Hydroiodic acid attachment kinetics as a chemical probe of gaseous protein ion structure: Bovine pancreatic trypsin inhibitor

The kinetics of attachment of hydroiodic acid (HI) to the (M + 6H)6+ ions of native and reduced forms of bovine pancreatic trypsin inhibitor (BPTI) in the quadrupole ion trap environment are reported. Distinctly nonlinear (pseudo first-order) reaction kinetics are observed for reaction of the native ions, indicating two or more noninterconverting structures in the parent ion population. The reduced form, on the other hand, shows very nearly linear reaction kinetics. Both forms of the parent ion attach a maximum of five molecules of hydroiodic acid. This number is expected based on the amino acid composition of the protein. There is a total of 11 strongly basic sites in the protein (i.e., six arginines, four lysines, and one N-terminus). An ion with protons occupying six of the basic sites has five available for hydroiodic acid attachment. The kinetics of successive attachment of HI to the native and reduced forms of BPTI also differ, particularly for the addition of the fourth and fifth HI molecules. A very simple kinetic model describes the behavior of the reduced form reasonably well, suggesting that all of the neutral basic sites in the reduced BPTI ions have roughly equal reactivity. However, the behavior of the native ion is not well-described by this simple model. The results are discussed within the context of differences in the three-dimensional structures of the ions that result from the presence or absence of the three disulfide linkages found in native BPTI. The HI reaction kinetics appears to have potential as a chemical probe of protein ion three-dimensional structure in the gas phase. Hydroiodic acid attachment chemistry is significantly different from other chemistries used to probe three-dimensional structure and hence, promises to yield complementary information.