METHODS Mass spectrometric analyses were conducted in a quadrupole time-of-flight (Q- TOF) spectrometer fitted with electrospray ionization source. Protonation sites have been proposed on the basis of the gas-phase basicity, proton affinity, atomic charges, and a molecular electrostatic potential map obtained on the basis of the quantum chemistry calculations at the B3LYP/6-31+ G (d, p) and G2 (MP2) levels. RESULTS Analysis of the ...