A peptide's serine residue has a pKa of 13.0 in a standard environment. If the microenvironment shifts the pKa to 12.0, how does this affect the charge at pH 7.4?

• A. The serine residue becomes positively charged.
• B. The serine residue becomes zwitterionic.
• C. The serine residue remains neutral.
• D. The serine residue becomes negatively charged.

Answer: (C)