Amino acids are the building blocks of proteins, and can act asboth a Brønsted acid and a Brønsted base through intramolecularproton transfer (see Chapter 16, pg. 709 of the textbook). Thesimplest amino acid known is glycine, NH2CH2CO2H (Ka = 4.5 ×10–3and Kb = 6.0 ×10–5 ), and it can exist in three forms inequilibrium with one another:
H2N––CH2––COOH +H3N––CH2––COOH +H3N––CH2––COO–H2N––CH2––COO–
Glycine cation zwitterion anion
(a) Write the equilibria corresponding to Ka and Kb ofglycine.
(b) Estimate the value of theequilibrium constant, K, for the intramolecular proton transfer toform the zwitterion in glycine: H2N––CH2––COOH (aq) K ?+H3N––CH2––COO– (aq)
(c) If the pH of an aqueoussolution containing glycine is 7.2, in what form is glycine mostabundant?
