Apparent proton affinities of highly charged peptide ions.

The apparent proton affinities (PA) of various charge states of three highly basic peptides [(KAP)10, (KAP)8, (KAA)8] were measured by the "bracketing" method. For all three peptides the apparent PA decreases as the charge state increases and the magnitude of the decrease is consistent with an increase in coulombic repulsion in the highly protonated species. Based on a simple electrostatic model, theoretical PAs were predicted for each charge state and the values for (KAP)10 and (KAP)8 were within 10 kcal/mol of the experimental values. The maximum charge state of these peptides was observed in all cases even when the most volatile solvent was sufficiently basic to deprotonate that charge state in the gas phase. In solution (KAP)8 exhibits a random coil secondary structure while (KAA)8 exhibits an α-helix structure. Comparison of measured and calculated apparent PAs suggests that (KAP)8 retains its solution random coil structure in the gas phase and (KAA)8 retains the solution compact α-helix structure in the lower charge states but opens up to a β structure in the gas phase to minimize electrostatic repulsions in higher charge states.