Complexes solution-phase-based noncovalent

One approach that can overcome immobilization-related problems involves the use of antigen (or ligand) covalently bound to biotin (Ag-B). The Ag-B is added to the solution at the desired concentration to allow solution-phase binding, so that the selection process is based mainly on the differential affinity of each antibody or other binding molecule. The mixture is then added to strepavidin-coated beads or wells, where the formation of the biotin-strepavidin noncovalent complex allows the selection of high affinity antigen-binding molecules. 

There are no unanimous clear-cut answers. In an excellent recent review of ESI-MS of noncovalent complexes, Loo5 states that, There are three camps of opinion believers, non-believers and undecided, based on their personal experience. There are reports in the literature that demonstrate a good correlation between the solution and gas phase properties, while others report on discrepancies between the results obtained in solution and by mass spectrometry. 

In positive-mode experiments, Mirza and Chait showed that the charge-state distribution was influenced by the type of anion (conjugate base) that was used to acidify the solution. These authors considered that the anion remained associated with the multiply protonated protein in solution and that charge removal could occur via dissociation of the neutral acid (anion plus proton in tow) in the later moments of the droplet lifetime. Shortly afterwards, LeBlanc et al. proposed that neutral nitrogen bases that were noncovalently attached to gramicidin S peptide molecules also could serve to remove charge as the complex underwent collisions in the gas phase. These conclusions were reiterated by Hiraoka et al. in their work with amino acids. 

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