Characterization of Protein−Hapten Conjugates. 2. Electrospray Mass Spectrometry of Bovine Serum Albumin−Hapten Conjugates

Fifteen hapten−bovine serum albumin (BSA) conjugates were prepared from five commercially available activated haptens. Each hapten was coupled to BSA at three different ratios. The conjugates were characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS−PAGE) and two mass spectrometry (MS) methods: matrix-assisted laser desorption ionization (MALDI) and liquid chromatography−electrospray ionization (LC−ESI). SDS−PAGE was useful in detecting protein cross-linking, but not in assessing hapten density. MALDI-MS and LC−ESI-MS gave comparable qualitative results, but LC−ESI-MS provided a clearer representation of the distribution of hapten−protein species present in the conjugates. Conjugate species substituted with up to 25 haptens per BSA were recorded by LC−ESI-MS.