092209-BCH311

092209-BCH311 - Acid-Base Chemistry of Amino acids In a...

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Unformatted text preview: Acid-Base Chemistry of Amino acids In a free amino acid, the carboxyl group and amino group of the general structure are charged at neutral pH (NH 3 + and COO- ) Amino acids without charged groups on their side chains exist in neutral solutions as zwitterions with no net charge A zwitterion has equal positive and negative charges; in solution, it is electrically neutral The carboxyl and amino groups of amino acids are titratable For example, at very low pH, alanine has a protonated (and thus uncharged) carboxyl group and a positively charged amino group that is also protonated - under these conditions, alanine has a net positive charge of +1 As base is added, the carboxyl group loses its proton to become a negatively charged carboxylate ion - alanine now has no net charge As the pH increases further, the protonated amino group (a weak acid) loses its proton, and the alanine molecule now has a net charge of -1 The titration curve of alanine is that of a diprotic acid Fig. 3-5a, p. 74 The ionic forms of amino acids The ionic forms of amino acids, shown without consideration of any ionizations of the side chain The cationic form is the low pH form, and the titration of the cationic species with base yields the zwitterion and nally the anionic form Fig. 3-6, p. 74 The titration curve of alanine At pH 2.34 (p K a1 ) there are equal concentrations of the cationic form and the zwitterion ( i.e. the weak acid and its conjugate base ) At pH 6.02 (pI - the isoelectric point), the molecule has no net charge - 100% zwitterion At pH 9.69 (p K a2 ) there are equal concentrations of the zwitterion and the anionic form The titration curve of histidine In histidine, the imidazole side chain also contributes a titratable group At very low pH values, the histidine molecule has a net positive charge of +2, because the imidazole and amino groups have positive charges As base is added and the pH increases, the carboxyl group loses a proton to become a carboxylate ion as before, and the histidine has a charge of +1 As more base is added, the charged imidazole group loses its proton, and this is the point at which the histidine has no net charge - zwitterion At still higher values of pH, the amino group loses its proton and the histidine molecule now has a net charge of -1 The titration curve of histidine is that of a triprotic acid Fig. 3-5b, p. 74 The titration curve of histidine Fig. 3-7, p. 75 The titration curve of histidine Amino acid titrations The titratable groups of each of the amino acids have characteristic p K a values The p K a values of -carboxyl groups are fairly low, around 2 The p K a values of amino groups are much higher, with values ranging from 9 to 10.5 The p K a values of side-chain groups, including side chain carboxyl and amino groups, depend on the groups chemical nature The classiFcation of an amino acid as acidic or basic depends on the p K...
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This note was uploaded on 12/05/2009 for the course BCH 311 taught by Professor Howlett during the Fall '08 term at Rhode Island.

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092209-BCH311 - Acid-Base Chemistry of Amino acids In a...

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