A modified kinetic method was used in combination with MIKE spectrometry to obtain the value of 191.7 +/- 2.3 kcal/mol for the gas phase basicity of the protonated octapeptide RPPGFSPF (des-Arg(9)-bradykinin). This was found to be 63.1 +/- 3.0 kcal/mol lower than the gas phase basicity of the neutral peptide (254.8 +/- 2.0 kcal/mol) determined by the kinetic method on the same instrument. Molecular mechanics studies support the conclusion that this difference reflects both conformational changes and interproton electrostatic repulsion. The experiments and the molecular modeling calculations suggest that protons in multiply charged peptides are not necessarily located on the most basic sites, if the alternative of greater intercharge distance is available.