Amino acids zwitterionic nature

Because of their zwitterionic nature, amino acids are generally soluble in water. Their solubility in organic solvents rises as the fat-soluble portion of the molecule increases. The likeliest impurities are traces of salts, heavy metal ions, proteins and other amino acids. Purification of these is usually easy, by recrystallisation from water or ethanol/water mixtures. The amino acid is dissolved in the boiling solvent, decolorised if necessary by boiling with Ig of acid-washed charcoal/lOOg amino acid, then filtered hot, chilled, and set aside for several hours to crystallise. The crystals are filtered off, washed with ethanol, then ether, and dried. 

Given that hydrolysis is a reversible reaction, the principle of microscopic reversibility implies that biocatalytic aminoacylation should also be applicable as a mild and efficient alternative method of introducing the side chain of both penicillin- and cephalosporin-based antibiotics. This is the case, with PGAs proving to be particularly effective biocatalysts towards the aminoacylation of both penicillin and cephalosporin nuclei with a variety of carboxyhc acids. Amoxicillin and cephalexin, two of the most important (3-lactam antibiotics, contain an (/ )-phenylglycine side chain which cannot be directly introduced as the amino acid due to its zwitterionic nature at the moderate pH values at... 

Amino acids are represented in two ways either as H2N-CHR-COOH or as the zwitterionic form H3N-CHR-COO . Although the second of these forms is overwhelmingly predominant in the crystal and in solution, it is generally more convenient to name them and their derivatives from the first form. They are normal organic compounds and are treated as such as far as numbering and naming are concerned, although trivial names are retained for all natural amino acids. 

Recognition of amino acids has been attempted more frequently with receptors that are likewise zwitterionic in nature (compare 35). Some early studies include the transport of phenylalanine by a merocyanine dye through a liposomal bilayer [53]. Rebek s dicarboxylate-complexing cleft 9 (see Sect. 2) turned out to be a selective (though achiral) binder of trypthophane, phenylalanine, and tyrosine methyl ether [54]. A reasonable structure for a phenylalanine complex of 2 1 stoichiometry, as deduced from NMR studies, is schematically represented in formula 39 [55]. 

This chapter deals with the very important a-amino acids, the building blocks of the proteins that are necessary for the function and structure of living ceils. Enzymes, the highly specific biochemical catalysts are proteins. or-Amino acids are dipolar ions (zwitterions), RCH(N" H,)COO , as is indicated by their crystallinity, high melting point, and solubility in water rather than in nonpolar solvents. The standard (naturally occurring) amino acids are listed in Table 21-1 those marked with an asterisk are essential amino acids that cannot be synthesized in the body and so must be in the diet. They have 1° NHj s except for proline and hydroxyproline (2°). They have different R groups. 

Re-examination of the radiolysis of aqueous solutions of alanine (absence of oxygen) shows that electrons react rapidly with the cationic form, less rapidly with the zwitterion, and much less rapidly with the anionic form. These conclusions have been confirmed by pulse radiolysis. Rate constants for amino acids, peptides, proteins, and numerous other substances have been obtained. Critical evaluation of these and correlation with molecular properties is now well under way. In living systems the reactions of the hydrated electron vary with the part of the cell concerned, with the developmental stage of the cell, and possibly with the nature of any experimentally added substances. 

The sapphyrin unit connected to natural ionophore lasalocid [78] has been used for the design of ligand 48. Membrane transport was measured for this ligand using a traditional U-tube arrangement. It was found that the ligand is able to transport amino acids in their zwitterionic form with a preference for the L-enantiomer of Phe. 

Amino acids, the building blocks of proteins, can exist in two principal tautomeric forms. In aqueous solution, the naturally occurring a-amino acids are found in their zwitterionic form 42Z, but in the gas phase, the neutral form 42N is the only structure observed. This substantial solvent effect is understood in terms of the dipole moments of the tautomers. The dipole moment of the zwitterion is very large and is favorably stabilized by electrostatic interactions with the polar water molecules. In the absence of the polar environment (the gas phase), the charge separation of the zwitterion is unsustainable and only the neutral form exists. 

Amphoteric nature of amino acids as they contain both acidic (COOH) and basic (NH2) groups. The amino acids possessing both positive and negative charges are called zwitterions. 

The reagents and product will be mostly as carboxylate anions in this solution but the proouc will be mostly zwitterion. The product is the amino acid alanine and it differs from natural alanu-. by being racemic. 

---