Gas-phase protonation thermochemistry of arginine

The gas-phase basicity (GB), proton affinity (PA), and protonation entropy (Δ_pS^o(M) = S^o(MH⁺) - S ^o(M)) of arginine (Arg) have been experimentally determined by the extended kinetic method using an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer. This method provides GB(Arg) = 1004.3 ± 2.2 (4.9) kJ·mol^-1 (indicated errors are standard deviations, and in parentheses, 95% confidence limits are given). Consideration of previous experimental data using a fast atom bombardment ionization tandem sector mass spectrometer slightly modifies these estimates since GB(Arg) = 1005.9 ± 3.1 (6.6) kJ·mol^-1. Lower limits of the proton affinity, PA(Arg) = 1046 ± 4 (7) kJ·mol ^-1, and of the "protonation entropy", Δ _pS^o(Arg) = S^o(ArgH⁺) - S ^o(Arg) = -27 ±7 (15) J·mol^-1·K ^-1, are also provided by the experiments. Theoretical calculations conducted at the B3LYP/6-311 +G(3df,2p)//B3LYP/6-31 +G(d,p) level, including 298 K enthalpy correction, predict a proton affinity value of ca. 1053 kJ·mol^-1 after consideration of isodesmic proton-transfer reactions with guanidine as the reference base. Computations including explicit treatment of hindered rotations and mixing of conformers confirm that a noticeable entropy loss does occur upon protonation, which leads to a theoretical Δ_pS^o(Arg) term of ca. -45 J·mol^-1·K^-1. The following evaluated thermochemical parameter values are proposed: GB(Arg) = 1005 ± 3 kJ·mol^-1; PA(Arg) = 1051 ± 5 kJ·mol ^-1, and Δ_PS^o-(Arg) = -45 ± 12 J·mol^-1·K^-1.