The molecular structure of a sample of solid, non-ionized L-phosphotyrosine is shown below. What is the net charge of L-phosphotyrosine when placed in an aqueous solution at physiological pH 7.4? (Note: Assume that the pK_a values of the phosphoryl group are 2.2 and 7.2.)

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11ecba2d_11f2_9d4c_a3bf_b58abbefd3b8_MD0008_00 The net charge of a molecule is the sum of all nonzero formal charges from each atom in the structure. Atoms often acquire a nonzero formal charge by acid-base interactions in solution. Functional groups that are acidic act as proton (H⁺) donors, but those that are basic act as proton acceptors. Each proton donated by an atom decreases its formal charge by one unit, but each proton accepted by an atom increases the formal charge by one unit. At physiological pH (7.4), amino acids have negatively charged carboxylic acid groups and positively charged amino groups. Carboxylic acid groups (weak acids) donate a proton to form negatively charged carboxylates. Conversely, the amino groups behave as bases and accept protons to form aminium cations. Groups attached to amino acids can also act as an acid or a base. A phosphoryl group behaves like an attached molecule of phosphoric acid, and at physiological pH, it also fully ionizes to acquire two negative charges because the pK_a values are lower than the pH. As a result, L-phosphotyrosine in a solution at pH 7.4 has two negative charges from the phosphoryl group, one negative charge from the carboxyl group, and one positive charge from the amino group which sum to a net charge of −2. (Choice A) Zero net charge would require that no ions formed in the structure (or that the charges formed cancelled out). The acid-base behavior of the phosphoryl, carboxyl, and amino groups excludes this possibility. (Choice B) A net charge of −1 would occur if only the amino and the phosphoryl groups formed ions. However, the acidic nature of carboxylic acids ensures that the carboxyl group will ionize also. (Choice D) A net charge of −3 would be possible if only the acidic phosphoryl and carboxyl groups formed ions. The basic nature of amines, however, causes the amino group to be protonated and form a +1 charge. Educational objective: Some amino acid functional groups act as proton donors or as proton acceptors. Each proton donated by an atom decreases its formal charge by one unit, but each proton accepted by an atom increases the formal charge by one unit. The net charge is the sum of all nonzero formal charges in a molecule.

The Molecular Structure of a Sample of Solid, Non-Ionized L-Phosphotyrosine