Characterization of Ultrathin Films of Cellulose Esters

The adsorption of cellulose acetate (CA), cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB) from solutions prepared in acetone onto silicon wafers led to ultrathin films, which were characterized by ellipsometry, atomic force microscopy (AFM) and contact angle measurements. The polysaccharides films were characterized in the air just after their formation and after annealing at temperatures higher than their glass transition temperature or melt temperature. The films thickness close to 2 nm and surface roughness did not vary significantly upon annealing. AFM images revealed the presence of small clumps dispersed on a homogeneous layer, which covered completely the Si wafers. Such topographic details were also observed after annealing. However, upon annealing the films surfaces changed from hydrophilic to hydrophobic, evidencing molecular re-orientation at the solid–air interface. The adhesion of bovine serum albumin (BSA) and lipase onto the cellulose esters films was quantified in order to evaluate the possibility of applying such films as selective support for biomolecules.     

Surface forces in foam films from ABA block copolymer: a dynamic method study

The thinning of foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (average molecular weight 14,000 g/mol) is studied experimentally. The dependence of the surface forces on film thickness is obtained by the dynamic method of Scheludko and Exerowa.The total surface force measured in foam films (radius 60–70 m) from 10^–5 M (0.014 wt%) polymer solution with 0.1 M NaCl is positive at thicknesses from about 800 down to 460 . The electrostatic repulsion is negligible while the contribution of van der Waals attraction is small (within 15%). Therefore a positive surface force component predominates. Most probably it arises from steric interactions between the hydrophilic polyethylene oxide tails of the polymer. The dynamic method appears to be a suitable technique for exploring the stabilization of foam films from ABA copolymers.     

辣根过氧化物酶活性膜结构及生物电催化性能

通过分子沉积法研究了在聚对苯二甲酸乙二醇酯（PET）表面及金电极表面组装辣根过氧化物酶（HRP）/聚对苯乙烯磺酸钠（PSS）多层生物活性膜，用原子力显微镜(AFM)研究了组装膜的表面形貌，并研究了组装膜的形貌、粗糙度和活性关系.应用循环伏安法（CV）研究了组装HRP膜后电极对H2O2的电化学催化还原作用.实验发现，采用亚甲基蓝（MB）溶液为介质，在H2O2浓度为0.2～5.0 mmol•L－1时，其响应电流对H2O2浓度变化基本呈线性.     

Dynamic imaging of single DNA-protein interactions using atomic force microscopy

Atomic force microscopy (AFM) imaging of static DNA-protein complexes, in air and in liquid, can be used to directly obtain quantitative and qualitative information on the structure of different complexes. For example, DNA length, the location of preferential binding sites for proteins and bending of DNA as a result of the complexation can all be measured. Recording consecutive AFM images of DNA and protein molecules under conditions that they are still able to move and interact, or dynamic AFM imaging, however, can reveal information on the dynamic aspects of the interactions between these molecules. Here, an overview is given of the technical challenges that need to be considered for successful dynamic AFM imaging studies of individual DNA-protein interactions. Necessary technical improvements to the AFM set-up and the development of new sample preparation methods are described in this paper.     

Free energy perturbation approach to the critical assessment of selective cyclooxygenase-2 inhibitors

The discovery of selective cyclooxygenase-2 (COX-2) inhibitors represents a major achievement of the efforts over the past few decades to develop therapeutic treatments for inflammation. To gain insights into designing new COX-2-selective inhibitors, we address the energetic and structural basis for the selective inhibition of COX isozymes by means of a combined computational protocol involving docking experiment, force field design for the heme prothetic group, and free energy perturbation (FEP) simulation. We consider both COX-2- and COX-1-selective inhibitors taking the V523I mutant of COX-2 to be a relevant structural model for COX-1 as confirmed by a variety of experimental and theoretical evidences. For all COX-2-selective inhibitors under consideration, we find that free energies of binding become less favorable as the receptor changes from COX-2 to COX-1, due to the weakening and/or loss of hydrogen bond and hydrophobic interactions that stabilize the inhibitors in the COX-2 active site. On the other hand, COX-1-selective oxicam inhibitors gain extra stabilization energy with the change of residue 523 from valine to isoleucine because of the formations of new hydrogen bonds in the enzyme-inhibitor complexes. The utility of the combined computational approach, as a valuable tool for in silico screening of COX-2-selective inhibitors, is further exemplified by identifying the physicochemical origins of the enantiospecific selective inhibition of COX-2 by -substituted indomethacin ethanolamide inhibitors.     

Second Dissociation Constants of 4-[N-morpholino]butanesulfonic Acid and N-[2-hydroxyethyl]piperazine-N′-4-butanesulfonic Acid from 5 to 55°C

The values of the second dissociation constant, pK ₂, for the dissociation of the NH⁺ charge center of the zwitterionic buffer compounds 4-(N-morpholino)butanesulfonic acid (MOBS), and N-(2-hydroxyethyl)piperazine-N-4-butanesulfonic acid (HEPBS) have been determined from 5 to 55°C, including, 37°C at intervals of 5°C. The electromotive-force (emf) measurements have been made utilizing hydrogen electrodes and silver–silver chloride electrodes. The value of pK ₂ for MOBS was found to be 7.702 ± 0.0005, and 8.284 ± 0.0004 for HEPBS, at 25°C, respectively. The related thermodynamic quantities, G ^o, H ^o, S ^o, and C _p ^o for the dissociation processes of MOBS and HEPBS have been derived from the temperature coefficients of pK ₂. Both the MOBS and HEPBS buffer materials are useful as primary pH standards for the control of pH 7.3 to 8.6 in the region close to that of physiological fluids.     

Extremality and the global Markov property II: The global markov property for non-FKG maximal Gibbs measures

We give a condition on a Gibbs measure for an attractive Markov specification, which assures extremality and the global Markov property. As an example of application we consider the class of attractive Markov specifications defined on a compact configuration space over a two-dimensional lattice by the interaction Hamiltonians (assumed to have a finite set of periodic ground configurations) satisfying Peierl's condition. We prove that each extremal Gibbs measure for such a specification, at sufficiently low temperature, has the global Markov property.On leave of absence from the Institute of Theoretical Physics, University of Wrocaw, Poland.     

AFM of adsorbed polyelectrolytes on cellulose I surfaces spin-coated on silicon wafers

Thin layers of cellulose I nanocrystals were spin-coated onto silicon wafers to give a flat model cellulose surface. A mild heat treatment was required to stabilize the cellulose layer. Interactions of this surface with polyelectrolyte layers and multilayers were probed by atomic force microscopy in water and dilute salt solutions. Deflection–distance curves for standard silicon nitride tips were measured for silicon, cellulose-coated silicon, and for polyelectrolytes adsorbed on the cellulose surface. Transfer of polymer to the tip was checked by running deflection–distance curves against clean silicon. Deflection–distance curves were relatively insensitive to adsorbed polyelectrolyte, but salt addition caused transfer of cationic polyelectrolyte to the tip, and swelling of the polyelectrolyte multilayers.     

Synthesis, structure and magnetic properties of the S=1/2, one-dimensional antiferromagnet, Y5Re2O12

Single crystals of Y₅Re₂O₁₂ have been grown, and the crystal structure has been determined by X-ray diffraction. This compound crystallizes in space group C2/m with cell dimensions of a=12.4081(10) Å b=5.6604(5)Å, c=7.4951(6) Å, β=107.837(3)°, Z=2. The final refinement led to R1=0.0238, WR2=0.0459 for 1053 observed reflections with F>4σ(F₀). Edge-sharing ReO₆ octahedra form infinite linear [ReO₂O_4/2]_n chains along the b direction with alternating short and long Re-Re distances. Three crystallographically independent yttrium atoms surround O2 to form OY₄ tetrahedra, which share edges and corners in the ab plane to form a two-dimensional Y₅O₄ network which separates the [ReO₂O_4/2]_n magnetic chains. This compound is therefore isostructural with the series Ln₅Re₂O₁₂Ln=Gd-Lu, which have been known since 1969. The average Re oxidation state is +4.5 in the chains and a reasonable, if qualitative MO scheme results in one unpaired electron per Re dimer. Consistent with this, magnetic susceptibility data can be fitted to the one-dimensional antiferromagnetic Heisenberg model with S=1/2 and parameters J^intra/k=−89(1)K, g=2.15(4) and χ(TIP)=5(1)×10⁻⁴ emu/mol. There is no sign of long-range magnetic order down to 2 K. These results are contrasted with those for the isostructural Y₅Mo₂O₁₂.     

Enzymatic degradation of highly phenolic lignin-based polymers (lignophenols)

Enzymatic degradation of two lignin-based polymers (lignophenols), lignocatechol and lignocresol, prepared by selectively grafting catechol and p-cresol to C_α positions of lignin, respectively, were carried out in aqueous organic solvents. Both lignophenols showed high reactivity in the peroxidase-catalyzed oxidation. Structural analyses by NMR spectroscopies revealed that the degraded lignophenols contained aliphatic chain content, which might be mainly formed in the reduction of the intermediate initially generated by the aromatic ring cleavage. Lower amount of aromatic units in the lignophenols after degraded by peroxidase also indicted the cleavage of aromatic rings. Due to the substitution of phenols at C_α positions of lignin, the degraded lignophenols did not have carbonyl structure, which was abundant in the biodegradation products of native lignin. The two lignophenols were also degraded by Rhus vernicifera laccase. But the degree of degradation was lower than that of the degradation by peroxidase, which might be due to the low activity of laccase on the lignin moieties in lignophenols.