On-Line Bioaffinity-Electrospray Mass Spectrometry for Simultaneous Detection, Identification, and Quantification of Protein-Ligand Interactions

We describe here an on-line combination of a surface acoustic wave (SAW) biosensor with electrospray ionization mass spectrometry (SAW-ESI-MS) that enables the direct detection, identification, and quantification of affinity-bound ligands from a protein-ligand complex on a biosensor chip. A trapping column was used between the SAW-biosensor and the electrospray mass spectrometer equipped with a micro-guard column, which provides simultaneous sample concentration and desalting for the mass spectrometric analysis of the dissociated ligand. First applications of the on-line SAW-ESI-MS combination include (1), differentiation of beta-amyloid (A beta) epitope peptides bound to anti-A beta antibodies; (2), the identification of immobilized Substance P peptide-calmodulin complex; (3), identification and quantification of the interaction of 3-nitrotyrosine-modified peptides with nitrotyrosine-specific antibodies; and (4), identification of immobilized anti-alpha-synuclein human alpha-synuclein complex. Quantitative determinations of protein-ligand complexes by SAW yielded dissociation constants (K-D) from micro-to low nanomolar sample concentrations. The on-line bioaffinity-ESI-MS combination presented here is expected to enable broad bioanalytical application to the simultaneous, label-free determination and quantification of biopolymer-ligand interactions, as diverse as antigen-antibody and lectin-carbohydrate complexes. (J Am Soc Mass Spectrom 2010, 21, 1643-1648) (C) 2010 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry