Calculated apparent gas-phase basicity as a function of charge state for arginine-containing peptides (RG)3, +; R6, ▴; (RRGG)3, •; R6G6, *; (RG)6, ○; (RG)10, ▪; (RĀ), R, □; (RG)15, ⋄; (RĀ)14R, ⊗. The GBapp of the n+ charge state reflects the proton transfer reactivity of the (n + 1)+ charge state ion. The dashed line indicates that GB of methanol (174.1 kcal/mol). [The GBapp of the 8 + charge state of (RG)15 is 0.5 kcal/mol greater than the GB of methanol.]