Discussion
An amino-acid has one proton donating group in the side chain (R). The pKCOOH, pKNH2 and pKR values for this amino-acid are 2.19, 9.67 and 4.25, respectively. Which one of the following statements about this amino-acid is CORRECT?
Answer: A
The correct statement is Majority of the molecules will have a net charge of -1 at pH of 7.0.
The values describe an acidic amino acid such as glutamate. At pH 7.0 the α-carboxyl (pK 2.19) and the side-chain carboxyl (pK 4.25) are both well below the pH, so both are deprotonated (-1 each), while the amino group (pK 9.67) is protonated (+1). Net charge = -1 + -1 + 1 = -1.
At pH 4.25 the side chain is exactly half dissociated, so half the molecules are 0 and half -1, not a majority at 0. pH 3.22 is the isoelectric point, \((2.19 + 4.25)/2\), where the side chain is still mostly protonated, so 'all deprotonated' is wrong. On titration with base, the group with the lowest pK, the α-COOH, loses its proton first, not the R group.
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