Acids, amino neutralization

Acid-Base Properties of Amino Acids with Neutral Side Chains... 

The isoelectric point of an amino acid depends on its structure, with values for the 20 common amino acids given in Table 26.1. The 15 neutral amino acids have isoelectric points near neutrality, in the pH range 5.0 to 6.5. The two acidic amino acids have isoelectric points at lower pH so that deprotonation of the side-chain -C02H does not occur at their pi, and the three basic amino acids, have isoelectric points at higher pH so that protonation of the side-chain amino group does not occur at their pi. 

What about tertiary structure Why does any protein adopt the shape it does The forces that determine the tertiary structure of a protein are the same forces that act on ail molecules, regardless of size, to provide maximum stability. Particularly important are the hydrophilic (water-loving Section 2.13) interactions of the polar side chains on acidic or basic amino acids. Those acidic or basic amino acids with charged side chains tend to congregate on the exterior of the protein, where they can be solvated by water. Those amino acids with neutral, nonpolar side chains tend to congregate on the hydrocarbon-like interior of a protein molecule, away from the aqueous medium. 

TRIFLUOROACETYLATION OF AMINES AND AMINO ACIDS UNDER NEUTRAL, MILD CONDITIONS N-TRIFLUOROACET ANILIDE AND N-TRIFLUOROACETYL-l-TYROSINE... 

In the gas phase, amino acids are neutral and the conformational properties of the backbone are determined by the torsions Tj(H-N-C-C), t2(N-C-C=0), and t3(0=C-0-H). In the planar structures of glycine, two meaningful arrangements can be expected for each of these torsions, in the vicinity of 60° or 120° for tp and 0° or 180° for t2 and t3. The possible combinations yield eight different conformers. The number is enhanced when planar or nonplanar geometries are considered in some cases. A summary of the possible conformations of glycine is given in Fig. 7.2. 

FIGURE 2.7 Structures of neutral, basic, and acidic amino acids. 

FIGURE 8.14 Salts of amino acids. (A) Acetic acid and aqueous ammonia form salts with the basic and acidic amino acids but (B) not with neutral amino acids. (C) Stronger acids and bases form salts with all amino, guanidino and carboxyl groups of amino acids. Ac = acetyl R = alkyl or cyclohexyl. 

In the prototype analysers, two columns were often needed to achieve complete separation of all the amino acids. A 50-100 cm column was used to separate the acidic and neutral amino acids and a 5-10 cm column for the basic amino acids, each with a diameter of 1 or 2 cm, but today s instruments use single columns with narrower diameters. As peak width is proportional to the square root of the column length, these glass or stainless steel columns give narrow peaks and improved separation of closely related amino acids. 

Sowden et al. [4] also did detailed amino acid and amino sugar analyses of the soils from the different dimatic regions. The following amino acids were determined acidic amino acids aspartic and glutamic acids basic amino acids arginine, histidine, lysine and ornithine neutral amino adds, phenylalanine, tyrosine, glycine, alanine, valine, leudne, isoleudne, serine, threonine, proline and hydroxyproline ... 

Several symport proteins have been identified in the luminal and basolateral surfaces of the proximal tubule cells, each with a specific transport function. For example, mechanisms exist for transport of (i) neutral amino acids, except glycine, (ii) glycine alone, (iii) acidic amino acids (glutamate and aspartate), (iv) basic amino acids... 

Putnam, W.S., Ramanathan, S., Pan, L., Takahashi, L.H., and Benet, L.Z., Functional characterization of monocarboxylic acid, large neutral amino acid, bile acid and peptide transporters, and P-glycoprotein in MDCK and Caco-2 cells, /. Pharm. Sci., 91, 2622, 2002. 

Armstrong et al. ° first introduced chiral stationary phases based on macrocyclic antibiotics. Vancomycin, ristocetin A, teicoplanin, avoparcin, rifamycin B and thiostrepton are used as chiral selectors. They posses a broad enantiorecognition range, similar to protein based CSPs. However, CSPs based on macrocyclic antibiotics show higher stability and capacities.Underivatized amino acids, N-derivatized amino-acids, acidic compounds, neutrals, amides, esters and amines can be separated.The first four of the above-mentioned chiral selectors appear to have the largest enantiorecognition range.The selectors can also be derivatized to obtain different enantioselectivities. 

Amino acids and some small peptides are absorbed into the enterocytes in the jejnnnm. The transport of amino acids from the lumen into the ceU is an active process, coupled to the transport of Na ions down a concentration gradient. There are at least six carrier systems with different amino acid specificities neutral amino acids (i.e. those with no net charge, e.g. branched-chain amino acids) neutral plus basic amino acids imino acids (proline, hydroxyproline) and glycine basic amino acids (e.g. arginine and lysine) P-amino acids and taurine acidic amino acids (glutamic and aspartic acids). 

---