Amphiprotic groups

In a very thoughtful investigation of solvent systems to model membrane characteristics, Leahy et al. (1989, 1992) have argued that two receptors sited in different tissues (or membranes) could exist in environments that are very different in hydrogen bonding character one may be surrounded by amphiprotic groups (as in a protein) or by proton donors the other may be surrounded by proton acceptors (as in a phospholipid membrane). 

When amphiprotic groups as NH2 and OH are placed in 6a position, the possibility of a third hydrogen bond appears, and as a result, the tendency is reversed, the homochiral complexes being more stable than their heterochiral counterparts, due to the third hydrogen bond formed in the homodimer. In the amino substituted compound, the discrimination energy favors the homochiral complex in 11.3 kJ mol-1. 

In cases where both types of compounds are present in significant amount, the use of a mixture of solvents, nucleophilic and electrophilic, was suggested. In such cases, an amphiprotic solvent could also be suitable (an amphiprotic solvent contains simultaneously an electron donating and electron accepting groups in a single molecule), for instance ethanolamine. 

In the section dealing with alumina and silica, the necessity of basic sites on the surface, which cooperate with acidic sites, has been stressed. Also, for both amorphous silica—alumina and zeolites, the simultaneous presence of acidic and basic sites has been proved and it has been suggested that OH groups act as amphiprotic centres according to the nature of the adsorbed species [49],... 

The name oxime may be considered to be derived from oxy-imine, >C=NOH. The oxime group is amphiprotic with a slightly basic nitrogen and a mildly acidic hydroxyl group. In his doctoral research under Hantzsch, Werner1 explained the observed isomerism of oximes on the basis of differences in spatial arrangement of the groups attached to the C=N moiety. 

Ligand-protein contact (LPC) data describe putative surface interactions between the ligand and the site residues and predict whether they are energetically favourable or unfavourable [18,19], Putative hydrogen bonds are labelled as backbone, functional group donors or acceptors or amphiprotic. Putative hydrogen bonds with bond lengths less than or equal to 3.5A are accepted and considered in this analysis. 

The simple amino acids are an important class of amphiprotic compounds that contain both a weak acid and a weak base functional group. When dissolved in water, an amino acid such as glycine undergoes a kind of internal acid/base reaction to produce a zwitterion—a species that bears both a positive and a negative charge. Thus,... 

Separation of Amphiprotic Species In isoelectric. separation of amphiprotic species, the separation is performed in a buffer mixture that continuously varic.s in pH along its length. This pH gradient is prepared from a mixture of several different ampholytes in an aqueous solution. Ampholytes arc amphoteric compounds usually containing carboxylic and amino groups. Ampholyte mixtures having different pH ranges can be prepared or arc available from several commercial sources. 

The earliest advantages recognized arose from the use of amphiprotic solvents, those that have both acidic and basic properties. The prototype is water. Significant differences in acid-base properties are seen in the case of either protogenic solvents (more acidic than water), for example acetic acid, or protophilic solvents (more basic than water), for example ethylenediamine. In the protogenic cases it was found that bases too weak to be titrated in water could be successfully titrated with a strong acid dissolved in the same solvent. For example, primary, secondary, and tertiary amines can be titrated in acetic acid with perchloric acid in acetic add as titrant. Medicinal sulfonamides, which have a primary amino group, can be titrated... 

Amino acids contain a carboxyl group and can therefore serve as acids. They also contain an NH2 group, characteristic of amines (Section 16.7), and thus they can also act as bases. Amino acids, therefore, are amphiprotic. For glycine, we might expect the acid and base reactions with water to be... 

The doubly ionized form, called a zwitterion, usually predominates at near-neutral pH values. This form is a result of the transfer of a proton from the carbox)1ic acid group to the amine group. (Section 16.10 Chemistry and Life The Amphiprotic Behavior of Amino Acids )... 

The important aspect of structure 58b is that the solvent hydroxyl group, whose proton serves as donor to the solute, acts on its oxygen as acceptor in a hydrogen bond with another R-OH molecule. Huyskens (170) has recently pointed out that, when an amphiprotic molecule acts simultaneously as hydrogen... 

An amphoteric species is a molecule or ion that can react as an acid as well as a base. One type of amphoteric species are amphiprotic molecules, which can either donate or accept a proton examples are amino acids and proteins, which have amine and carboxylic groups. The competition between the acid-base equilibria of these groups leads to additional complications in their physical behavior but can also present some advantages due to the variation of their charge as a function of pH. 

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