(A) kcat/Km-pH profiles for PAO with N,N'-dibenzyl-1,4-diaminobutane (●), N,N'-bis(4-CH3O-benzyl)-1,4-diaminobutane (□), and N,N'-bis(4-CF3-benzyl)-1,4-diaminobutane (▲) as substrates. The average error in the data is 15%. The lines are from fits of the data to eq 1. (B) pH distribution of N,N'-benzyl-1,4-diaminobutane with zero, one, or two protonated nitrogens.