Interaction of myo-inositol hexakisphosphate with biogenic and synthetic polyamines.

Within all the eukaryotic cells myo-inositol phosphates (InsPs) are an important group of biomolecules that are potentially related to signaling functions. The most abundant member of this family in nature is InsP6 (phytate, L(12-) in its fully deprotonated form). The complicated chemical behavior of this molecule demands a great effort to understand its function in the cell medium. In this work we follow our earlier studies on the interaction of InsP6 with metal cations by inclusion of polyamines (both biogenic and synthetic) as potential agents to produce stable adducts. The stability constants of InsP6-amine adducts and the relevant thermodynamic parameters ΔG°, ΔH°, and ΔS° were determined at 37.0 °C and 0.15 M ionic strength by means of potentiometric titrations and isothermal titration calorimetry (ITC). The biogenic amines studied were 1,4-diaminobutane (putrescine, put), 1,5-diaminopentane (cadaverine, cad), N-(3-aminopropyl)-1,4-diaminobutane (spermidine, spd), N,N'-bis(3-aminopropyl)-1,4-diaminobutane (spermine, spm), and 1-(4-aminobutyl)guanidine (agmatine, agm), while the synthetic models of longer polyamines were 1,19-dimethyl-1,4,7,10,13,16,19-heptaazanonadecane (Me2hexaen), 1,22-dimethyl-1,4,7,10,13,16,19,22-octaazadocosane (Me2heptaen), 1,25-dimethyl-1,4,7,10,13,16,19,22,25-nonaazapentacosane (Me2octaen) and N,N'-bis(3-aminopropyl)-1,3-propanediamine (3,3,3-tet). With the aid of molecular modeling, we also studied the structural aspects of molecular recognition in operation. The final result is a balance between many parameters including charge of the species, flexibility of the amines, H-bonds in the adduct, and desolvation processes.