Abstract
The relative non‐covalent binding behavior of two small molecule ligands to the tetrameric protein streptavidin was investigated by electrospray ionization mass spectrometry (ESI‐MS). An extended m/z range quadrupole mass spectrometer was employed to observe the intact multimeric form of the protein and to probe the relative stabilities of the tetrameric protein‐ ligand complexes in the gas phase. Various protein:ligand molar ratio concentrations and incubation times were studied for the ligands biotin (K~d~ ≈ 10^−15^ M) and iminobiotin (K~d~ ≈ 10^−7^ M), in combination with the adjustment of variables in the ESI atmosphere‐vacuum interface region. Positive‐ion ESI‐MS of a 1:7 molar ratio streptvidin–biotin sample produced peaks corresponding to the intact tetrameric complex (16+ to 14+ charge states) with four ligands binding to the active from of the protein. Only the expected specific non‐covalent complexes of four ligand molecules binding to the protein tetramer were detected, consistent with known solution behavior and without the appearance of any random aggregation. However, under identical interface conditions, complete iminobiotin binding to the protein was not observed, even when higher ligand concentrations, longer complexation times and gentler interface conditions were employed. The thermally induced dissociation (TID) of the non‐covalent complexes was studied by applying more severe conditions in the ESI interface region. In addition, observation of an intact streptavidin–biotinylated oligonucleotide non‐covalent complex (17– to 14– tetrameric charge states) by negative‐ion ESI‐MS at m/z 4500–5000 is presented. The results show that the complexes observed in the mass spectrometer are representative of those that exist in solution, under the ESI conditions employed, and that the ability to observe such complexes is at least qualitatively related to stability in solution.