EXPERIMENTAL-MEASUREMENT OF COULOMB ENERGY AND INTRINSIC DIELECTRIC POLARIZABILITY OF A MULTIPLY PROTONATED PEPTIDE ION USING ELECTROSPRAY-IONIZATION FOURIER-TRANSFORM MASS-SPECTROMETRY

A method to quantitatively measure the Coulomb energy in gas-phase ion s with two or more protons by measuring gas-phase basicity is demonstr ated. From these measurements and estimates of distance between charge sites, the intrinsic dielectric polarizability (epsilon(r)) of isolat ed peptide and protein ions can be obtained. This method is demonstrat ed with the peptide gramicidin S; for the (M + 2H)(2+) ion, we find th e Coulomb energy is >27.9 kcal/mol. The distance between the two charg es on this ion (9.5 Angstrom) is determined from the lowest energy con figuration obtained by molecular modeling. The proposed gas-phase stru cture of this ion is consistent with experimentally determined rates o f H/D exchange for the singly and doubly protonated molecular ions wit h D2O. From the Coulomb energy and distance between charges, an upper limit of epsilon(r) < 1.2 is obtained. This is decidedly less than the theoretical value of 2-4 predicted for peptides and proteins.