MULTIPLY-PROTONATED PROTEIN IONS IN THE GAS-PHASE - CALCULATION OF THE ELECTROSTATIC INTERACTIONS BETWEEN CHARGED SITES

A macroscopic approach for calculation of electrostatic interactions i n proteins, initially developed for solution, was extended to describe isolated multiply-charged protein ions in the gas phase. It was combi ned with a Monte Carlo algorithm for determination of the most probabl e protonated sites for a different charge. Several quantities characte rizing the behavior of protein ions in vacuum were calculated for nati velike lysozyme and ubiquitin. Among these are the apparent gas-phase basicity for various charge states, as well as the intrinsic basicitie s and the probabilities of protonation of individual sites. The contri butions of the intramolecular solvation term and the peptide dipole-ch arge interactions to the intrinsic gas-phase basicity of each site wer e estimated. It was shown that the peptide dipoles may essentially inf luence the intrinsic gas-phase basicity of individual protonation site s. The approach can be successfully used to probe electrostatic intera ctions of gas-phase protein ions, provided information on the three-di mensional structure of these ions is available.