Dynamic Monte Carlo simulation of aggregation of nanoparticles in the presence of diblock copolymer

The aggregation of hydrophobic nanoparticles in the presence of diblock copolymers is investigated using dynamic Monte Carlo simulation on a simple cubic lattice. One nanoparticle occupies one lattice site, one block copolymer (A(m)B(m)) occupies 2m sequentially linked sites with m segments of A and m segments of B, and solvents are represented by any unoccupied sites. All of them are self-avoiding and nearest-neighbor interactions are considered. A compact big aggregate, dispersed aggregates wrapped by polymer chains, and an ordered lamellar structure are obtained by varying the concentration of copolymer. The structures are seen to be controlled by competing forces between the interaction of copolymer with nanoparticles and the self-assembly of copolymer in solution. The critical concentration of copolymer needed to form the lamellar structure, C(p,L), decreases with the chain length. It is also found that C(p,L) decreases roughly linearly with the concentration of nanoparticles C(n), which can be approximately expressed as C(p,L)=0.764-0.857C(n) when m=2. The simulation demonstrates that addition of diblock copolymer can effectively control the aggregation of nanoparticles and lead to the formation of a variety of nanostructures.     

A detailed mechanistic study of the substitution behavior of an unusual seven-coordinate iron(III) complex in aqueous solution

A detailed mechanistic study of the substitution behavior of a 3d metal heptacoordinate complex, with a rare pentagonal-bipyramidal structure, was undertaken to resolve the solution chemistry of this system. The kinetics of the complex-formation reaction of [Fe(dapsox)(H(2)O)(2)]ClO(4) (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with thiocyanate was studied as a function of thiocyanate concentration, pH, temperature, and pressure. The reaction proceeds in two steps, which are both base-catalyzed due to the formation of an aqua-hydroxo complex (pK(a1) = 5.78 +/- 0.04 and pK(a2) = 9.45 +/- 0.06 at 25 degrees C). Thiocyanate ions displace the first coordinated water molecule in a fast step, followed by a slower reaction in which the second thiocyanate ion coordinates trans to the N-bonded thiocyanate. At 25 degrees C and pH <4.5, only the first reaction step can be observed, and the kinetic parameters (pH 2.5: k(f(I)) = 2.6 +/- 0.1 M(-1) s(-1), DeltaH(#)(f(I)) = 62 +/- 3 kJ mol(-1), DeltaS(#)(f(I)) = -30 +/- 10 J K(-1) mol(-1), and DeltaV(#)(f(I)) = -2.5 +/- 0.2 cm(3) mol(-1)) suggest the operation of an I(a) mechanism. In the pH range 2.5 to 5.2 this reaction step involves the participation of both the diaqua and aqua-hydroxo complexes, for which the complex-formation rate constants were found to be 2.19 +/- 0.06 and 1172 +/- 22 M(-1) s(-1) at 25 degrees C, respectively. The more labile aqua-hydroxo complex is suggested to follow an I(d) or D substitution mechanism on the basis of the reported kinetic data. At pH > or =4.5, the second substitution step also can be monitored (pH 5.5 and 25 degrees C: k(f(II)) = 21.1 +/- 0.5 M(-1) s(-1), DeltaH(#)(f(II)) = 60 +/- 2 kJ mol(-1), DeltaS(#)(f(II)) = -19 +/- 6 J K(-1) mol(-1), and DeltaV(#)(f(II)) = +8.8 +/- 0.3 cm(3) mol(-1)), for which an I(d) or D mechanism is suggested. The results are discussed in terms of known structural parameters and in comparison to relevant structural and kinetic data from the literature.     

Stabilities and isomeric equilibria in aqueous solution of monomeric metal ion complexes of adenosine 5'-diphosphate (ADP3-) in comparison with those of adenosine 5'-monophosphate (AMP2-)

Under experimental conditions in which the self-association of the adenine phosphates (AP), that is, of adenosine 5'-monophosphate (AMP(2-)) and adenosine 5'-diphosphate (ADP(3-)), is negligible, potentiometric pH titrations were carried out to determine the stabilities of the M(H;AP) and M(AP) complexes where M(2+)=Mg(2+), Ca(2+), Sr(2+), Ba(2+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), or Cd(2+) (25 degrees C; I=0.1 M, NaNO(3)). It is concluded that in the M(H;AMP)(+) species M(2+) is bound at the adenine moiety and in the M(H;ADP) complexes at the diphosphate unit; however, the proton resides in both types of monoprotonated complexes at the phosphate residue. The stabilities of nearly all the M(AMP) and M(ADP)(-) complexes are significantly larger than what is expected for a sole coordination of M(2+) to the phosphate residue. This increased complex stability is attributed, in agreement with previous (1)H NMR shift studies and further information existing in the literature, to the formation of macrochelates of the phosphate-coordinated metal ions with N7 of the adenine residues. On the basis of recent measurements with simple phosphate monoesters and phosphonate ligands (R-MP(2-)) as well as with diphosphate monoesters (R-DP(3-)), where R is a noncoordinating and noninhibiting residue, the increased stabilities of the M(AMP) and M(ADP)(-) complexes due to the M(2+)-N7 interaction could be evaluated and the extent of macrochelate formation calculated. The results show that the formation degrees of the macrochelates for the complexes of the alkaline earth ions are small (about 15 % at the most), whereas for the 3d metal ions as well as for Zn(2+) and Cd(2+) the formation degrees vary between about 15 % (Mn(2+)) and 75 % (Ni(2+)) with values of about 40 and 50 % for Zn(2+) and Cu(2+), respectively. It is interesting to note, taking earlier results for M(ATP)(2-) complexes also into account (ATP(4-)=adenosine 5'-triphosphate), that for a given metal ion in nearly all instances the formation degrees of the macrochelates are within the error limits the same for M(AMP), M(ADP)(-) and M(ATP)(2-) complexes; except for Co(2+) and Ni(2+) it holds M(AMP) > M(ADP)(-) approximately M(ATP)(2-). This result is astonishing if one considers that the absolute stability constants of these complexes, which are determined largely by the affinity of the phosphate residues, can differ by more than two orders of magnitude. The impact and conclusions of these observations for biological systems are shortly lined out.     

Boronate-containing copolymers: polyelectrolyte properties and sugar-specific interaction with agarose gel

Copolymers of N-acryloyl-m-aminophenylboronic acid (NAAPBA) with acryamide (AA), N,N-dimethylacrylamide (DMAA), and N-isopropylacrylamide (NIPAM) were found to adsorb on cross-linked agarose gel (Sepharose CL-6B) in the pH range from 7.5-9.2, due to specific boronate-sugar interactions. The molar percentages of phenylboronic acid (PBA) groups in the boronate-containing copolymers (BCCs), as estimated by 1H NMR spectroscopy, were 13, 10, and 16%, respectively, whereas the apparent ionization constants, the pKa values, of the copolymers were similar and equal to 9.0 +/- 0.2 at 20 degrees C. The copolymers adsorption capacities were in the range of 15-30 mg x ml(-1) gel (14-36 micromol pendant PBA ml(-1) gel) at pH 9.2 and decreased with decreasing pH value. The interaction of monomeric NAAPBA with Sepharose CL-6B was characterized by an equilibrium association constant of 53 +/- 17 M(-1), the chromatographic capacity factor k' = 1.8, and a total content of binding sites of 27 +/- 10 micromol x ml(-1) gel at pH 9.2. The weak reversible binding of monomeric NAAPBA and almost irreversible binding of NAAPBA copolymers to the gel at pH 9.2 suggested a multivalent character of the copolymer adsorption. At pH 7.5, the maximal adsorption capacity was displayed by the AA-NAAPBA copolymer (15 mg x ml(-1) gel). All the BCCs could be completely desorbed from the gel by 0.1 M fructose in aqueous buffered media with pH values from 7.5-9.2. The strong adsorption of AA-NAAPBA on agarose gel probably relates to the conformation of the copolymer in aqueous solution and provides opportunities for biomedical applications of the copolymer under physiological conditions. Multivalent, weak-affinity adsorption of BCCs to the agarose gel seems to be a tentative model for the copolymers' binding to oligo- and polysaccharides of cell membranes and mucosal surfaces.     

Living polymerization of ethylene catalyzed by titanium complexes having fluorine-containing phenoxy-imine chelate ligands

Seven titanium complexes bearing fluorine-containing phenoxy-imine chelate ligands, TiCl(2)[eta(2)-1-[C(H)=NR]-2-O-3-(t)Bu-C(6)H(3)](2) [R = 2,3,4,5,6-pentafluorophenyl (1), R = 2,4,6-trifluorophenyl (2), R = 2,6-difluorophenyl (3), R = 2-fluorophenyl (4), R = 3,4,5-trifluorophenyl (5), R = 3,5-difluorophenyl (6), R = 4-fluorophenyl (7)], were synthesized from the lithium salt of the requisite ligand and TiCl(4) in good yields (22%-76%). X-ray analysis revealed that the complexes 1 and 3 adopt a distorted octahedral structure in which the two phenoxy oxygens are situated in the trans-position while the two imine nitrogens and the two chlorine atoms are located cis to one another, the same spatial disposition as that for the corresponding nonfluorinated complex. Although the Ti-O, Ti-N, and Ti-Cl bond distances for complexes 1 and 3 are very similar to those for the nonfluorinated complex, the bond angles between the ligands (e.g., O-Ti-O, N-Ti-N, and Cl-Ti-Cl) and the Ti-N-C-C torsion angles involving the phenyl on the imine nitrogen are different from those for the nonfluorinated complex, as a result of the introduction of fluorine atoms. Complex 1/methylalumoxane (MAO) catalyst system promoted living ethylene polymerization to produce high molecular weight polyethylenes (M(n) > 400 000) with extremely narrow polydispersities (M(w)/M(n) < 1.20). Very high activities (TOF > 20 000 min(-1) atm(-1)) were observed that are comparable to those of Cp(2)ZrCl(2)/MAO at high polymerization temperatures (25, 50 degrees C). Complexes 2-4, which have a fluorine atom adjacent to the imine nitrogen, behaved as living ethylene polymerization catalysts at 50 degrees C, whereas complexes 5-7, possessing no fluorine adjacent to the imine nitrogen, produced polyethylenes having M(w)/M(n) values of ca. 2 with beta-hydrogen transfer as the main termination pathway. These results together with DFT calculations suggested that the presence of a fluorine atom adjacent to the imine nitrogen is a requirement for the high-temperature living polymerization, and the fluorine of the active species for ethylene polymerization interacts with a beta-hydrogen of a polymer chain, resulting in the prevention of beta-hydrogen transfer. This catalyst system was used for the synthesis of a number of unique block copolymers such as polyethylene-b-poly(ethylene-co-propylene) diblock copolymer and polyethylene-b-poly(ethylene-co-propylene)-b-syndiotactic polypropylene triblock copolymer from ethylene and propylene.     

Equilibrium and Kinetics of Bromine Hydrolysis

Equilibrium constants for bromine hydrolysis, K(1) = [HOBr][H(+)][Br(-)]/[Br(2)(aq)], are determined as a function of ionic strength (&mgr;) at 25.0 degrees C and as a function of temperature at &mgr; approximately 0 M. At &mgr; approximately 0 M and 25.0 degrees C, K(1) = (3.5 +/- 0.1) x 10(-)(9) M(2) and DeltaH degrees = 62 +/- 1 kJ mol(-)(1). At &mgr; = 0.50 M and 25.0 degrees C, K(1) = (6.1 +/- 0.1) x 10(-)(9) M(2) and the rate constant (k(-)(1)) for the reverse reaction of HOBr + H(+) + Br(-) equals (1.6 +/- 0.2) x 10(10) M(-)(2) s(-)(1). This reaction is general-acid-assisted with a Br?nsted alpha value of 0.2. The corresponding Br(2)(aq) hydrolysis rate constant, k(1), equals 97 s(-)(1), and the reaction is general-base-assisted (beta = 0.8).     

Binding of amino acids to "smart" sorbents: where does hydrophobicity come into play?

Poly(N-isopropylacrylamide) (PNIPA)-based sorbents have been successfully used as sorbents in temperature-sensitive chromatography. Yet, the mechanisms controlling the binding of biochemicals to these sorbents and, therefore, the separation process are not fully understood. In the current work, the role of hydrophobic interactions in the binding of amino acids of different hydrophobicities to PNIPA microgels was studied. Binding experiments were conducted both below (25 degrees C) and above (37 degrees C) the volume-phase transition temperature of the gel. At 25 degrees C, no straightforward correlation between the partition coefficient and the hydrophobicity could be suggested for low hydrophobicity values. Contrary, at higher hydrophobicities the partition coefficient increases with increasing hydrophobicity. This correlation holds for the whole hydrophobicity range at 37 degrees C; however, the binding data suggests two different binding mechanisms of the hydrophilic amino acids and the hydrophobic ones. Isothermal titration calorimetry measurements confirmed this suggestion: The binding of hydrophobic amino acids seems to be driven by hydrophobic interactions, as evident from the positive binding enthalpy and the clear correlation between the amino acid's hydrophobicity and the binding entropy. Contrary, the binding of the hydrophilic amino acids was exothermic, implying a binding mechanism based on specific interactions, most probably hydrogen bonding.     

Analysis of the interaction between human plasma fibronectin and gelatin by affinity electrophoresis

The interaction between human plasma fibronectin and gelatin was analyzed by affinity electrophoresis, in which the fibronectin was subjected to electrophoresis in a 4% polyacrylamide gel in the presence and absence of gelatin, as an affinity ligand, and the fibronectin band was stained by an immunoblotting method. The apparent dissociation constants (Kd) of fibronectin for gelatin were calculated from affinity plots based on the original affinity equation at different pHs, urea concentrations, and temperatures. The fibronectin exhibited much lower affinity in the presence of urea. The Kds at 37 degrees C were 1.49 X 10(-7) M, 2.50 X 10(-6) M, and 3.58 X 10(-6) M with 2 M, 3 M, and 4 M urea, respectively. The van't Hoff plots of Kd values against absolute temperature (T) showed that the value of log Kd decreased in proportion to the increase in the value of 1/T within the range of 15-50 degrees C. The standard enthalpy, the standard free energy change at 37 degrees C, and the entropy change at 37 degrees C for association were calculated to be -124.7 kJ/mol, -33.23 kJ/mol, and -295.1 J/mol/deg, respectively. These results suggest that a hydrophilic interaction, such as hydrogen bond or van der Waals interaction, plays an important role in the binding of plasma fibronectin to gelatin.     

Comparison of the binding of cadmium(II), mercury(II), and arsenic(III) to the de novo designed peptides TRI L12C and TRI L16C

Designed alpha-helical peptides of the TRI family with a general sequence Ac-G(LKALEEK)(4)G-CONH(2) were used as model systems for the study of metal-protein interactions. Variants containing cysteine residues in positions 12 (TRI L12C) and 16 (TRI L16C) were used for the metal binding studies. Cd(II) binding was investigated, and the results were compared with previous and current work on Hg(II) and As(III) binding. The metal peptide assemblies were studied with the use of UV, CD, EXAFS, (113)Cd NMR, and (111m)Cd perturbed angular correlation spectroscopy. The metalated peptide aggregates exhibited pH-dependent behavior. At high pH values, Cd(II) was bound to the three sulfurs of the three-stranded alpha-helical coiled coils. A mixture of two species was observed, including Cd(II) in a trigonal planar geometry. The complexes have UV bands at 231 nm (20 600 M(-1) cm(-1)) for TRI L12C and 232 nm (22 600 M(-1) cm(-1)) for TRI L16C, an average Cd-S bond length of 2.49 A for both cases, and a (113)Cd NMR chemical shift at 619 ppm (Cd(II)(TRI L12C)(3)(-)) or 625 ppm (Cd(II)(TRI-L16C)(3)(-)). Nuclear quadrupole interactions show that two different Cd species are present for both peptides. One species with omega(0) = 0.45 rad/ns and low eta is attributed to a trigonal planar Cd-(Cys)(3) site. The other, with a smaller omega(0), is attributed to a four-coordinate Cd(Cys)(3)(H(2)O) species. At low pH, no metal binding was observed. Hg(II) binding to TRI L12C was also found to be pH dependent, and a 3:1 sulfur-to-mercury(II) species was observed at pH 9.4. These metal peptide complexes provide insight into heavy metal binding and metalloregulatory proteins such as MerR or CadC.     

Supercritical carbon dioxide extraction equilibrium of gallium(III) with 2-methyl-8-quinolinol and 2-methyl-5-butyloxymethyl-8-quinolinol

This work performed fundamental studies for the extraction of gallium(III) with 2-methyl-8-quinolinol (HMQ) and 2-methyl-5-butyloxymethyl-8-quinolinol (HMO(4)Q) into supercritical carbon dioxide (SF-CO(2)) from a weakly acidic solution. The distribution constants of HMO(4)Q between aqueous and SF-CO(2) phases were determined at 45 degrees C, 8.6-20.4 MPa and I=0.1 M (H, Na)NO(3) (M=mol dm(-3)). At 45 degrees C and 15.7 MPa, gallium(III) was hardly extracted with HMQ into SF-CO(2), but was quantitatively extracted with HMO(4)Q in the pH range of 2.20-2.84. The extraction constant, K(ex, SF-CO(2)) (=[Ga(OH)(MO(4)Q)(2)](SF-CO(2))[H(+)](3)[Ga(3+)](-1)[HMO(4)Q](SF-CO(2))(-2)), of gallium(III) with HMO(4)Q was determined to be 10(-2.6+/-0.1) at 45 degrees C, 15.7 MPa and I=0.1 M (H, Na)NO(3), which was 63 times larger than that in heptane at 45 degrees C and 0.10 MPa. It was also found that the addition of 3,5-dichlorophenol as a synergist enhanced the extractability of gallium(III) with HMO(4)Q into SF-CO(2).     

Temperature responsive complex coacervate core micelles with a PEO and PNIPAAm corona

In aqueous solutions at room temperature, poly( N-methyl-2-vinyl pyridinium iodide)- block-poly(ethylene oxide), P2MVP 38- b-PEO 211 and poly(acrylic acid)- block-poly(isopropyl acrylamide), PAA 55- b-PNIPAAm 88 spontaneously coassemble into micelles, consisting of a mixed P2MVP/PAA polyelectrolyte core and a PEO/PNIPAAm corona. These so-called complex coacervate core micelles (C3Ms), also known as polyion complex (PIC) micelles, block ionomer complexes (BIC), and interpolyelectrolyte complexes (IPEC), respond to changes in solution pH and ionic strength as their micellization is electrostatically driven. Furthermore, the PNIPAAm segments ensure temperature responsiveness as they exhibit lower critical solution temperature (LCST) behavior. Light scattering, two-dimensional 1H NMR nuclear Overhauser effect spectrometry, and cryogenic transmission electron microscopy experiments were carried out to investigate micellar structure and solution behavior at 1 mM NaNO 3, T = 25, and 60 degrees C, that is, below and above the LCST of approximately 32 degrees C. At T = 25 degrees C, C3Ms were observed for 7 < pH < 12 and NaNO 3 concentrations below approximately 105 mM. The PEO and PNIPAAm chains appear to be (randomly) mixed within the micellar corona. At T = 60 degrees C, onion-like complexes are formed, consisting of a PNIPAAm inner core, a mixed P2MVP/PAA complex coacervate shell, and a PEO corona.     

Microcalorimetric study on the influence of temperature on bacterial coaggregation

Binding isotherms and heats of interaction have been determined at 15, 25, and 40 degrees C for a coaggregating and a non-coaggregating oral bacterial pair. Heats of interaction were measured upon three consecutive injections of streptococci into an actinomyces suspension using isothermal titration calorimetry. After each injection, the number of streptococci injected remaining free in suspension was quantified microscopically and the degree of binding between the two bacterial strains was established. The coaggregating pair shows positive cooperative binding. The highest cooperativity, at 25 degrees C, correlates with a strong, macroscopically visible coaggregation. The non-coaggregating pair shows low cooperativity and lacks macroscopically visible coaggregation. Interactions between the coaggregating partners seem to be mainly due to specific, enthalpically saturable and favorable binding sites. Even though the enthalpic part of the interaction is saturated, cooperativity increases with consecutive injections, implying that the coaggregation phenomenon is driven by entropy gain. The change in heat capacity (DeltaC(p)) is positive for the non-coaggregating pair from 15-40 degrees C as well as for the coaggregating pair beyond 25 degrees C. At lower temperatures the coaggregating pair causes a negative DeltaC(p). The decrease in heat capacity together with an increase in entropy is considered to be indicative of hydrophobic interactions playing an important role in the formation of large coaggregates as observed for the coaggregating pair at 25 degrees C.     

The demicellization of alkyltrimethylammonium bromides in 0.1 M sodium chloride solution studied by isothermal titration calorimetry

The demicellization of the cationic detergents dodecyltrimethylammonium bromide, tetradecyltrimetylammonium bromide, and cetyltrimethylammonium bromide was studied at temperatures between 20 and 60 degrees C in 0.1 M NaCl (pH 6.4) using isothermal titration calorimetry (ITC). We determined the critical micellization concentration (cmc) of the cationic detergents which show a minimum at temperatures between 20 and 34 degrees C. In accordance with the lengthening of the hydrophobic tail of the detergents the cmc decreases with increasing alkyl chain length. The thermodynamic parameters describing the changes of enthalpy (DeltaH(demic)), the changes of entropy (DeltaS(demic)) and the Gibbs free energy change (DeltaG(demic)) for demicellization were first obtained using the pseudophase-separation model. The aggregation number n at the cmc as well as the demicellization enthalpy, entropy and Gibbs free energy change were also calculated using a simulation based on the mass-action model. Furthermore, we investigated the demicellization of CTAB in deionized water in comparison to demicellization in sodium chloride solution to determine the influence of counter ion binding on the demicellization.     

Thermodynamics of micelle formation in water, hydrophobic processes and surfactant self-assemblies

The critical micelle concentration (c.m.c.) for four cationic surfactants, alkyl-trimethyl-ammonium bromides, was determined as a function of temperature by conductivity measurements. The values of the standard free energy of micellisation DeltaG degrees(mic) at different temperatures were calculated by using a pseudo-phase transition model. Then, from the diagram (-DeltaG degrees(mic)/T)=f(1/T), the thermodynamic functions DeltaH(app) and DeltaS(app) were calculated. From the plots DeltaH(app)=f(T) and DeltaS(app) = f(ln T) the slopes DeltaC(p) = n(w(H))C(p,w) and DeltaC(p)=n(w(S))C(p,w) were calculated, with the numbers n(w(H)) and n(w(S)) negative and equal and therefore defined simply as n(w). The number n(w)<0, indicating condensed water molecules, depends on the reduction of cavity that takes place as a consequence of the coalescence of the cavities previously surrounding the separate aliphatic or aromatic moieties. The analysis, based on a molecular model consisting of three forms of water, namely W(I), W(II), and W(III), respectively, was extended to several other types of surfactants for which c.m.c. data had been published by other authors. The results of this analysis form a coherent scheme consistent with the proposed molecular model. The enthalpy for all the types of surfactant is described by DeltaH(app)= -3.6 + 23.1xi(w)-xi(w)C(p,w)T and the entropy by DeltaS(app)= +10.2+428xi(w)-xi(w)C(p,w) ln T where xi(w)= |n(w)| represents the number of molecules W(III) involved in the reaction. The term Deltah(w)= +23.1 kJ mol(-1) xi(w)(-1) indicates an unfavourable endothermic contribution to enthalpy for reduction of the cavity whereas the term Deltas(w)= +428 J K(-1) mol(-1) xi(w)(-1) represents a positive entropy contribution for reduction of the cavity, what is the driving force of hydrophobic association. The processes of non polar gas dissolution in water and of micelle formation were found to be strictly related: they are, however, exactly the opposite of one another. In micelle formation no intermolecular electronic short bond is formed. We propose, therefore, to substitute the term "hydrophobic bond" with that of "hydrophobic association".     

Thermodynamic and kinetic studies on reactions of Pt(II) complexes with biologically relevant nucleophiles

The effect of different N-N spectator ligands on the reactivity of platinum(II) complexes was investigated by studying the water lability of [Pt(diaminocyclohexane)(H2O)2]2+ (Pt(dach)), [Pt(ethylenediamine)(H2O)2]2+ (Pt(en)), [Pt(aminomethylpyridine)(H2O)2]2+ (Pt(amp)), and [Pt(N,N'-bipyridine)(H2O)2]2+ (Pt(bpy)). Some of the selected N-N chelates form part of the coordination sphere of Pt(II) drugs in clinical use, as in Pt(dach) (oxaliplatin), or are models, regarding the nature of the amines, with higher stability in terms of substitution and hydrolysis of the diamine moiety, as in Pt(en) (cisplatin) and Pt(amp) (AMD473). The effect of pi-acceptors on the reactivity was investigated by introducing one (Pt(amp)) or two pyridine rings (Pt(bpy)) in the system. The pK(a) values for the two water molecules (viz., Pt(dach) (pK(a1) = 6.01, pK(a2) = 7.69), Pt(en) (pK(a1) = 5.97, pK(a2) = 7.47), Pt(amp) (pK(a1) = 5.82, pK(a2) = 6.83), Pt(bpy) (pK(a1) = 4.80, pK(a2) = 6.32) show a decrease in the order Pt(dach) > Pt(en) > Pt(amp) > Pt(bpy). The substitution of both coordinated water molecules by a series of nucleophiles (viz., thiourea (tu), L-methionine (L-Met), and guanosine-5'-monophosphate (5'GMP-) was investigated under pseudo-first-order conditions as a function of concentration, temperature, and pressure using UV-vis spectrophotometric and stopped-flow techniques and was found to occur in two subsequent reaction steps. The following k1 values for Pt(dach), Pt(en), Pt(amp), and Pt(bpy) were found: tu (25 degrees C, M(-1) s(-1)) 21 +/- 1, 34.0 +/- 0.4, 233 +/- 5, 5081 +/- 275; L-Met (25 degrees C) 0.85 +/- 0.01, 0.70 +/- 0.03, 2.15 +/- 0.05, 21.8 +/- 0.6; 5'GMP- (40 degrees C) 5.8 +/- 0.2, 3.9 +/- 0.1, 12.5 +/- 0.5, 24.4 +/- 0.3. The results for k2 for Pt(dach), Pt(en), Pt(amp), and Pt(bpy) are as follows: tu (25 degrees C, M(-1) s(-1)) 11.5 +/- 0.5, 10.2 +/- 0.2, 38 +/- 1, 1119 +/- 22; L-Met (25 degrees C, s(-1)) 2.5 +/- 0.1, 2.0 +/- 0.2, 1.2 +/- 0.3, 290 +/- 4; 5'GMP- (40 degrees C, M(-1) s(-1)) 0.21 +/- 0.02, 0.38 +/- 0.02, 0.97 +/- 0.02, 24 +/- 1. The activation parameters for all reactions suggest an associative substitution mechanism. The pK(a) values and substitution rates of the complexes studied can be tuned through the nature of the N-N chelate, which is important in the development of new active compounds for cancer therapy.     

On the Binding Strength Sequence for Nucleic Acid Bases and C(60) with Density Functional and Dispersion-corrected Density Functional Theories: Whether C(60) could protect nucleic acid bases from radiation-induced damage?

The major objective of this paper is to address a controversial binding sequence between nucleic acid bases (NABs) and C(60) by investigating adsorptions of NABs and their cations on C(60) fullerene with a variety of density functional theories including two novel hybrid meta-GGA functionals, M05-2x and M06-2x, as well as a dispersion-corrected density functional, PBE-D. The M05-2x/6-311++G** provides the same binding sequence as previously reported, guanine(G) > cytosine(C) > adenine (A) > thymine (T); however, M06-2x switches the binding strengths of A and C, and PBE-D eventually results in the following sequence, G>A>T>C, which is the same as the widely accepted hierarchy for the stacking of NABs on other carbon nanomaterials such as single-walled carbon nanotube and graphite. The results indicate that the questionable relative binding strength is due to insufficient electron correlation treatment with the M05-2x or even the M06-2x method. The binding energy of G@C(60) obtained with the M06-2x/6-311++G(d,p) and the PBE-D/cc-pVDZ is -7.10 and -8.07 kcal/mol, respectively, and the latter is only slightly weaker than that predicted by the MP2/6-31G(d,p) (-8.10kca/mol). Thus, the PDE-D performs better than the M06-2x for the observed NAB@C(60) π-stacked complexes. To discuss whether C(60) could prevent NABs from radiation-induced damage, ionization potentials of NABs and C(60), and frontier molecular orbitals of the complexes NABs@C(60) and (NABs@C(60))(+) are also extensively investigated. These results revealed that when an electron escapes from the complexes, a hole was preferentially created in C(60) for T and C complexes, while for G and A the hole delocalizes over the entire complex, rather than a localization on the C(60) moiety. The interesting finding might open a new strategy for protecting DNA from radiation-induced damage and offer a new idea for designing C(60)-based antiradiation drugs.     

The solubilisation behaviour of some dichloroalkanes in aqueous solutions of PEO-PPO-PEO triblock copolymers: a dynamic light scattering, fluorescence spectroscopy, and SANS study

The aggregation behaviour of PEO-PPO-PEO triblock copolymers in water and in water + chlorinated additive mixtures was studied by means of fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The copolymers were chosen such as to investigate the effects of molecular architecture (L35 and 10R5) and molecular weight by keeping constant the hydrophilic/hydrophobic balance (F88 and F108). 1,2-Dichloroethane was used as a prototype of water basins contaminants. The hydrodynamic radius of the block copolymer aggregates (R(h,M)) and the intensity ratio of pyrene of the first and the third vibrational band (I(1)/I(3)) were determined as a function of temperature (10-45 degrees C) and concentration. The copolymer architecture essentially does not affect R(h,M) in the entire range of temperature and concentration investigated. At a given temperature, increasing macromolecular size leads to a decrease of R(h,M). With rising temperature R(h,M) also decreases. According to the DLS results, the I(1)/I(3) change with temperature clearly detects the aggregation only for F88 and F108. The presence of 1,2-dichloroethane, at concentrations close to its solubility in water, does not lead to changes in the distribution of hydrodynamic radii for L35 and 10R5. Larger quantities of additive induce the formation of quite polydisperse mixed aggregates for L35 and of networks for 10R5. In the case of F88 and F108, low concentrations of additive lead to formation of mixed aggregates with smaller R(h,M). The SANS results corroborate the DLS and fluorescence findings proving enhancement of the copolymer aggregation through the presence of 1,2-dichloroethane. The DLS findings combined with those from the fluorescence spectroscopy provide some insight into the site of solubilisation of the additive in the aggregates.     

The synthesis of biodegradable graft copolymer cellulose-graft-poly(L-lactide) and the study of its controlled drug release

According to the concept of green chemistry and sustainable development, a new biodegradable copolymer comprised of hydrophobic poly(l-lactide) (PLLA) segments and hydrophilic cellulose segment (cellulose-g-PLLA) was designed and synthesized. The structure of the copolymer was characterized by (1)H NMR, FT-IR, (13)C NMR, DSC and WAXD. The cytotoxicity study shows that cellulose-g-PLLA exhibits good biocompatibility. The copolymer can self-assemble into micelles in water with the hydrophobic PLLA segments at the cores of micelles and the hydrophilic cellulose segments as the outer shells. Transmission electron microscopy (TEM) shows that the micelles exhibit nanospheric morphology within a size range of 30-80nm. The drug loaded micelles formed by the copolymer in aqueous media show sustained drug release which indicates their potential applicability in drug carrier.     

Poly(N-isopropylacrylamide). I. Interactions with sodium dodecyl sulfate measured by conductivity

Conductometric titration of poly(N-isopropylacrylamide) (polyNIPAM) with sodium dodecyl sulfate (SDS) gave two apparent transitions labeled C1 and C2. The C1 transition was independent of polyNIPAM concentration in the 0.05–0.3 wt % range, whereas C2 was proportional to the polymer concentration. C1 corresponded to the onset of binding of surfactant with polymer. Arguments based on a simple mass action model for micellization are presented to show that C2, the second transition, is not due to any simple explanation such as being the point above which only free micelles are formed with surfactant addition. The cloud point of polyNIPAM increased with the amount of bound surfactant. This was attributed to electrostatic contribution of bound sulfate groups to the increased solubility of polyNIPAM. © 1993 John Wiley & Sons, Inc.     

Speciation of Aluminum(III) Complexes with Oxidized Glutathione in Acidic Aqueous Solutions

The structural speciation aspects, including the binding sites, species, complexation abilities and effects of the oxidized glutathione (GSSG) with aluminum(III) in aqueous solutions, have been studied by means of many analytical techniques: pH-potentiometry (25 degrees C, 0.1 M KCl and 37 degrees C, 0.15 M NaCl medium) was used to characterize the stoichiometry and stability of the species formed in the interactions of the Al(III) ion and the peptide GSSG, while multinuclear ((1)H, (13)C, (27)Al) nuclear magnetic resonance (NMR) and electrospray mass spectroscopy (ESI-MS) were applied to characterize the binding sites and species of the metal ion in the complexes. Two-dimensional ((1)H, (1)H-NOESY) was also employed to reveal the difference in the conformational behavior of the peptide and its complexes. The following results were obtained: (1) Aluminum(III) can coordinate with the important biomolecule GSSG through the following binding sites: glycyl and glutamyl carboxyl groups to form various mononuclear 1:1 (AlLH(4), AlLH(3), AlLH(2), AlLH, AlL, AlLH(-1), AlLH(-2)) and several binuclear 2:1 (Al(2)LH(4), Al(2)LH(2), Al(2)L) species (where H(6)L(2+) denotes the totally protonated oxidized glutathione) in acidic aqueous solutions. (2) It indicates that the COO(-) groups at low level of preorganization in such small peptide are not sufficient to keep the Al(III) ion in solution and to prevent the precipitation of Al(OH)(3) in the physiological pH range. (3) It also suggests that the occurrence of an Al-linked complexation, the conformation of the peptide GSSG in aqueous solutions appeared to change a little, relative to the initial structure.