Competitive binding to the oligopeptide binding protein, OppA: In-trap cleanup in an Fourier transform ion cyclotron resonance mass spectrometer

This communication demonstrates that gentle infrared laser heating can remo ve unwanted buffer adducts from a gas-phase protein complex without dissoci ating the complex itself. Specifically, noncovalent complexes of the oligop eptide-binding protein, OppA, bound to either (Ala)(3) or LysTrpLys were el ectrosprayed from aqueous buffer solution into a 9.4 tesla Fourier transfor m ion cyclotron resonance mass spectrometer. Ln addition to the intact comp lexes, several additional buffer adduct species were produced under the con ditions of the experiment. Irradiation of the trapped ion population with a continuous-wave infrared CO2 laser at relatively low power (2.5 W) for 1 s dissociated the buffer adducts but retained the intact protein:peptide com plexes. Adduct-free complex(es) were then readily identified, and signal-to -noise ratio also increased by an order of magnitude because the same numbe r of protein ions are distributed over fewer species. Higher IR power (5 W for 1 s) dissociated the adduct-free complex(es) without internal fragmenta tion. The present in-trap clean-up technique may prove especially useful fo r identifying and screening the combinatorial library ligands most strongly bound to a receptor in the gas phase. (C) 2000 American Society for Mass S pectrometry.