Thermodynamic and electronic and electron spin resonance spectroscopic investigation of the coordinating properties of 4-azaoctane-1,8-diamine (spermidine) in aqueous solution

The thermodn. of Cu2+ complex formation of spermidine [124-20-9] were studied to det. the reason for the exchange of Cu(II)-contg. proteins with spermidine according to a nonimmunogenic mechanism for retardation of tumor growth. Of the 2 complexes found in aq. soln. at physiol. pH, [Cu(HL)]3+ (L = spermidine) [66139-45-5] and [CuL]2+ (L = spermidine) [66139-46-6], only the latter has the possibility of exchange. The high stability of [CuL]2+ arose from a favorable enthalpy contribution due to the full coordination of the 3 N atoms in a tetragonally distorted octahedral structure. ESR and electronic spectra showed that [CuL]2+ contained a 7-membered chelate ring fused to a 6-membered ring and that the primary N of the tetramethylene chain was protonated in [Cu(HL)]3+. The thermodn. of protonation of spermidine were studied and the successive enthalpy and entropy values were explained on the basis of a tautomeric process which exists in sym. triamines.