Final answer:
A protein molecule at a pH above its isoelectric point will have a net negative charge due to excess negative charges from deprotonated amino acid side chains. The option (B) is correct.
Step-by-step explanation:
To answer the question of how a protein molecule behaves at a pH above its isoelectric point (pI), we must understand that the isoelectric point of a protein is the pH at which the protein has no net charge. At this point, the amount of positive charges on the amino acid chains is equal to the amount of negative charges, and the molecule behaves as a zwitterion. When the pH is higher than the protein's isoelectric point, the protein tends to take on a net negative charge. This happens because, at higher pH levels, protons are removed from the protein, leading to an excess of negative charges.
This is because the proteins will gain more negative charges due to the deprotonation of amino acid side chains, which outweigh the positive charges present. Thus, the correct answer to the question is b) Net negative charge.