Chemical structures of 3-[(trimethylammonio)methyl]catechol (1), 4-hydroxy-3-[(trimethylammonio)methyl]catechol (2), 5-hydroxy-3-[(trimethylammonio)methyl]catechol (3), N,N-dimethylepinephrine (4), 6-hydroxy-N,N-dimethylepinephrine (5), N,N′-(2,3-dihydroxybenzyl)-N,N,N′,N′-tetramethyl-1,6-hexanediamine dibromide (DTH, 6), N,N′-(2,3-dihydroxybenzyl)-N,N,N′,N′-tetramethyl-1,10-decanediamine dibromide (DTD, 7), hemicholinium-15 (HC-15, 8) and hemicholinium-3 (HC-3, 9).