Zwitterions amino acids

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

Reaching the equilibrium where the amino acid zwitterion predominates is a slow process. After acidifying to pH 6.5, the solution is allowed to stir at 0°C for 25 min during which time the pH of the solution slowly increases. The pH is readjusted to pH 6.5 by slow addition of 2.0N HC1 at 0°C. Repetition of this procedure as many as ten times may be necessary to insure the pH value of the aqueous solution remains at 6.5. 

Fig. 8.—Favored Conformation of Some o-Amino Acid Zwitterions as Indicated by Molecular-orbital Calculations. ...

Abraham, D. J., Leo, A. J. Extension of the fragment method to calculate amino acid zwitterion and side chain partition coefficients. Proteins 1987, 2,130-152. 

The composition of the products from the alkylation of lithiated 3,6-dialkoxy-2,5-dihydropy-razines with bis-alkylating reagents depends on the equivalents of electrophile employed. For example, with three equivalents of bis-alkylating electrophiles, (25)-2,5-dihydro-2-isopropyi-3,6-dimethoxy-5-methylpyrazine (I) gives high yields of the 1 1 adducts 2 or 5 with > 97% de2. These products can be further converted to the bicyclic systems (3 and 6. respectively) which, upon hydrolysis, give optically pure cyclic amino acids (4 and 7). Methyloxirane is used as an acid scavenger in order to release the free amino acid zwitterion. 

Amino acids, zwitterions, cysteine Peptides, protein structures Enzyme classification Enzyme activity Third hour exam Chemical messengers... 

Proteins contain, on average, more acceptor than donor sites [596]. Similar proton deficiency in the amino acid zwitterion crystal structures results in the formation of three-center hydrogen bonds rather than in unsatisfied acceptor sites [74] (Part I A, Chap. 2.6). There is less flexibility in the orientation of hydrogen-bond donor and acceptor groups in proteins which would lead to a relatively larger number of unsatisfied acceptor sites. Some of the more unsymmetrical three-center bonds which might have been missed in the survey [596] because of the 3.5 A X - A cut-off limit will also contribute to reduce the number of unsatisfied acceptors in side-chains. 

Internal water molecules tend to form clusters. In general, internal water molecules in protein structures are not found isolated but are assembled in clusters. Their hydrogen-bonding scheme could be derived in actinidin (Fig. 19.13), in lysozyme, and in penicillopepsin, based oh the assumption that water molecules act as double donors and acceptors. In some of the protein structures, which have been analyzed in greater detail, an internal water is associated with three acceptor sites indicating three-center bonding as observed in the amino acid zwitterion crystal structures (see Part IB, Chap. 8). 

Table 11. Fluorescence properties of the aromatic amino acid zwitterions in aqueous solution at room temperature...

Amino acid zwitterions Kirschner 1998 Ammonium, alkylammonium Liang 1997a... 

Amino acid zwitterions are internal salts and therefore have many rf the physical properties associated with salts. They have large dipolef moments, are soluble in water but insoluble in hydrocarbons, and are cry - ... 

The infrared spectrum of cycloserine presented in Fig. 1 was taken in a KBr pellet. A spectrum of the same sample taken in a Nujol Mull is essentially identical to the one presented. Hidy- -, Kuehl, and Stammer showed that the solid state spectrum of cycloserine has two ionizable groups with pKa, equal to. A -. 5 and pKa2 equal to 7 Spectral bands typical of an amino acid zwitterion (2200 cm assigned to the -NH +) and a resonance stabilized hydroxamafe anion (1600 to 1500 cm-1) are in agreement with the peaks represented in Fig. 1. 

Fluorescent signalling of the brain neurotransmitter 7-aminobutyric acid and related amino acid zwitterions, A. P. de Silva, H. Q. Nimal Gunaratne, C. McVeigh, G. E. M. Maguire, P. R. S. Maxwell and E. O Hanlon, Chem. Commun., 1996, 2191. 

Sohmuck. C. Self-assembly of 2-(guanidiniocarbonyl)-4-carboxylate in dimethyl sulfoxide An entropy driven oligermerization. Tetrahedron 2001. 57 (15). 3063-3067. Silva. A.P. Gunaratne. H.Q.N. McVeigh, C. Maguire, G.E.M. Maxwell. P.R.S. O Hanlon. E. Fluorescent signalling of the brain neurotransmitter y-aminobutyric acid and related amino acid zwitterions. Chem. Commun. 1996. 18. 2191-2192. 

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