Optimizing Charge: Determining the Charged Amino Acids in Molecule A at pH 7 Based on pKa Values

Which amino acids in “a” would be charged at pH 7?

Asp, Glu, Arg, His, Lys

To determine which amino acids in molecule “a” would be charged at pH 7, we need to know the pKa values of their functional groups. At pH 7, the amino group (NH3+) would be protonated and carry a positive charge, while the carboxyl group (COO-) would be deprotonated and carry a negative charge. The side chains of some amino acids also have functional groups that can be ionized at different pH values.

Here are the three amino acids in molecule “a” and their relevant pKa values:

1. Histidine (pKa = 6.0): At pH 7, the imidazole group in histidine would be partially protonated, giving it a net charge of +1.

2. Aspartic acid (pKa = 3.9): At pH 7, the carboxyl group in aspartic acid would be fully deprotonated, giving it a net charge of -1.

3. Lysine (pKa = 10.8): At pH 7, the amino group in lysine would be fully protonated, giving it a net charge of +1.

Therefore, at pH 7, histidine and lysine in molecule “a” would be positively charged, while aspartic acid would be negatively charged.

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