Conformation of native, reduced and [5-55]Ala bovine pancreatic trypsin inhibitor in the gas phase.

Collision cross sections have been measured for gas phase ions of native oxidized bovine pancreatic trypsin inhibitor (BPTI), native reduced BPTI and a mutant form of BPTI containing a single disulphide bond between residues 5 and 55 ([5-55]Ala BPTI). Cross sections for [5-55]Ala BPTI and reduced BPTI were 9% and 17% greater respectively than those for native BPTI. Cross sections for native BPTI were smaller than previous estimates from the crystal structure but in reasonable agreement with values calculated from the radius of gyration determined by x-ray scattering from solution BPTI. The increase in cross section for reduced BPTI over native BPTI is similar to that seen in molecular dynamics simulations. The results show that the disulphide bonds of BPTI contribute to the folding of the gas phase ions, but that even in the absence of disulphide bonds the protein ions maintain compact structures. Comparisons to relative areas calculated from hydrodynamic radii of BPTI in solution suggest that when disulphide linkages are removed, BPTI in the gas phase unfolds less than BPTI in solution.