PK values of amino acids

In general, the environment of polymer domain influences activities of nucleophile, electrophile and general base catalysis. It is easily understood that unusual pK values of amino acid residues in enzymes are given, as is shown in Table 15 (92). 

Table 15. pK values of amino acid residues in several enzymes... 

An alternative model for ESI was developed by the group of Siu [24-25]. They correlated the ion envelope of a protein with the predicted distribution of charge states of the protein in solution. The latter depends on pH and pK values of the acidic and basic amino acids in the protein. From the good correlation obtained, they conclude that the ion envelope nicely reflects the abundances of the preformed ions of the protein in solution. Based on further experiments, they postulate that the droplet evaporation is not that important and that the ESI-MS spectrum results from ions directly emitted from the Taylor cone [25-26]. 

There is a linear correlation between the stability constants for the 1 1 zirconium complexes with the amino-acids glycine, alanine, P-alanine, lysine, asparagine, m-aminobenzoic acid, y-aminobutyric acid, and the pK values of the acids. Spectroscopic evidence for binding to zirconium through both the amino- and carboxy-groups has been obtained. A method for the preparation of the l-alanine complexes M(0H)2(ala)2X2,3H20 (M = Zr or Hf X = halide or NOJ) has been patented. ... 

The isoelectric point for histidine is the pH value halfway between the pK values for the two nitrogen-containing groups. The p.Aa and pi values of amino acids in peptides and proteins differ somewhat from those of free amino acids, principally because most of the a-amino and a-carboxyl groups are not ionized but are covalently joined in peptide bonds. 

Relative pK calculations are the main method used to determine pK shifts in biomolecular systems. Such calculations have error bars of several pK units for the determination of the pK values for amino acid side chains on a protein [155-157], Because biomolecular systems such as proteins are very large, molecular dynamics simulations use empirical potentials such as AMBER, CHARMM, and OPLS [52], Correspondingly, solvation models are fitted [158] for use with empirical potentials, such as Sharma and Kaminski s fuzzy-border continuum model [159],... 

Histidine is one of the 20 amino acids found in proteins. Shown here is a fully protonated histidine molecule where the numbers denote the pK values of the acidic groups. 

If a sample contains groups that can take up or lose a proton, (N//, COO//), then one must expect the pH and the concentration to affect the chemical shift when the experiment is carried out in an acidic or alkaline medium to facilitate dissolution. The pH may affect the chemical shift of more distant, nonpolar groups, as shown by the amino acid alanine (38) in neutral (betaine form 38a) or alkaline solution (anion 38b). The dependence of shift on pH follows the path of titration curves it is possible to read off the pK value of the equilibrium from the point of inflection... 

The isoelectric points of the amino acids in Table 27.3 are midway between the pK values of the zwitterion and its conjugate acid. Take two exanples aspartic acid and lysine. Aspartic acid has an acidic side chain and a pi of 2.77. Lysine has a basic side chain and a pi of 9.74. 

The amino acid L-histidine has three acid-base equilibria, with pK, values of 1.78, 5.97, and 8.97. 

Nuclear magnetic resonance (NMR) is a widely utilized technique, which detects the reorientation of nuclear spins in a magnetic field. It can potentially be used to determine the 3-D structure of the protein itself, as well as supplying information on kinetics and dynamics, ligand binding, determination of pK- values of individual amino acid residues, on electronic structure and magnetic properties, to mention only some of the applications. In addition, it can be selectively applied to specific nuclei—1H, 13C, 15N, 19F (often substituted for H as a... 

Distinct changes in several properties of lysozyme occur after reaction with ozone. The lytic activity of the ozonized lysozyme shows the same trend at various pHs as the native enz3mie (Fig. 2) this may suggest that the pK values of the ionizable groups involved in catalysis have not been altered by ozonplysis. The amino acid composition of ozonized lysozyme differs from that of the native enz3mie in three residues — methionine, tryptophan and t3H osine. None of the other amino acids is affected by ozone. The extensive loss of enz5miic activity must be ascribed to the oxidative modification of these three amino acid residues in the lysozyme. 

Arginase (EC 3.5.3.T) mediates the hydrolytic cleavage of L-canavanine to produce L-canaline and urea. L-Canaline, 2-amino-4-aminooxybutyric acid, bears the same structural analogy to L-ornithine as canavanine does to arginine. The aminooxy group of canaline with its pK value of 3.96 differs markedly from the 6-amino function of ornithine (pK = 10.76). 

In nitrogen heteroaromatics, upfield protonation shifts are found for carbons a to nitrogen, while those in / and y positions are deshielded on protonation [94, 99,100]. This is shown in Fig. 3.5 for quinoline [94]. The protonation shifts for C-/1 and C-y can be rationalized in terms of the cannonical formulae of protonated pyridine [73 d], while the upfield shifts for C-a are probably due to the lower n character of the N — C-a bond. The curves in Fig. 3.5 representing the pH dependence of 13C shifts resemble titration curves. pK values and, in the case of amino acids, the isoelectric points pi can be obtained from the point of inflection of the (5 versus pH plot for each individual carbon [84, 94, 98]. 

The acid-base behavior of peptides (10) is determined by the free a-amino group on the N-terminal residue, by the free a-carboxyl group on the C-terminal residue, and by the ionizable R groups located at intermediate positions. The pK values of the terminal a-carboxyl groups are somewhat higher and those of the a-amino groups somewhat lower than those of the corresponding free amino acids (Table 1) (11). 

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