Question

At \(\mathrm{pH}\) 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

Answer: At pH 7.4, the majority of protein-derived amino acids carry a net negative charge.

Step-by-step solution

Understanding pKa values

pKa values represent the pH at which an amino acid side chain gains or loses a proton. When the pH is below the pKa value, the side chain will carry a proton and have a net positive charge. When the pH is above the pKa value, the side chain will lose a proton and have a net negative charge. Thus, comparing the pKa values and the given pH, we can determine the net charge on the protein-derived amino acids.

Collecting pKa values of amino acid side chains

Here are some representative pKa values of the side chains for some amino acids: - Aspartic acid: pKa = 3.9 - Glutamic acid: pKa = 4.1 - Arginine: pKa = 12.5 - Lysine: pKa = 10.5 - Histidine: pKa = 6

Comparing pKa values to the given pH

At pH 7.4 (blood plasma pH), comparing the pKa values: - Aspartic acid (pKa = 3.9) and glutamic acid (pKa = 4.1): pH > pKa, so these amino acids will have a net negative charge. - Arginine (pKa = 12.5) and lysine (pKa = 10.5): pH < pKa, so these amino acids will have a net positive charge. - Histidine (pKa = 6): pH > pKa, so this amino acid will have a net negative charge.

Determining the net charge of protein-derived amino acids

As we can see from the above comparison, at pH 7.4, two of the amino acids (aspartic acid and glutamic acid) will have a net negative charge, while two others (arginine and lysine) will have a net positive charge. Histidine also has a net negative charge at pH 7.4. Although not all amino acids are listed here, the majority of protein-derived amino acids will tend to follow a similar trend. Thus, we can conclude that at pH 7.4, the majority of protein-derived amino acids will bear a net negative charge.