Ch_322b_24.02

Ch_322b_24.02 - Amino Acids as Dipolar Ions The properties...

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Unformatted text preview: Amino Acids as Dipolar Ions The properties of amino acids indicate they exist as dipolar ions (zwitterions) in the solid state and in solution in water. Amino acids are nonvolatile, crystalline solids with high melting points. They are soluble in water,insoluble in nonpolar solvents . Aqueous solutions of amino acids behave like solutions of high ionic strength. Acidity and basicity constants (K a and K b ) are consistent with the zwitterionic structures. Acidity and Basicity Constants Typically, RCOOH + H 2 O RCO 2- + H 3 O + K a ~ 10-5 RNH 2 + H 2 O K b ~ 10-4 RNH 3 + + HO- But for amino acids, K a ~ 10-10 and K b ~ 10-12 . Example: Glycine H 2 NCH 2 CO 2 H K a = 1.6 x 10-10 , K b = 2.5 x10-12 . These values are consistent with a zwitterionic structure : H 3 NCH 2 CO 2 +- acid function base function where acid and base strength are determined by H 3 N CH 2 CO 2 +- + H 2 O H 2 NCH 2 CO 2- + H 3 O + K a H 3 NCH 2 CO 2 +- + H 2 O H 3 NCH 2 CO 2 H + HO- K b + Amino Acids as Dipolar Ions The structure of the species in solution depends on the pH: H 3 NCHCO 2 +- R-H + +H + H 2 NCHCO 2- H 3 NCHCO 2 H-H + +H + + in strong acid in strong base dipolar ion (no net charge) cation anion pH < 3 pH > 10 R R Isoelectric Point In the presence of an electric field , chemical species with a net charge migrate towards the pole of the electric field with opposite charge. migrate towards the pole of the electric field with opposite charge....
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Ch_322b_24.02 - Amino Acids as Dipolar Ions The properties...

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