Simultaneous detection of multiply and singly phosphorylated peptides using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is challenging because of suppression effects during ionization. In oder to overcome this problem, this study presents a new approach to improve the detection of phosphopeptides by stepwise enrichment using polyarginine-coated (PA-coated) and titanium dioxide-coated (TiO2-coated) nanodiamonds for fractionation of multiply and singly phosphorylated peptides prior to on-probeMALDI MS analysis. The feasibility of this approach was demonstrated using synthetic peptides containing different numbers of phosphate groups, tryptic digests of α-casein, β-casein, and complex protein mixtures. The high specificity of the approach is shown in its effective enrichment and fractionation of phosphopeptides from the digest of β-casein and bovine serum albumin at a molar ratio as low as 1:1000, which out-performs the commercial Fe3+-IMAC and TiO2 isolation kits. It offers a simple and effective alternative for the fractionation and identification of multiply and singly phosphorylated peptides by MALDI MS and allows for deduction of more information from limited starting materials.
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