High resolution analysis of protein phosphorylation using capillary isoelectric focusing - electrospray ionization - mass spectrometry.

On-line capillary isoelectric focusing (CIEF)-electrospray ionization - mass spectrometry (ESI-MS) as a two-dimensional separation system is employed for high resolution analysis of ovalbumin phosphorylation. On the basis of their differences in isoelectric point (pI), the mono- and diphosphoovalbumins are separated and resolved in CIEF. The focused protein zones of mono- and diphosphoovalbumins are eluted by combining gravity with cathodic mobilization. At the end of the CIEF capillary, the mobilized ovalbumin zones are analyzed by mass spectrometry coupled on-line to an electrospray interface with a coaxial sheath flow configuration. Additional ovalbumin variants within each of the mono- and diphosphoovalbumins, differing in their molecular masses due to glycosylation microheterogeneity, are easily distinguished by ESI-MS.