What conclusions can be drawn from the titration curves of amino acids?

  • Carboxyl group of glycine is over 100 times more acidic (more easily ionized) than the carboxyl group of acetic acid (pKa=4.76)—for a carboxyl group attached to an unsubstituted aliphatic hydrocarbon. The decrease in pKa of glycine is caused by repulsion between the departing proton and the nearby positively charged amino group on the α-carbon atom. The opposite charges on the resulting zwitterion are stabilizing, moving the equilibrium farther to the right. 
  • Similarly, the pKa of the amino group in glycine is decreased relative to the average pKa of an amino Group in methylamine. This effect is due partly to the electronegative oxygen atoms in the carboxyl groups, which tend to pull electrons toward them, increasing the tendency of the amino group to give up a proton. Hence, the α-amino group has a pKa that is lower than that of methylamine.
  • Amino acids have two regions of buffering power. One will correspond to the relatively flat portion near pK1 and the other near pK2. For glycine, they will be around pH equal to 2.4 and 9.6.

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