Acid-Base Properties of Amino Acids

What are amino acids? Amino acids are the building blocks of proteins. In the body, they exist as zwitterions. Zwitterions can behave as both an acid or a base.

Today we will: Study the acid-base properties of amino acids, We will perform a simple titration. Start the titration with the amino acid in acidic form. As we slowly increase the pH we should be able to plot a graph similar to the one on the right. mL NaOH pH 1 2 3

Remember from Chem 103L… At point Z…? At point X…? pKa = - log Ka Ka = X -pKa Henderson-Hasselbalch HCl + NaOH NaCl + H 2 O

The titration curve points… 1 – where half of the original acidic amino acid had been titrated and became a zwitterion. 2 - where the amino acid is entirely in the zwitterion form. 3 – where half of the amino acid is in the zwitterion form and half is in the basic form Volume NaOH, mL pH

What does each point mean? The pH of the midpoint of the first leg (1) is the pK value of the carboxylic acid group. The midpoint of the second leg (2) is known as the isoelectric point. All the amino acids are in zwitterion form at this point. The pH of the midpoint of the third leg (3) is equal to the pK of the – NH Volume NaOH, mL pH

Today we will… Titrate our amino acid solution with NaOH to see the pH curve as it relates to the amount of NaOH added. Use pH meters to monitor pH changes during our titration. Construct a graph of pH vs. Volume, mL of NaOH added.

Caution! Be sure to monitor your buret carefully as to not put too much base into your amino acid solution at once.

Potential Health Concerns Inhalation: irritation, burns, coughing, corrosion, sneezing, choking, sore throat, runny nose, pneumonitis, inflammation of the upper respiratory system, pulmonary edema, circulatory failure and death Ingestion: large oral doses may result in nausea, corrosion, pain and burns of the mouth, throat, esophagus and gastrointestinal tract, vomiting, scarring of tissue, bleeding, diarrhea, low blood pressure, and death Skin contact: corrosion, redness, pain, burns, scarring, ulcers and discoloration of the skin Eye contact: irritation, tearing, pain, redness, corrosion, soreness, burns, impaired vision, permanent eye damage and blindness Tumorigen, mutagen and reproductive effector