Thermodynamic data for the protonation reaction of the side chain of some amino acids.

Internal energy change (), proton affinity (PA), and gas-phase basicity (GPB) values (in kJ/mol) are reported for two limiting situations: (i) protonated and deprotonated species in their lowest-energy conformer (labeled as “folded”), and (ii) in an extended, all-trans conformer (labeled as “linear”). Differences between these quantities calculated for the folded and linear conformations are also given.