Glycine is optically inactive, simplest amino acid because it has no asymmetric carbon atom. Acid-Base titration involves the gradual addition (or) removal of protons. It has three different stages when the Glycine undergoes acid-base titration.
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Steps of Titration curve of Glycine
Stage 1: At low pH
At very low pH, the predominant ionic species of Glycine is the fully protonated form, +H3N-CH2-COOH. For Glycine, the pH at the midpoint is 2.34, thus it’s –COOH group has pKa of 2.34.
Stage 2: Reaching middle pH
As the titration proceeds, another important point is reached at pH 5.97. Here there is another point of infection, at which removal of the first proton is essentially complete and removal of the second proton has just begun.
Stage 3: Releasing Proton
the third stage of the titration corresponds to the removal of a proton from the –NH3+ group of Glycine. The pH at the midpoint of this stage is 9.60, equal to the pKa for the –NH3+ group.
From this titration curve of glycine, we can derive several important pieces of information.
- It gives a quantitative measure of the pKa of each of the two ionic groups; 2.34 for the –COOH group and 9.60 for the –NH3+ group.
- The perturbed pKa of Glycine is caused by repulsion between the departing proton and the nearby positively charged amino group on the α-carbon atom.
- The titration curve of Glycine has two regions of buffering power. At pKa 2.34, glycine is a good buffer near this pH. The other buffering zone is centered on pH of 9.60.
- Glycine is not a good buffer at the pH of intracellular fluid (or) blood, about 7.4. To calculate the buffering ranges, we can use the Handerson-Hasselbalch equation.