Mario Barteri

Coupling between binding-induced conformational phenomena and stereospecific effects in asymmetric reactions

The oxidation by H2O2 of L-(+)-ascorbate anion in the presence of 2,2′,2″,2‴-tetrapyridineiron(III) complex ions anchored to poly(L-glutamate)(FeL) or poly(D-glutamate)(FeD) has been studied at pH 7.0 and varying complex-to-polymer-residue ratio [C]/[P]. The reaction follows two parallel routes; one corresponds to an electron-transfer process within a substrate–catalyst adduct and the other refers to an uncatalysed pathway to the dehydroascorbic acid. Unusual phenomena are observed in the catalysis in the sense that only the conformational dissymmetry of the active sites, arising from the binding-induced coil-to-α-helix transition of polypeptide matrices by FeIII complex counter-ions, is able to impart stereospecific effects in the reaction.Evidence is produced to show that stereoselectivity is driven by activation entropy. The effect probably arises from the formation of a rather rigid precursor complex, with the optically active substrate molecules bound to the chiral residues of the ordered polymer surrounding the active sites. The stereochemistry of such an intermediate allows the reaction to proceed only by a remote electron-transfer pathway, through the quaterpyridine ligand of the metal chelate. Evidence suggests that the asymmetric [Fe(tetpy)(OH)2]+–polyelectrolyte systems also behave as environmental controllers of the uncatalysed oxidation of the L-(+)-ascorbate anion. This effect is briefly discussed in terms of the role played by macroions in ionic reactions in solution.

Stereoselective catalysis by Fe(III) complex ions supported on asymmetric polymers: Oxidation of ascorbic acid in aqueous solution

Abstract Quaterpyridyneiron (III) complex ions anchored to partially ordered poly (L-glutamate) or poly (D-glutamate) were used as (enantiomeric) catalysts for the H 2 O 2 -oxidation of L(+) ascorbic acid at pH 7. When the α-helical fraction of polypeptide matrices was low, the configuration dissymmetry of the active sites was unable to impart any stereoselective effect in the catalysis, i.e. k = 3.66 x 10 3 M −1 ▿sec −1 (25.9°C) with both catalysts. On the contrary, by increasing the amount of α-helix in the polymeric supports the stereoselectivity increases, the second-order rate constants k FeD being definitely higher than k FeL . Implications of the role played by the conformational dissymmetry of the active sites in the stereospecificity of the process are briefly discussed.

Coupling between pH-induced conformational phenomena and stereospecific effects in electron-transfer reactions

The oxidation by H2O2 of L-(+)-ascorbate anion in the presence of 2,2′,2″,2‴-tetrapyridineiron(III) complex ions anchored to poly(L-glutamate)(FeL) or poly(D-glutamate)(FeD) has been studied at a complex-to-polymer-residue ratio of 0.10 and in the pH range 6–8. Evidence is produced that the reaction is a composite process reflecting contributions from parallel routes; one of these corresponds to a catalytic, [H2O2]-independent pathway and the other refers to an uncatalysed electron-transfer process between ascorbate anion and hydrogen peroxide. Stereospecific effects in the catalysis are observed with decreasing pH, accompanied by an increase in the amount of α-helix in the polypeptide supports (ƒh). Thus at pH 7.8 (ƒh≈ 0.13), kFeD= 1382.3 ± 113.2 and kFeL= 1034.4 ± 79.3 dm3 mol–1 s–1 and the activation energy is 3.8 ± 0.3 kcal mol–1 with both enantiomeric catalysts, whereas at pH 6.3 (ƒh≈ 0.84), kFeD= 70.9 ± 5.5 and kFeL= 13.6 ± 1.1 dm3 mol–1 s–1 and the activation energy is 18.0 ± 1.3 kcal mol–1 in both cases. The results indicate that stereoselectivity is an entropy-controlled phenomenon. The effect is probably caused by conformational rigidity of the precursor complex, which arises from interactions between the optically active substrate molecules and the chiral residues of the ordered polymer surrounding the active centres.Effects of the stereochemical features of the substrate–catalyst adduct on the mechanism of electron transfer are discussed. The evidence suggests that the asymmetric[Fe(tetpy)(OH)2]+–polyelectrolyte systems also behave as environmental controllers of the uncatalysed oxidation of ascorbate anion.

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.

Binding equilibria and catalase-like reactivity of deuteroferrihaem–poly-L-lysine complexes

At pH 10.5, 25 °C, and ionic strength 0.1 mol dm–3, deuteroferrihaem binds to poly-L-lysine as a monomeric haem species with a single axial lysine ligand. The observed rate constant for catalytic decomposition of H2O2 by deuteroferrihaem increases initially with increasing [poly-L-lysine] but then falls rapidly at higher [poly-L-lysine]. The results imply that the catalytic rate constant for poly-L-lysine bound deuteroferrihaem monomer is closely similar to that of free deuteroferrihaem monomer at low [poly-L-lysine] but decreases sharply (by almost 103 fold) at higher [poly-L-lysine].

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.