Zwitterion form, of amino acids

In polar solvents, the structure of the acridine 13 involves some zwitterionic character 13 a [Eq. (7)] and the interior of the cleft becomes an intensely polar microenvironment. On the periphery of the molecule a heavy lipophilic coating is provided by the hydrocarbon skeleton and methyl groups. A third domain, the large, flat aromatic surface is exposed by the acridine spacer unit. This unusual combination of ionic, hydrophobic and stacking opportunities endows these molecules with the ability to interact with the zwitterionic forms of amino acids which exist at neutral pH 24). For example, the acridine diacids can extract zwitterionic phenylalanine from water into chloroform, andNMR evidence indicates the formation of 2 1 complexes 39 such as were previously described for other P-phenyl-ethylammonium salts. Similar behavior is seen with tryptophan 40 and tyrosine methyl ether 41. The structures lacking well-placed aromatics such as leucine or methionine are not extracted to measureable degrees under these conditions. 

FIGURE 3-9 Nonionic and zwitterionic forms of amino acids. The... 

Becauce of their biological relevance, most of the work published in this field refers to the recognition, and the elosely related problem of transport across liquid membranes. of the zwitterionic form of amino acids, which is the most abundant form in solution at pH values around neutrality (see Eq. 1 below). 

UV irradiated for 20 min. followed by being visible light irradiated for 5 min, a significant transfer of phenylalanine from the liposome to the outer solvent took place. No transport of amino acid was observed in the dark or in the absence of 1. The system suffers from several drawbacks, including the question of the integrity of the membrane under the conditions of the experiment and the photochemical instability of the photospiran 1. However, in spite of such limitations. Sunamoto s work established for the first time the simple concept of charge complementarity for recognition and transport of the zwitterionic form of amino acids, as schematically depicted in Eq. 2 (see below). 

As shown by the results of this combined investigation in an aqueous solution the zwitterionic form of amino acids solvated by water molecules is more stable than the neutral form [22-27]. The activation energy required for the neutral form of glycine to transfer into the zwitterionic in water amounts to 16.85 kcal/mol [22]. 

A way of examining the zwitterionic behavior of amino acids is to study their titration. Suppose, for example, that we begin with a solution of glycine hydrochloride, in which both groups are in their acidic forms. Addition of sodium hydroxide brings about an increase in the pH of the solution, and at the same time, dissociated protons react with the added hydroxyl ions to form water thus allowing further dissociation to take place, as shown in Fig. 3-2. 

An important subset of biologically relevant molecules is that of natural amino acids, the so-called building blocks of peptides and proteins. Amino acids have long been studied in solids [78-82] and in solution [83-85], where they are stabilized as doubly charged species or zwitterions (R-CH(NH3 )-COO-). Gas-phase studies have the advantage of providing information on the neutral forms of amino acids (R-CH(NH2)-COOH, the canonical forms present in peptide chains) and on their inherent molecular properties free from the intermolecular interactions occurring in the condensed media. Furthermore, gas-phase data can be easily contrasted with theoretical models and used to refine the latter. 

Isoelectric point (Section 27.3) pH at which the concentration of the zwitterionic form of an amino acid is a maximum. At a pH below the isoelectric point the dominant species is a cation. At higher pH, an anion predominates. At the isoelectric point the amino acid has no net charge. 

Zero order reaction A reaction whose rate is independent of reactant concentration, 289,295-298, 317q Zinc, 86-87,550 Zn-Cn2+ voltaic cell, 481-485 Zwitterion Form of an amino acid in which there is a separation of charge between the nitrogen atom of the NH2 group (+) and one of the oxygen atoms of the COOH group (—), 623-624... 

Some amphoteric softeners such as amino acids (10.237) and sulphobetaines (10.238) are more effective and durable than the nonionic types but less durable than the cationics moreover, they tend to be expensive. Other amphoteric types include the zwitterionic forms of quaternised imidazolines (10.239) long-chain amine oxides (10.240) also exhibit softening properties. 

When derivatives of amino acids are formed, their zwitterionic character is destroyed, a process which requires the supply of energy. We shall first discuss chemical activation, as it is important for the understanding of hypotheses dealing with prebiotic protein formation. In the case of amino acids which are activated at the carbonyl group, the amino group remains unsubstituted. The derivatives are able to react with nucleophilic residues (Y) ... 

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

Figure 10.6 Dipolar or zwitterionic form of an amino acid. Amino acids exist in a charged form in aqueous solution, the carboxyl group being dissociated and the amino group associated. Some amino acids also have an extra ionizable group present in their side chain (R group). The ionization of each group is pH-dependent and for each amino acid there is a pH at which the charges are equal and opposite and the molecule bears no net charge. This is called the iso-ionic pH (p/).

Example ion-pair liquid chromatography of amino acids. Amino acids are zwitterions. The amino group can form an ion-pair with an alkanesulfonate ion (such as octanesulfonate), and the carboxyl group can form an ion-pair with a tetrabutylammonium ion, depending on the pH of the solution. 

The general structure of the zwitterionic form of an L-amino acid. R represents the side chain. COO H3N - C - H R... 

Reaction 24 does not occur with the protonated amine (i.e. with the zwitterionic form of the dihydroxyethylated amino acid) in the absence of pyridine. It seems most likely that the catalytic role of pyridine involves release of the lone-pair electrons of the nitrogen atom, that facilitates the formation of the transannular Si- —N bond and, consequently, the silatrane ring110. 

Using the native cyclodextrin, the enantiomers of amino acid derivatives were enantioselectively complexed [21]. Further, for a more detailed analysis, zwitterionic tryptophan was employed [22]. For the complexation studies performed on this molecule the a-cyclodextrin was used, as its inner cavity is the smallest. The H NMR measurements showed that (R)-tryptophan formed a stronger complex with a-cyclodextrin compared with the (S) enantiomer. This is due to the number of hydrogen bonds which can be formed between each enantiomer and the host molecule. The NMR studies showed another very interesting fact the amino acid is very likely forming no intracavity complex. It has been suggested that it is coordinated near the rim of the cyclodextrin. 

The Isoelectric Point The isoelectric point (pi) or isoelectric pH is the pH where the net charge on the amino acid is zero. At pi, the zwitterion is the dominant species, with an alpha value very close to 100%. The tiny equilibrium amounts of the cationic form and the anionic form are equal, so their opposite charges cancel each other out. The net charge on a collection of amino acid molecules at a pH less than pi is positive. The net charge on a collection of amino acid molecules at a pH greater than pi is negative. 

The substitution of an a-hydrogen on a carboxylic acid by an amino-group decreases the reactivity. The rate constants of glycine and alanine are 8 x 106 and 6 x 10 M-1 sec-1, respectively (Davis et al., 1965a). This low reactivity of amino acids is expected because they are predominantly in the form of a zwitterion. Other amino acids exhibit higher reactivity, owing to other reactive functional groups (Braams, 1965,... 

When simulating the interaction of amino acids with a silica surface in a biological medium, the zwitterionic form as well as hydration should also be taken into consideration.12 Shielding of functional groups (amino, carboxyl, and thiol) of sulfur-containing amino acid molecules by water molecules results in... 

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