Amine-functionalized sol-gel-based lab-in-a-pipet-tip approach for the fast enrichment and specific purification of phosphopeptides in MALDI-MS applications

Amine-functionalized sol-gels were investigated for the enrichment and purification of phosphopeptides from digested protein mixture solutions. Tetramethylorthosilicate (TMOS) and N'3-(trimethoxysilyl)-propyl]-diethylenetriamine (TPDA) were used in a 1:1 mole ratio in the production of amine-functionalized sol-gels. The sol-gel network was then used for phosphopeptide enrichment. Phosphopeptide enrichment onto the synthesized amine-functionalized sol-gels was performed using an enolase digested peptide mixture, a β-casein digested peptide mixture, as well as these digested peptide mixtures contaminated 50-fold with bovine serum albumin (BSA). Moreover, phosphopeptide enrichment was successfully performed using nonfat milk as a highly contaminated and complex material. In each phosphopeptide enrichment and purification process, only phosphopeptides were enriched and separated from the other digested peptides. Phosphopeptides were adsorbed onto the amine-functionalized sol-gels at pH 4.0 and eluted at pH 1.0 using trifluoroacetic acid (TFA). For phosphopeptide analysis by MALDI-MS, a 2,5-dihydroxybenzoic acid matrix containing 1.0% phosphoric acid was used to overcome the degradation of phosphopeptides and provide high intensity phosphopeptide protonated molecular ion signal intensities. It was also found that phosphopeptide detection limits were improved to approximately 10 femtomoles. For rapid and specific phosphopeptide enrichment and purification, sol-gel materials were placed in a 10 μL pipet tip with glass wool on either side. Phosphopeptide enrichment from digested peptide mixtures was performed in a very short time (less than 1 min) at subpicomole levels using this novel lab-in-a-pipet-tip approach.