Binding of polyanions by biogenic amines. III. Formation and stability of protonated spermidine and spermine complexes with carboxylic ligands

The formation and stability of protonated spermidine and spermine-carboxylic ligand complexes (seven systems) were studied potentiometrically (H+-glass electrode). ALH(r) species are formed (A = spermidine, spermine, Lz- = acetate, malonate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate; r = 1...m + n - 1, where m and n are the maximum degree of protonation of the amine and of the carboxylic ligand, respectively), and their stability is a function of charges involved in the formation reaction. For the equilibrium H(i)A(i+) + HjL(j-z) = ALH(i+j)((i+j-z)) the linear relationship log K-ij = 0.46 zeta (zeta = \i(z - j)\) was found. By means of this simple equation a mean free energy value can be obtained per salt bridge (n), namely - Delta G degrees = 5.25 +/- 0.15 kJ mol(-1) n(-1). Species formed by the highest charged ligands are quite stable (K > 10(3) mol(-1) dm(3)) and potentially play an important role in the speciation of biofluids, as shown by speciation diagrams and simulated experiments. (C) 1999 Elsevier Science B.V. All rights reserved.