Measuring the isoelectric point of peptides by potentiometric titration

Purpose The isoelectric point (pI) is the pH at which biological amphoteric molecules such as peptides and proteins carry no net electrical charge. Biopharmaceutical drug formulations that are intended for subcutaneous injection are often prepared at about pH 4 to enhance shelf life but when they are injected into the body the pH swiftly moves to 7.4. If the isoelectric point is between these pH values, there is increased risk of precipitation and aggregation after injection, which could diminish bioavailability. Isoelectric point values can be calculated precisely for molecules with only two ionizable groups, but are harder to estimate as the number of acidic and basic groups increases. Isoelectric point values are traditionally measured by electrophoresis, but here we present the measurement by potentiometric titration. This method has the advantage of high precision.