acids exist in aqueous solution primarily in the form of a dipolar ion, or zwitterion (German zwitter, meaning "hybrid"). 

Structure B cannot exist in aqueous solution because at any pH low enough to protonate the carboxyl group the amino group would also be protonated. Similarly, at any pH sufficiendy high for an uncharged amino... 

The most common linker molecules are pyrene derivatives, which have been used, for example, to link Au [64,65] and Pd [66] NPs, as well as QDs [67] to CNTs. Martin et al. chemically attached a n-extended tetrathiafulvalene (exTFF) group to pyrene prior to n-n hybridization on SWCNTs in order to investigate donor-acceptor interactions between the SWCNT and electron rich exTFF [68], More recently, the same group has synthesized a complex molecule consisting of two exTFF anchors connected via a flexible linkage that also consists of a second generation carboxylic terminated den-dron [69]. Here, n interactions between the exTFF anchors and the SWCNTs lead to hybridization while the carboxylic acid groups of the dendron (when de-protonated) lead to increased solubility in aqueous solutions [69]. 

An even more extreme case is the reaction of hydroxide with acetonitrile. In the ICR spectrometer, the bare hydroxide ion yields a simple proton transfer product, by way of reaction (9). In contrast, in aqueous solution the bulk-solvated hydroxide ion reacts to hydrolyze the nitrile group to give the carboxylate ion plus... 

The main fluorescent pH indicator probes are based on fluorescein and therefore it is important to understand the pH-dependent ionic equilibria of it and its derivatives, hi aqueous solutions above pH 9 the phenolic and carboxylic acid functional groups in the molecule are almost totally ionised (Figure 3.14). Upon acidification of the dianion, firstly, protonation of the phenolic group occurs (pK 6.4) to yield the monoanion followed by the carboxylic acid (pA < 5), giving the neutral species of fluorescein. On further acidification the fluorescein cation pK 2.1) is generated. In strongly acidic environments fluorescein is non-fluorescent, only the mono-anion and di-anions are fluorescent, with quantum yields of 0.37 and 0.93, respectively. The pH-dependent absorption spectrum of fluorescein exhibits a blue-shift and... 

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... 

Using the data of Wilson and Cannan (18), Cleaves (81) was able to show that the rate of formation of pyrrolidone carboxylic acid from glutamic acid in aqueous solution depends directly on the concentration of the ionic species of glutamic acid in solution. Thus, the reactive species are (I), (II), and (IV), while (III) is relatively unreactive. Protonation of the amino group and dissociation of the y-carboxyl group thus makes these groups less reactive carboxylate ion resonance apparently hinders nucleophilic attack by the amino nitrogen. 

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