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

摘要

A method to quantitatively measure the Coulomb energy in gas-phase ions with two or more protons by measuring gas-phase basicity is demonstrated. From these measurements and estimates of distance between charge sites, the intrinsic dielectric polarizability (epsilon(r)) of isolated peptide and protein ions can be obtained. This method is demonstrated with the peptide gramicidin S; for the (M + 2H)(2+) ion, we find the Coulomb energy is >27.9 kcal/mol. The distance between the two charges on this ion (9.5 Angstrom) is determined from the lowest energy configuration obtained by molecular modeling. The proposed gas-phase structure of this ion is consistent with experimentally determined rates of H/D exchange for the singly and doubly protonated molecular ions with 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. [References: 65]