Basilio Pispisa

Binding-induced conformational transition of sodium poly-L-glutamate by iron(III) complex ions in aqueous solution

The binding of pseudo-octahedral trans- and cis-FeIII complex ions by sodium poly-L-glutamate (PLG) and dextransulphate (DS) in aqueous solution at about pH 7 has been studied. Equilibrium dialysis and “phase-separation” data show that the affinity for the complex counterions by both polyelectrolytes follows the order trans-[Fe(tetpy)(OH)2]+ > cis-[Fe(pmen)(OH)2]+. Evidence of a specific site binding, leading to a marked “renaturation” effect on the charged polypeptide, is produced in the case of the trans-FeIII-quaterpyridine compound. The binding isotherm of the trans-complex + PLG system and the circular dichroism patterns of the polypeptide as a function of the bound-trans-complex to polymer-residue ratio were successfully treated by a two-state model for the polyelectrolyte and a preferential association of the complex ions to the helical conformation of the polymeric matrix. All these features are examined in the light of the structural characteristics of the interacting species. The implications of the different stereochemistry of the other complex ions studied on the binding process with PLG are also discussed.

Oxo-bridged binuclear iron(III) complexes with NN′-bis(2-methylpyridyl)ethylenediamine and 2,2′ : 6′,2″ : 6″,2‴-tetrapyridyl

Two oxo-bridged complexes of iron(III), having different configurations depending on the different conjugative capacity of the quadridentate nitrogen ligands used, have been isolated and characterized. They are formulated as cis-[{Fe(bmen)}2O(H2O)2][SO4]2·H2O and trans-[{Fe(tetpy)}2O][SO4]2·7H2O, in which pairs of iron(III) ions (S= 5/2) interact antiferromagnetically [bmen =NN′-bis(2-methylpyridyl)ethylenediamine and tetpy = 2,2′:6′,2″:6″,2‴-tetrapyridyl]. Evidence is produced that the dimeric form of both complexes also predominates in an aqueous medium. As the pH is increased, the stability between the oxo-bridged units of the two complexes in solution differs; this is briefly discussed in the light of the different stereochemical features of the molecu les.

Environmental control of reactions. Influence of poly(L-glutamate) on the kinetics of decomposition of hydrogen peroxide catalysed by quaterpyridineiron (III) complex ions

The decomposition of hydrogen peroxide catalysed by quaterpyridineiron(III) complex ions has been studied within the pH range 6.5–8. The reaction obeys total third-order kinetics with a partial order of two with respect to the substrate. At pH ≈ 7.6, where the reaction velocity exhibits a maximum asymptotic value, the apparent activation energy is 29.3±2.9 kJ mol–1. Addition of poly (L-glutamate)(PLG) in solution determines a change in the overall kinetics. In this case, the process follows a total second-order equation and has an apparent activation energy of 53.1±6.3 kJ mol–1. At a fixed concentration of complex, saturation phenomena are observed in all cases on increasing the concentration of substrate. As expected, the Lineweaver–Burk plot of the polymer-free complex-catalysed reaction is nonlinear, whereas it is linear when the complex + PLG system is used. At 25°C, the rate constant for the irreversible decomposition of the Michealis “complex”, which represents the rate-determining step, is k3= 4.9±0.2 and 2.7±0.2 s–1 for [Fe(tetpy) X2]n+ and [Fe(tetpy) X2]n++ PLG catalytic systems, respectively. Implications of the role played by the poly-peptide matrix as environmental controller of the catalysis under study are briefly discussed.

Stereochemistry and spin configuration of NN′-bis(2-pyridylmethyl)-ethyleneddiamine complexes of iron(III) in the solid state and in aqueous solution

High-spin [Fe(pmen)Cl2]Cl and low-spin [Fe(pmen)(OH2)(OH)][ClO4]2 complexes (pmen is the title ligand) have been prepared and characterized by spectroscopic and magnetic measurements. Electronic and e.p.r. spectra suggest that the complexes have a cis-type geometry both in the solid state and in aqueous solution. Peculiar features observed in solution are aquation of the cation of the chloride salt which effects electron pairing, whilst a change from low to high spin, accompanied by a structural rearrangement, occurs when the interaction of the complexes in aqueous solution with an optically active polyelectrolyte (Which does not absorb over the wave-length region explored), make possible an assignment of cis-α geometry to the high-spin form and cis-β geometry to the low-spin species, in agreement with predictions from qualitative steric considerations. Implications of stereochemical demands of the complexes on variation of the spin configuration of the iron are also discussed.

Circular dichroism studies on the interactions between tris (L-alaninato)cobalt(III) complexes and basic polyelectrolytes in solution

The circular dichroism spectra of the poly-L-lysine complexes with abc(–), abd(–), and abd(+)Co(L-ala-O)3 in water and isopropyl alcohol–water (1 : 1), at different pH values and over a range of complex to polymer molar ratios, have been measured. Evidence of a specific site binding is presented for abc(–)Co(L-ala-O)3 in the mixed solvent medium. Of the diastereoisomers used, only in the presence of this material does the macroion assume an α-helical conformation at pH values where the coil form normally predominates. The greater the complex to polymer ratio, the greater the proportion of material of α-helical conformation there is present. Directional modes in the binding, very likely involving hydrogen bonding interactions, are indicated by the changes in the visible c.d. spectra of abc(–)-Co(L-ala-O)3 in PLL-50% isopropyl alcohol solutions. In contrast, the same complex destabilizes the α-helix structure of poly-L-ornithine. All these features are examined in the light of the structural characteristics of the interacting species. The influence of solvent composition is also considered. Implications of the different stereochemical features of abd-diastereoisomers on the association process with the polypeptides are discussed.