Final answer:
Most leucine molecules exist in the zwitterion form at physiological pH of 7.4, as the amino group is protonated and the carboxyl group is deprotonated, resulting in a neutral molecule.
Step-by-step explanation:
The percentage of leucine molecules in the zwitterion form is highly dependent on the pH of the solution in which the leucine is dissolved. At physiological pH, which is typically around 7.4, most of the amino acids, including leucine, exist predominantly in their zwitterionic form. This is because at this pH, the amino group (NH3+) is protonated and carries a positive charge, while the carboxyl group (COO-) loses a proton and carries a negative charge, resulting in the overall neutral zwitterion form of the amino acid.