Hydroxyapatite affinity chromatography for the highly selective enrichment of mono- and multi-phosphorylated peptides in phosphoproteome analysis.

The most challenging analytical task facing phosphoproteome determination requires the isolation of phosphorylated peptides from the myriad of unphosphorylated species. In the past, several strategies for phosphopeptide isolation have been proposed in combination with subsequent mass spectrometric investigations. Among these techniques, immobilized metal affinity chromatography and titanium dioxide have been recognized as the most effective. Here, we present an alternative method for the enrichment of phosphopeptides based on hydroxyapatite (HAP) chromatography. By taking advantage of the strong interaction of HAP with phosphate and calcium ions, we developed an efficient method for the selective separation and fractionation of phosphorylated peptides. The effectiveness and efficiency of recovery for this procedure was assayed using tryptic digests of standard phosphorylated protein mixtures. Based on the higher affinity of multi-phosphorylated peptides for HAP surfaces, the introduction of a phosphate buffer gradient for stepwise peptide elution resulted in the separation of mono-, di-, tri-, and multi-phosphorylated peptides. Thus, we demonstrated that this technique is highly selective and independent of the degree of peptide phosphorylation.