Basicity constants amino acids

Figure 7.4 Basic structure of an IgG molecule. Two heavy chains (440 residues) and two light chains (214 residues) are joined by disulphide bonds and each shows a relatively constant amino acid sequence in one section (C-terminal end) and a variable sequence section (N-terminal end). The variable regions of both heavy and light chains are involved in the formation of the antigen-binding site.

More specifically, the pi of any amino acid is the average of the two acid-dissociation constants that involve the neutral zwitterion. For the 13 amino acids with a neutral side chain, pi is the average of pKal and p/amino acids with either a strongly or weakly acidic side chain, pi is the average of the two lowest pKa values. For the three amino acids with a basic side chain, pi is the average of the two highest pKz values. 

None of the other reactions so far discussed involve interaction between a pair of charged species. This is but another instance of the electrostatic effect shown by Kirkwood and Westheimer to be responsible for the disparity between the first and second ionization constants of dibasic acids, for the effect of the carboxylate ion on the basicity of an a-amino acid, and for the difference in reactivity of ionic compounds compared with analogous nonionic species in acid- or base-catalyzed reactions. ... 

Blue copper proteins, 36 323, 377-378, see also Azurin Plastocyanin active site protonations, 36 396-398 charge, 36 398-401 classification, 36 378-379 comparison with rubredoxin, 36 404 coordinated amino acid spacing, 36 399 cucumber basic protein, 36 390 electron transfer routes, 36 403-404 electron transport, 36 378 EXAFS studies, 36 390-391 functional role, 36 382-383 occurrence, 36 379-382 properties, 36 380 pseudoazurin, 36 389-390 reduction potentials, 36 393-396 self-exchange rate constants, 36 401-403 UV-VIS spectra, 36 391-393 Blue species... 

Despite their very short sequence (7 to 38 amino acid residues for the 12 histatins identified so far), the histidine-rich salivary protein histatins have also been reported to precipitate tannins, eventually more efficiently than proline-rich proteins, especially at neutral pH and high tannin concentration. A detailed NMR analysis of the binding between EGCG and histatin 5, a 24-mer that is very rich in basic His, Lys, and Arg residues ( 60%) and devoid of secondary structure, has revealed noncooperative binding of six to seven flavanol molecules with a dissociation constant of 1 mM (pH 3.0, 25°C). ... 

In the Horner-Emmons reaction (Scheme 3), the sulfonylphosphonate carbanion 5 is formed in the presence of NaH and then reacts with an aldehyde to produce the intermediate 6 that undergoes in situ elimination to yield the vinyl sulfones and phosphonate anion. The sulfonyl group can stabilize the anion in the sulfonylphosphonate 5. The vinyl sulfones that are produced by this method using aldehydes as starting materials are exclusively the E (trans) isomers. The E-isomers of the vinyl sulfones are shown in the NMR spectra based on the coupling constants of the vinylic protons. Although strongly basic conditions are used in the Horner-Emmons reaction and a-amino aldehydes are easily racemized, the amino acid vinyl sulfones prepared by this method still show substantial optical activity. However, the enantiomeric purity of these compounds has not been determined. 5 ... 

Potentiometric determination of the pH of L-histidine solutions with and without D-glucose at 40° showed that, at initial pH values between 5 and 6.5, the pH remained constant for both samples. When the initial pH was above 6.5, alkali had to be added to the D-glucose-histidine mixture in order to maintain the pH.137 Chromatographic evidence shows that other basic amino acids (such as lysine and arginine) also react238 with D-glucose... 

However, some amino acids have an acidic or basic functional group in their side chains. In these cases there is another acidity constant to consider. Let s examine the case of aspartic acid, which has two carboxylic acid groups and an amino group. At low pH, aspartic acid is present in a cationic form ... 

With these newer methods of protein separation and amino acid analysis he prepared serum protein fractions by serial salting out with ammonium sulfate and by the Sober and Peterson DEAE cellulose columns (42), using the sera of reptile, fowl, and mammalian blood. Some of the amino acid analyses were carried out by the automatic amino acid methods of Hirs, Moore, and Stein (18). Fortified with this plethora of data, Block now had the opportunity to re-examine not only the ratio of the basic amino acids, but at least 12 amino acids in a variety of protein fractions prepared by at least two different procedures. With the aid of a statistician he determined the significance of the constancy of the molar ratios of pairs of amino acids and found that in spite of the marked variation of the absolute amounts of an amino acid, the molar ratios of certain pairs remain relatively constant among the numerous protein components of animal sera. 

Stability constant measurements have been undertaken to show that in basic solution mixed amino acid ligands with two amino acid moieties present, i.e. NH2—C(R)—C—N—C(R )— COOH), yield complexes with log K values of the order of 6 to 7 384 such compounds appear to bind the amine nitrogen.385... 

In synthetic peptides, the presence of the two adjacent basic amino acids is an essential requirement for the peptide to be phosphorylated with kinetic constants comparable to those of the physiological substrates (31). It has also been observed that in addition to the requirement for the two adjacent basic residues, the nature of the amino acid C-terminal to the phosphoserine is involved as a specificity determinant. When serine was replaced by threonine, the synthetic peptides were no longer active as substrate. 

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