Atomic scale investigation of the volume phase transition in concentrated PNIPAM microgels

Combining elastic incoherent neutron scattering and differential scanning calorimetry, we investigate the occurrence of the volume phase transition (VPT) in very concentrated poly-(N-isopropyl-acrylamide) (PNIPAM) microgel suspensions, from a polymer weight fraction of 30 wt. % up to dry conditions. Although samples are arrested at the macroscopic scale, atomic degrees of freedom are equilibrated and can be probed in a reproducible way. A clear signature of the VPT is present as a sharp drop in the mean square displacement of PNIPAM hydrogen atoms obtained by neutron scattering. As a function of concentration, the VPT gets smoother as dry conditions are approached, whereas the VPT temperature shows a minimum at about 43 wt. %. This behavior is qualitatively confirmed by calorimetry measurements. Molecular dynamics simulations are employed to complement experimental results and gain further insights into the nature of the VPT, confirming that it involves the formation of an attractive gel state between the microgels. Overall, these results provide evidence that the VPT in PNIPAM-based systems can be detected at different time- and length-scales as well as under overcrowded conditions.

On the molecular origin of the cooperative coil-to-globule transition of poly(N-isopropylacrylamide) in water

The cooperativity of PNIPAM coil-to-globule transition in water arises from the structuring of solvent in proximity to hydrophobic groups.

By means of atomistic molecular dynamics simulations we investigate the behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures below and above the lower critical solution temperature (LCST), including the undercooled regime. The transition between water soluble and insoluble states at the LCST is described as a cooperative process involving an intramolecular coil-to-globule transition preceding the aggregation of chains and the polymer precipitation. In this work we investigate the molecular origin of such cooperativity and the evolution of the hydration pattern in the undercooled polymer solution. The solution behaviour of an atactic 30-mer at high dilution is studied in the temperature interval from 243 to 323 K with a favourable comparison to available experimental data. In the water soluble states of PNIPAM we detect a correlation between polymer segmental dynamics and diffusion motion of bound water, occurring with the same activation energy. Simulation results show that below the coil-to-globule transition temperature PNIPAM is surrounded by a network of hydrogen bonded water molecules and that the cooperativity arises from the structuring of water clusters in proximity to hydrophobic groups. Differently, the perturbation of the hydrogen bond pattern involving water and amide groups occurs above the transition temperature. Altogether these findings reveal that even above the LCST PNIPAM remains largely hydrated and that the coil-to-globule transition is related with a significant rearrangement of the solvent in the proximity of the surface of the polymer. The comparison between the hydrogen bonding of water in the surrounding of PNIPAM isopropyl groups and in the bulk displays a decreased structuring of solvent at the hydrophobic polymer–water interface across the transition temperature, as expected because of the topological extension along the chain of such interface. No evidence of an upper critical solution temperature behaviour, postulated in theoretical and thermodynamics studies of PNIPAM aqueous solution, is observed in the low temperature domain.

Solution behaviour of poly(N-isopropylacrylamide) stereoisomers in water: a molecular dynamics simulation study

The water affinity of poly(N-isopropylacrylamide), PNIPAM, is tuned by tacticity, since the hydrophobicity rises with the increase of the degree of isotacticity. On the basis of this experimental evidence, atomistic molecular dynamics simulations of pairs of PNIPAM stereoisomers in 1.6% w/w polymer aqueous solution, a condition intermediate between the dilute and semidilute regimes, were carried out to comparatively investigate the solution behaviour and hydration of atactic and isotactic-rich PNIPAMs, both below and above the lower critical solution temperature, LCST. 30-mers with contents of meso dyads, m, of 45% and 59%, built assuming a Bernoullian dyad distribution, are used as models since their stereochemical composition corresponds to that of experimentally characterized PNIPAM stereoisomers. The simulation results at 283 K, below the LCST, show a slight influence of tacticity on the chain size, but a higher propensity for inter-chain association of the meso-dyad-rich system, in agreement with the experimental results. Junctions between chains are formed because of hydrophobic interactions and are stabilized by a layer of hydrogen bonded water molecules, whose mobility is reduced as compared to that observed for the same meso-dyad-rich stereoisomer at infinite dilution. At 323 K, above the LCST, simulations detect both the coil-globule transition and the aggregation of chains. Under these conditions, the influence of tacticity on the characteristics of PNIPAM aggregate is negligible.

Study of the interactions of D- and L-polylysine enantiomers withpectate in aqueous solutions

The interaction between D- and L-enantiomers of polylysine and potassium pectate was studied by means of CD, microcalorimetry, and osmometry. Upon binding with pectate, only poly(L-lysine) undergoes a coil to alpha-helix transition, while poly(D-lysine) remains in the disordered state. This suggest that the energetics of the interaction is influenced by stereochemical constraints besides electrostatic forces. Experimental findings from microcalorimetry suggest that a contribution to the overall enthalpy of binding comes from the polysaccharidic moiety. Stoichiometry of the macromolecular complexes studied by osmometry gives a polylysine:pectate ratio of 3:1, in agreement with the respective degree of polymerization of the two polyelectrolytes.

Mutations of Gly to Ala in human glutathione transferase P1-1 affect helix 2 (G-site) and induce positive cooperativity in the binding of glutathione

Previous kinetic studies on human glutathione transferase P1-1 have indicated that the motions of an irregular alpha-helix (helix 2) lining the glutathione (GSH) binding site are viscosity dependent and may modulate the affinity of GSH binding. The effect of single amino acid residue substitutions (Gly to Ala) in this region is investigated here by site-directed mutagenesis. Three mutants (Gly41Ala, Gly50Ala and Gly41Ala/Gly50Ala) were overexpressed in Escherichia coli, purified, and characterized by kinetic, structural, and spectroscopic studies. All these mutant enzymes show kcat values similar to that of the wild-type enzyme, while the [S]0.5 for GSH increases about eight-fold in the Gly41Ala mutant and more than 100-fold in the Gly41Ala/Gly50Ala double mutant. This change in affinity towards GSH is accompanied by an induced positive cooperativity as reflected by Hill coefficients of 1.4 (Gly41Ala) and 1.7 (Gly41Ala/Gly50Ala) upon substrate binding. Taken together, these data suggest that the region around helix 2 is markedly altered leading to the observed intersubunit communication. Molecular modeling of the Gly41Ala/Gly50Ala mutant and of the inactive oxidized form of the native enzyme provides a structural explanation of our results.

Structural flexibility modulates the activity of human glutathione transferase P1-1. Role of helix 2 flexibility in the catalytic mechanism

Presteady-state and steady-state kinetic studies performed on human glutathione transferase P1-1 (EC 2.5.1.18) with 1-chloro-2, 4-dinitrobenzene as co-substrate indicate that the rate-determining step is a physical event that occurs after binding of the two substrates and before the final sigma-complex formation. It may be a structural transition involving the ternary complex. This event can be related to diffusion-controlled motions of protein portions as kcat degrees /kcat linearly increases by raising the relative viscosity of the solution. Similar viscosity dependence has been observed for Km GSH, while Km CDNB is independent. No change of the enzyme structure by viscosogen has been found by circular dichroism analysis. Thus, kcat and Km GSH seem to be related to the frequency and extent of enzyme structural motions modulated by viscosity. Interestingly, the reactivity of Cys-47 which can act as a probe for the flexibility of helix 2 is also modulated by viscosity. Its viscosity dependence parallels that observed for kcat and Km GSH, thereby suggesting a possible correlation between kcat, Km GSH, and diffusion-controlled motion of helix 2. The viscosity effect on the kinetic parameters of C47S and C47S/C101S mutants confirms the involvement of helix 2 motions in the modulation of Km GSH, whereas a similar role on kcat cannot be ascertained unequivocally. The flexibility of helix 2 modulates also the homotropic behavior of GSH in these mutants. Furthermore, fluorescence experiments support a structural motion of about 4 A occurring between helix 2 and helix 4 when GSH binds to the G-site.

Structural flexibility modulates the activity of human glutathione transferase P1-1. Influence of a poor co-substrate on dynamics and kinetics of human glutathione transferase

Presteady-state and steady-state kinetics of human glutathione transferase P1-1 (EC 2.5.1.18) have been studied at pH 5.0 by using 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, a poor co-substrate for this isoenzyme. Steady-state kinetics fits well with the simplest rapid equilibrium random sequential bi-bi mechanism and reveals a strong intrasubunit synergistic modulation between the GSH-binding site (G-site) and the hydrophobic binding site for the co-substrate (H-site); the affinity of the G-site for GSH increases about 30 times at saturating co-substrate and vice versa. Presteady-state experiments and thermodynamic data indicate that the rate-limiting step is a physical event and, probably, a structural transition of the ternary complex. Similar to that observed with 1-chloro-2, 4-dinitrobenzene (Ricci, G., Caccuri, A. M., Lo Bello, M., Rosato, N. , Mei, G., Nicotra, M., Chiessi, E., Mazzetti, A. P., and Federici, G.(1996) J. Biol. Chem. 271, 16187-16192), this event may be related to the frequency of enzyme motions. The observed low, viscosity-independent kcat value suggests that these motions are slow and diffusion-independent for an increased internal viscosity. In fact, molecular modeling suggests that the hydroxyl group of Tyr-108, which resides in helix 4, may be in hydrogen bonding distance of the oxygen atom of this new substrate, thus yielding a less flexible H-site. This effect might be transmitted to the G-site via helix 4. In addition, a new homotropic behavior exhibited by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole is found in Cys-47 mutants revealing a structural intersubunit communication between the two H-sites.

Association complexes between Fe(III) or Cu(II) ions and chitosan derivatives. A thermodynamic and spectroscopic investigation

Association complexes between iron(III) or copper(II) ions and deoxylactit-1-yl (1), 2-substituted pentanedioic acid (2), or 2-substituted propanoic acid (3) derivatives of chitosan were prepared and characterized by thermodynamic and spectroscopic measurements. Complex solutions did not show any precipitate or even opalescence, owing to the hydrolysis of free metal ions, within a wide range of [Me(n+)]/[P] molar ratio, even at a pH as high as 10.5 (Me(n+) = Fe3+ or Cu2+). Both equilibrium dialysis and Job plot experiments suggest that the functional groups in each monomeric residue are an effective site of binding for one metal ion. Reduction potentials, as obtained by cyclic voltammetric measurements, indicate that (i) coordination of the aforementioned polymeric ligands to Cu2+ ions stabilizes the oxidized species, and (ii) iron complexes have an oxidation power definitely higher than that of the corresponding copper compounds. Electron paramagnetic resonances (100 or 6 K) and Mössbauer (r.t.) spectra suggest that the order of increasing distortion from idealized geometry is Me(n+)-chitosan approximately Me(n+)-(3) < Me(n+)-(2) < or = Me(n+)-(1). These results are discussed briefly in the light of a few general considerations concerning the structural features of association complexes between macromolecules and transition metal ions.

Copper complexes immobilized to chitosan

Polymeric ligands, such as 2-substituted pentanedioic acid (2), 2-substituted propanoic acid (3), and deoxylactit-1-yl (4) derivatives of chitosan (1), were used to prepare copper complexes that are widely soluble in aqueous solution. EPR results (100 K) show that all association complexes basically have a tetragonal symmetry. Visible CD spectra suggest, however, that the order of increasing departure from this geometry is Cu-(1) approximately Cu-(3) less than Cu-(2) less than or equal to Cu-(4), the lack of sterically constraining side-chains in (1) and (3) allowing a more symmetric arrangement of ligands around the central metal ion. Results on the catalytic activity of the association complexes for air oxidation of catechol derivatives are also presented.

Branched-chain analogues of linear polysaccharides: a spectroscopic and conformational investigation of chitosan derivatives

The solution properties and conformational features of 2-substituted propanoic acid (I) and 2-substituted pentanedioic acid (II) derivatives of chitosan were investigated over a wide range of pH by potentiometric, optical and chiroptical measurements, and by theoretical conformational analysis. No significant change is observed in the solution properties of I upon pH variations, in agreement with computational results showing that the conformational features of the polymer do not vary with respect to the charge state of the ionizable groups. In contrast, spectroscopic titration and preliminary 1H-n.m.r. data indicate that conformational equilibria in II are pH-dependent. Consistently, computed models show that both the charge state of the ionizable groups and the chirality of the carbon atom in the side chain control the structural features of the polymer.