Q1: Which of the following pairs of amino acids would carry a negative charge on their side chain at pH 8.0?

ANS:D - Aspartate & Glutamate

pKr of Glutamate is 4.25.

pKr of Aspartate is 3.65.

If pH, for example the pH of 8, is greater than the pKr, then the amino acid loses a proton and bears its negative charge.

If the pH is less than the pKr value, then the amino acids keeps its proton, bearing a neutral charge.

PH of 8 is greater than the pKr values of Glu and Asp, so they both lose a proton and bear that negative charge on their respective are group.