net charge

Meaning: Net charge is the total of all positive and negative charges on a molecule at a given pH. In amino acids and peptides, each ionizable group counts: it is mostly protonated when pH is below its pKa and mostly deprotonated when pH is above it. A neutral amino acid is +1 at low pH (NH₃⁺, COOH), a zwitterion with net charge 0 near its pI, and −1 at high pH (NH₂, COO⁻).

Equation: net charge = (number of positive charges) − (number of negative charges), in units of the elementary charge e = 1.60 × 10⁻¹⁹ C. For a neutral amino acid, pI = (pKa1 + pKa2)/2, where pKa1 and pKa2 are unitless.

On the MCAT: You are given pKa values and a pH and asked for the net charge, or asked which way a peptide moves in gel electrophoresis or ion-exchange chromatography. Below its pI it is positive and moves toward the cathode; above its pI it is negative and moves toward the anode. If a side chain ionizes, average the two pKa values on either side of the neutral form to find the pI.

Common confusion: A net charge of zero does not mean there are no charges. A zwitterion carries both a + and a − charge. Also, the pI is not 7 in general: acidic amino acids have a low pI and basic ones have a high pI.