A new mesoporous silica structure with cubic Iad symmetry has been synthesized. The structure has very large unit cell dimensions, up to 250 A in the as-synthesized form and 222 A after calcination, and the surface area is around 700 m2/g. The syntheses were done according to well-established synthesis conditions with the triblock copolymers Pluronics P103 or P123, except for the addition of NaI to the synthesis mixture. Small-angle X-ray scattering revealed that the sample has Iad symmetry. According to electron micrographs, the structure is similar to that of MCM-48, and we thus propose that the structure lies on the gyroid minimal surface.

Adsorption of uncharged homopolymers of various flexibilities in good solvent onto planar surfaces at various polymer-surface interaction strengths have been investigated by employing a coarsegrained bead-spring polymer model using simulation techniques. The polymer flexibility ranged from fully flexible to rod-like polymers, and the adsorption strength varied from weak to strong adsorption. Equilibrium adsorption properties were determined by Monte Carlo simulations, and adsorption processes were investigated by Brownian dynamic simulations. In the latter case, the initial systems were composed of a polymer solution and a surface separated by a slab of polymer-free solution. The equilibrium properties of the interfacial systems have been analyzed by monitoring bead and polymer density profiles, number of adsorbed beads and polymers, the components of the radius of gyration perpendicular and parallel to the surface as well as tail, loop, and train statistics. Flexible polymers adsorbed in two layers, and at an increasing surface attraction the number of adsorbed beads and polymers increased and the adsorbed polymers become flatter, whereas rod-like polymers adsorbed in a single and thin layer with a nematic-like order. At increasing polymer stiffness at fixed surface attraction strength, the number of adsorbed beads increased, whereas the number of adsorbed polymers, the polymer extension perpendicular to the surface, and the fraction of beads in tails all displayed nontrivial maxima at similar persistence length. The dynamic analysis showed that the initial adsorption was diffusion controlled, but soon became governed by the probability of a polymer to be captured by the surface attraction. Flexible polymers became flattened after attaching, but their final relaxation mechanism involved an increased perpendicular extension with fewer adsorbed beads and longer tails driven by the surface pressure originating from the surrounding adsorbed polymers. The stiff polymers displayed a much slower final relaxation to their equilibrium state; this relaxation predominately constituting a packing of the rod-like polymers in a 2-dimensional nematic order. Furthermore, we have defined an integration time denoting the adsorption time for adsorbed polymers to become fully integrated into the adsorbed layer. Integration times and residence times of integrated polymers became longer with increasing polymer stiffness and increasing bead-surface attraction.

The dynamics of adsorption and desorption of uncharged homopolymers have been investigated using a coarse-grained model comprised of a bead−spring chain and a planar surface. Brownian dynamics simulation has been used to examine the adsorption process for polymers released near the surface and with bulk properties as well as Monte Carlo simulations to explore equilibrium adsorption structures of the polymer. Systems with varying polymer contour length, polymer stiffness, and polymer−surface interaction potential have been considered. Investigations have been made on polymer extensions perpendicular and parallel to the surface and also characterized the adsorbed state in terms of loops, tails, and trains. After a diffusion of the polymer to the vicinity of the surface, three succeeding phases were identified:  distortion, attachment, and relaxation phase. In the distortion phase, the polymer starts to experience the potential of the surface and becomes elongated perpendicular to the surface. In the attachment phase, the polymer makes direct contact with the surface and becomes contracted perpendicular to the surface and starts to spread on the surface. Finally, in the extended relaxation phase, the polymer continues to spread on the surface.

Polymer adsorption of uncharged homopolymers onto planar surfaces has been investigated by employing a coarse-grained bead-spring polymer model using simulation techniques. Polymer solutions of two different densities and polymers of two different contour lengths have been examined. The dynamics of the adsorption process appearing in systems composed of a polymer solution placed near attractive, but initially polymer-free, surfaces were determined by Brownian dynamics simulations and equilibrium properties of corresponding systems by Monte Carlo simulations. The properties of the systems have been analyzed by monitoring the number of adsorbed polymers, bead density profiles, time and frequency of polymer attachments, spatial extension of polymers perpendicular and parallel to the surface, and configurational characteristics. Initially, the polymers diffuse toward the surfaces, and at shorter distances the attractive surface potential starts to pull the polymers toward the surface, making them slightly stretched perpendicular to the surface. Thereafter, the polymers collapse onto the surface with multiple anchoring, and a slower relaxation increasing the extension of the polymers parallel to the surfaces appears. Finally, at even longer times, and connected to the slow relaxation of the number of adsorbed polymers, the extension of the polymer coils parallel to the surface is reduced, and the perpendicular extension is increased, with associated changes of the number of beads residing in tails, loop, and trains. The adsorption process becomes faster at decreasing polymer length and at increasing polymer density.

ABSTRACT:The catalytic copper site in nitrite reductase contains a Cu 2ϩ ion bound to three histidine (His) ligands and a solvent molecule. Sites from various sources show a conspicuous variation in the structure. In some proteins, it is close to tetrahedral (even more so than are blue copper proteins), whereas in other proteins, it has a structure more similar to that expected for a type 2 copper site. We have studied this site with a number of theoretical methods, ranging from vacuum optimizations, combined quantum and molecular mechanics (QM/MM) optimization, quantum refinement (X-ray crystallography supplemented by quantum chemical calculations), and accurate energy calculations. We show that the difference in the structure arises from a movement of the solvent molecule and that this movement is determined by a compromise between its hydrogen bond interactions and the intrinsic preferences of the copper site. If the solvent molecule is deprotonated, the two structures have a similar energy, whereas if it is protonated, the more tetrahedral structure is energetically favorable. Neither of the structures involves a interaction as in the blue copper proteins; instead, both are strongly distorted tetragonal structures with bonds to all four ligands. We have also examined the position of hydrogen atoms shared between second-sphere carboxylate groups and the firstsphere solvent molecule and one of the His ligands. In the oxidized state, the structure with the solvent deprotonated but the His residue protonated seems to be most stable.