a. What is the net charge on valine at pH 1?
b. at pH 7?
c. at pH 14?
To determine the net charge on valine at different pH levels, we need to consider the pKa values of the amino acids involved in the valine molecule.
Valine has three ionizable functional groups: the alpha-amino group (NH3+), the alpha-carboxyl group (COOH), and the side chain amino group (NH2).
a. At pH 1: At very low pH, all three functional groups are protonated and will carry a positive charge. Since the pH is below the pKa values of the functional groups, the valine molecule will be in its fully protonated form. Therefore, the net charge on valine at pH 1 will be +3.
b. At pH 7: At neutral pH, the alpha-amino group and the alpha-carboxyl group will be mostly deprotonated and carry no charge. The pKa values of these functional groups are around 9 and 2, respectively. However, the side chain amino group has a pKa value of around 9.7, so it will be partially protonated and carry a positive charge. Therefore, the net charge on valine at pH 7 will be +1.
c. At pH 14: At high pH, all three functional groups will be deprotonated and carry no charge. Since the pH is above the pKa values of all the functional groups, the valine molecule will be in its fully deprotonated form. Therefore, the net charge on valine at pH 14 will be 0.
Remember, the pKa values used are approximate values and can vary slightly depending on the specific conditions.