Citrate boosts the performance of phosphopeptide analysis by UPLC-ESI-MS/MS

Incomplete recovery from the LC column is identified as a major cause for poor detection efficiency of phosphopeptides by LC-MS/MS. It is proposed that metal ions adsorbed on the stationary phase interact with the phosphate group of phosphopeptides via an ion-pairing mechanism related to IMAC (IMAC: immobilized metal ion affinity chromatography). This may result in their partial or even complete retention. Addition of phosphate, EDTA or citrate to the phosphopeptide sample was tested to overcome the detrimental phosphopeptide suppression during gradient LC-MS/MS analysis, while the standard solvent composition (water, acetonitrile, formic acid) of the LC system was left unchanged. With the use of UPLC, a citrate additive was found to be highly effective in increasing the phosphopeptide detection sensitivity. Addition of EDTA was found to be comparable with respect to sensitivity enhancement, but led to fast clogging and destruction of the spray needle and analytical columns due to precipitation. In contrast, a citrate additive is compatible with prolonged and stable routine operation. A 50 mM citrate additive was tested successfully for UPLC-MS analysis of a commercial four-component phosphopeptide mixture, a tryptic β-casein digest, and several digests of the 140 kDa protein SETDB1. In this protein, 27 phosphorylation sites could be identified by UPLC-MS/MS using addition of citrate, including the detection of several phosphopeptides carrying 3-5 pSer/pThr residues, compared to identification of only 10 sites without citrate addition. A 50 mM citrate additive particularly increases the recovery of multiply phosphorylated peptides, thus, extending the scope of phosphopeptide analysis by LC-MS/MS.