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, L(z-)=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+)+H(j)L((j-z))=ALH(i+j)((i+j-z)) the linear relationship logK(ij)=0.46 zeta (zeta=mid R:i(z-j)mid R:) was found. By means of this simple equation a mean free energy value can be obtained per salt bridge (n), namely -DeltaG 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.