Conformations of biopolymers in the gas phase: a new mass spectrometric method

A method is developed for measuring collision cross sections of gas-phase b iomolecules using a slightly modified commercial triple quadrupole instrume nt. The modifications allow accurate stopping potentials to be measured for ions exiting the collision region of the instrument. A simple model allows these curves to be converted to cross sections. In order to account for ce rtain poorly defined experimental parameters (exact ion energy, absolute pr essure in the collision cell, etc.) variable parameters are included in the model. These parameters are determined on a case by case basis by normaliz ing the results to the well known cross section of singly charged bradykini n, Two relatively large systems were studied (cytochrome c and myoglobin) s o comparisons could be made to literature values. A number of new peptide s ystems were then studied in the 9 -14 residue range. These included singly and doubly charged ions of luteinizing hormone releasing hormone (LHRH) sub stance P, and bombesin in addition to bradykinin. The experimental cross se ctions were in very good agreement with predictions from extensive molecula r dynamics modeling. One interesting result was the experimental observatio n that the cross section of the doubly charged ions of LHRH, substance P, a nd bombesin were all smaller than those of the corresponding singly charged ions. Molecular dynamics did not reproduce this result, predicting doubly charged cross sections of the same magnitude or slightly larger than for th e singly charged species. The experimental results appear to be correct, ho wever. Possible shortcomings in the modeling procedure for multiply charged ions were suggested that might account for the discrepancy. (C) 2000 Elsev ier Science B.V.