Question

At pH 7.4, the \(\mathrm{pH}\) of blood plasma, do the majority of protein- derived amino acids bear a net negative charge or a net positive charge? Explain.

Short answer

At a pH of 7.4, the majority of protein-derived amino acids carry a net positive charge, as there are more positively charged basic side chains (lysine, arginine, and histidine) than negatively charged acidic side chains (aspartate and glutamate).

Step-by-step solution

Understanding pH and pKa values

The pH of a solution is a measure of its acidity or alkalinity. It is calculated as \(\mathrm{pH} = -\log_{10} [\mathrm{H}^{+}]\), where \([\mathrm{H}^{+}]\) represents the concentration of hydrogen ions (protons) in the solution. A pKa value represents the pH at which an acidic group (such as a carboxyl or amino group) is 50% ionized. When the pH of a solution is equal to the pKa value of an acidic group, the concentration of the protonated form of the group will be equal to the concentration of the ionized form.• Amino groups (NH3+) have a pKa value around 9.0, meaning they will be mostly protonated (positively charged) at pH 7.4.• Carboxyl groups (COOH) have a pKa value around 2.0, meaning they will be mostly ionized (negatively charged) at pH 7.4.

Considering the charge of amino acid side chains

Amino acids can be classified based on the properties of their side chains (R groups). In this case, we will focus on the side chains that can carry a charge: acidic, basic, and neutral.• Acidic side chains (e.g., aspartate and glutamate) have a carboxyl group with a pKa value around 4.0. Thus, they will be mostly ionized (negatively charged) at pH 7.4.• Basic side chains (e.g., lysine, arginine, and histidine) have an amino group with a pKa value between 6.0 and 12.0. They will be mostly protonated (positively charged) at pH 7.4.• Neutral side chains (e.g., polar, non-polar, and special) do not carry a charge under physiological conditions.

Determining the net charge of amino acids at pH 7.4

To determine the net charge of an amino acid at pH 7.4, we will consider the charges of its amino group, carboxyl group, and side chain (R group):• Amino group (NH3+): mostly protonated, thus positively charged.• Carboxyl group (COOH): mostly ionized, thus negatively charged.• Side chain (R group): Depending on the type of the side chain, the charge may be positive, negative, or neutral.

Comparing the net charges of the majority of amino acids

Considering the charges of amino and carboxyl groups, as well as the variety of side chains with different charges, we can conclude that at pH 7.4, the majority of protein-derived amino acids carry a net negative charge or a net positive charge. To determine this, we compare the number of positively charged basic side chains to the number of negatively charged acidic side chains:• There are three basic (positively charged) side chains: lysine, arginine, and histidine.• There are two acidic (negatively charged) side chains: aspartate and glutamate.Since there are more positively charged basic side chains (three) than negatively charged acidic side chains (two), the majority of protein-derived amino acids at pH 7.4 bear a net positive charge.