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The increasing order of pKa of the following amino acids in aqueous solution is: Gly Asp Lys Arg

1. Asp < Gly < Arg < Lys
2. Gly < Asp < Arg < Lys
3. Asp < Gly < Lys < Arg
4. Arg < Lys < Gly < Asp

Correct answer: Asp < Gly < Lys < Arg

Solution

Ordering by characteristic pKa: Asp side chain ~3.7 < Gly (alpha-NH3+ ~9.6) < Lys epsilon-NH3+ ~10.5 < Arg guanidinium ~12.5. Since the guanidinium group of arginine has the highest pKa, the increasing order is Asp < Gly < Lys < Arg.

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