The R-group of histidine is a weak base with a pKb of 6.0. Calculate the percent ionization at pH 7.4.
What is the net charge of the following amino acids (single letter codes used) at pH7.4?
Glycine
Serine
Glutamic Acid
Lysine
Please help. I'm not sure how I'm supposed to know these problems. My professor did not go over it in class.
To calculate the percent ionization at pH 7.4 for the R-group of histidine, you need to know the pKb value of the R-group and the pH of the solution.
The percent ionization can be calculated using the Henderson-Hasselbalch equation:
Percent ionization = (Concentration of ionized form / Total concentration of R-group) * 100
The pKb value of the R-group of histidine is given as 6.0. Since pKb is the negative logarithm of the equilibrium constant for the base dissociation, you can convert it to Kb:
Kb = 10^(-pKb)
Kb = 10^(-6.0)
Next, you need to calculate the ratio of the concentration of the ionized form to the total concentration of the R-group. This can be determined by assessing the pH of the solution in relation to the pKb.
If the pH is less than the pKb, the majority of the R-group will be in its basic, non-ionized form. If the pH is greater than the pKb, the majority of the R-group will be in its conjugate acid, ionized form.
In this case, the pH of the solution is 7.4, which is higher than the pKb of 6.0. Therefore, the R-group of histidine will be mostly ionized at pH 7.4.
To calculate the percent ionization, substitute the values into the Henderson-Hasselbalch equation and solve:
Percent ionization = (Concentration of ionized form / Total concentration of R-group) * 100
Since the majority of the R-group will be in the ionized form, the percent ionization will be close to 100%.
Moving on to the net charge of the given amino acids at pH 7.4:
1. Glycine (G) - Glycine has no additional functional groups besides the amino and carboxyl groups. At neutral pH, the amino group gains a proton (H+) and becomes positively charged, while the carboxyl group loses a proton and becomes negatively charged. Therefore, the net charge of glycine at pH 7.4 is 0.
2. Serine (S) - Serine has an additional hydroxyl group (-OH). At neutral pH, the amino group gains a proton (H+) and becomes positively charged, while the carboxyl group loses a proton and becomes negatively charged. The hydroxyl group can lose a proton as well, making the net charge of serine at pH 7.4 0.
3. Glutamic Acid (E) - Glutamic acid has an additional carboxyl group (-COOH). At neutral pH, both the amino group and the additional carboxyl group gain a proton (H+) and become positively charged. Therefore, the net charge of glutamic acid at pH 7.4 is -1.
4. Lysine (K) - Lysine has an additional amino group (-NH2). At neutral pH, both the amino groups gain a proton (H+) and become positively charged. Therefore, the net charge of lysine at pH 7.4 is +2.
Remember, these calculations are based on the pKa values of the functional groups in the amino acids, and the effect of pH on their ionization. It is important to have knowledge of the functional groups present in each amino acid and their respective pKa values to accurately determine the net charge at a given pH.