A new screening method to detect protein῍ligand interactions has been developed through a combination of mixed self-assembled monolayer (SAM) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The mixed SAM surface was prepared on flat gold plate from a mixture of 20-mercapto-3,6,9trioxaicosanol and a ligand-bound derivative of 29-mercapto-3,6,9,12,15,18-hexaoxanonacosanol. This surface is very suitable for observing the molecular interaction accurately, because there

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... is very low levels of steric hindrance between immobilized ligands and the non-charged hydroxyl groups of polyethylene glycol which resists from the non-specific adsorption of biomolecules. Because the surface coated with the mixed SAM is washable with water, it can be used directly as a probe for MALDI-MS, a technique commonly used for the annotation of protein function. As an example we report here the selective detection of the molecular interaction between a Src homology 3 (SH3) domain of human p85a phosphatidylinositol 3-kinase (PI3K) and its ligand peptide (H-RKLPPRPAF-OH) attached covalently to mixed SAM using MALDI-MS.