Glycine ionization

Figure 1.1. Mathematical operation with Excel. Glycine ionizes according to O O O...

Strack, D. et al., Cyanidin 3-oxalylglncoside in orchids, J. BioscL, 41, 707, 1986. Choung, M.-G. et al.. Isolation and determination of anthocyanins in seed coats of black soybean (Glycine max (L.) Merr.), J. Agric. Food Chem., 49, 5848, 2001. Covey, T., Analytical characteristics of the electrospray ionization process, in Biochemical and Biotechnological Applications of Electrospray Ionization Mass Spectrometry, ACS Symposium Series, Snyder, A.P. and Anaheim, C. A., Eds., Washington, D.C., 1995, chap. 2. 

Depke, G., N. Heinrich, and H. Schwarz. 1984. On the Gas Phase Chemistry of Ionize Glycine and Its Enol. A Combined Experimental and Ab Initio Molecular Orbital Study. Int. J. Mass Spectrom. Ion Porcesses 62, 99-117. 

With multiple ionizable groups, such as in amino acids and proteins, each group titrates separately according to its pKa. The titration curves shown in Fig. 23-5 are for the amino acids glycine, histidine, and glutamate. 

For all other amino acids, with the exception of glycine (Gly), the a-carbon is bonded to four different groups, and the two stereoisomers are mirror images that cannot be superimposed. Eukaryotic proteins are always composed of L-amino acids although D-amino acids are found in certain peptide antibiotics and some peptides of bacterial cell walls. ° ° The physical properties of amino acids are influenced by the degree of ionization at different pH values. 

Selenoprotein A is remarkably heat stable, as seen by the loss of only 20% of activity on boiling at pH 8.0 for lOmin (Thrner and Stadtman 1973). Although selenoprotein A contains one tyrosine and no tryptophan residues, it contains six phenylalanine residues and thus has an unusual absorbance spectrum (Cone et al. 1977). Upon reduction, a unique absorption peak emerges at 238 nm, presumably due to the ionized selenol of selenocysteine, which is not present in the oxidized enzyme. The activity of selenoprotein A was initially measured as its ability to complement fractions B and C for production of acetate from glycine, in the presence of reducing equivalents (e.g., dithiothreitol). Numerous purification schemes were adopted for isolation of selenoprotein A, all of which employed the use of an anion exchange column to exploit the strongly acidic character of the protein. 

The kinetics of the alkaline hydrolysis of 2-methylpentyl salicylate (24) have been studied in various aqueous propanol and r-butanol mixtures and in mixtures of water and ethane-l,2-diol. ° Further smdies of the aminolysis of ionized phenyl salicylate (25) have been reported, in which it was observed that, in mixed acetonitrile-water solvents, glycine, 1,2-diaminoethane and 3-aminopropanol all reacted as did simple amines, via an intramolecular general-base-catalysed process. ... 

The ionization potentials of four sydnones (1) have been compared with those of glycines and iV-nitrosoglycines. The results suggest that in the gas phase the sydnones (1) exist as the nitrosoketen isomer (47). 

In as much as pK = — log K, the values for glycine are pKL = 2.34 and pK2 = 9.60 (in aqueous solution at 25eC), The homologous amino acids indicate similar values. The pH at which acidic ionization balances basic ionization is termed the isoelectric point (pHi). (corresponding to... 

Table 10 Ionization Constants of Some Coordinated Glycine Peptides, Showing Metal-ion-promoted Peptide Deprotonations ...

NH2CH2COO" (aq.). Louguinine1 found Q=2.97 for the reaction of aqueous aminoacetic acid with aqueous OH- to form aqueous glycinate, or aminoacetate, ion. Branch and Miyamoto,1 from conductivity measurements, calculated the heat of ionization of aqueous aminoacetic acid into aqueous H+ and aqueous glycinate ion to be —10.70. 

When amino acid ester prodrugs of acetaminophen were prepared (Kovach et al., 1975 Pitman, 1976), the hydrobromide salt of the glycine ester showed enhanced solubility in water, but the hydrochloride salt of th -aspartic acid ester exhibited a solubility lower than that of the parent compound. The enhanced solubility resulted from the formation of a salt, while the parent drug is a weakly acidic phenol and behaves as essentially a neutral molecule in solution. The reduced solubility in the case of th0-aspartic acid ester resulted from ionization of the terminal carboxylic acid, which, with the protonated amine, gives a zwitterionic compound. The zwitterion also behaved as a molecule with an overall neutral character, as is commonly observed with zwitterion behavior in aqueous media, but its larger size resulted in a further reduced solubility. 

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