Isoelectric Point (IEP) of Proteins

With the Nicomp 380 ZLS it is possible to not only size proteins but also determine their isoelectric point (IEP). Ampules of bovine serum albumin (BSA) were purchased from Sigma Aldrich and diluted 1:100 with deionized water. Figure A below shows the volume-weighted mean particle diameter to be 5.5 nm. Aggregates of ~25nm were also detected. For determination of the IEP, this BSA solution was brought to pH 8 with 0.1M KOH and titrated with 0.01M HCl to a final pH of 3.75. Zeta potential readings were taken and the results are shown in Figure B. The IEP for BSA was found to be at pH 5.07.

Figure A

Figure B