Acids, amino strong

Just as individual amino acids have isoelectric points, proteins have an overall p/ because of the acidic or basic amino acids they may contain. The enzyme lysozyme, for instance, has a preponderance of basic amino acids and thus has a high isoelectric point (p/= 11.0). Pepsin, however, has a preponderance of acidic amino acids and a low- isoelectric point pi 1.0). Not surprisingly, the solubilities and properties of proteins with different pi s are strongly affected by the pH of the medium. Solubility- is usually lowest at the isoelectric point, where the protein has no net charge, and is higher both above and below the pi, where the protein is charged. 

Formation of a novel binding site novel complexes may be formed between a SUMOylated protein and an effector protein that contains a SUMO-interacting motif (SIM or SBM). Proteins that contain two binding sites, a SIM and a weak binding motif to protein X, will bind more strongly to this protein if it is SUMOylated (Fig. 3b). Short peptides that contain the hydrophobic core motif [V/I]-X-[V/I]-[V/I] or [V/I-[V/I]]-X-[V/I] can act as a SIM and bind to SUMO. This core is often flanked by acidic amino acids and/or serine residues. 

The solubilization of amino acids in AOT-reversed micelles has been widely investigated showing the importance of the hydrophobic effect as a driving force in interfacial solubihzation [153-157]. Hydrophilic amino acids are solubilized in the aqueous micellar core through electrostatic interactions. The amino acids with strongly hydrophobic groups are incorporated mainly in the interfacial layer. The partition coefficient for tryptophan and micellar shape are affected by the loading ratio of tryptophan to AOT [158],... 

Although the order of affinity of PF-4 for different glycosaminogly-cans, and dissociation of their complexes with salts, are typical of nonspecific, electrostatic interactions, PF-4 is not strictly a cationic protein.452 It is probable that heparin binds to clusters of basic amino acids (two lysine pairs) near the carboxyl terminal of a polypeptide chain that has an overall preponderance of acidic amino acid residues.457 High-molecular-weight heparin species can bind two PF-4 molecules, with formation of complexes 10 to 100 times as strong as those with antithrombin.217... 

R Liardon, R lost. Racemization of free and protein-bound amino acids in strong mineral acid. Int J Pept Prot Res 18, 500, 1981. 

Glutamic acid is an example of an amino acid with an acidic side-chain. The pATa values are 2.19 (CO2H), 4.25 (Y-C02H), and 9.67 (NH2). Here, the y-carboxyl is more typical of a simple carboxylic acid. In strongly acidic solution, glutamic acid will... 

For the hydrolysis of peptides linked to resins, the release of the C-terminal amino acid is strongly dependent upon the type of resin and amino acid residue. For this purpose the use of propanoic acid/12M HC1 (1 1 )[6 is recommended. More studies, however, indicate hydrolysis with TFA/12M HC1 (1 3) at 160°C is more appropriate since quantitative release of the most stable Phe residue from benzhydrylamine- (BHA-), 4-methylbenzhydrylamine-(pMeBHA-), and 4-(hydroxymethyl)phenylacetamidomethyl-resins occurs in 20, 6, and 5 h, respectively, compared to the 30,18, and 18h required with propanoic acid/12M HC1 (1 1)) ... 

Amino acids with basic side chains Basic amino acids are the opposite of acidic amino acids. Their most important role is to form ionic bonds to negative ions—phosphate and the like. Lysine is a simple example. The side chain contains four CH2 groups and terminates in —NH3 +. Arginine has an even more basic, if somewhat more complicated and larger, side chain. Conversely, the side chain of histidine is not as basic as that of lysine and the concentrations of the unprotonated and protonated forms of histidine are almost equal at biological pH. Strong Lewis acid-Lewis base complexes between the unprotonated form of histidine and metal ions is very common in proteins. Histidine side chains are also involved in moving protons from one atom to another. 

ZWITTERION. An ion carrying charges of opposite sign, which thus constitutes an electrically neutral molecule with a dipole moment looking like a posilive ion at one end and a negative ion at the other. Most aliphatic amino acids form such dipolar ions, hence react with both strong acids and strong bases. 

The biosynthesis of alkaloids has been extensively studied, and although for a time it was thought that alkaloids arose primarily from amino acid precursors, strong evidence now is available that ethanoate also is involved. The mode of alkaloid biosynthesis is not yet as well understood as that of the terpenes and steroids. One experimental problem is the difficulty of feeding suitably labeled precursors to plants. 

The data in Chapter 20.2 show that the naturally occurring amino acids are strong chelators of the transition metal ions, the presence of the N donor atom considerably enhancing the thermodynamic stabilities relative to those of the carboxylic acids. 

As a final example of an acid-base titration, let s consider the gradual addition of NaOH to the protonated form of the amino acid alanine (H2A+), a substance that acts as a diprotic acid. Amino acids (which are discussed in more detail in Chapter 24) are both acidic and basic and can be protonated by strong acids such as HC1, yielding salts such as H2A+C1 . The protonated form of the amino acid has two... 

That is, basic and neutral amino acids follow the pattern long recognized for strongly dissociated solutions which promote transformation, whereas acidic amino acids form a protective overgrowth on the aragonite surface via carboxyl groups. 

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