Role of Arginine in Mediating Protein–Carbon Nanotube Interactions

Arginine-rich proteins (e.g., lysozyme) or poly-l-arginine peptides have been suggested as solvating and dispersing agents for single-wall carbon nanotubes (CNTs) in water. In addition, protein structure–function in porous and hydrophobic materials is of broad interest. The amino acid residue, arginine (Arg⁺), has been implicated as an important mediator of protein/peptide–CNT interactions. To understand the structural and thermodynamic aspects of this interaction at the molecular level, we employ molecular dynamics (MD) simulations of the protein lysozyme in the interior of a CNT, as well as of free solutions of Arg⁺ in the presence of a CNT. To dissect the Arg⁺–CNT interaction further, we also perform simulations of aqueous solutions of the guanidinium ion (Gdm⁺) and the norvaline (Nva) residue in the presence of a CNT. We show that the interactions of lysozyme with the CNT are mediated by the surface Arg⁺ residues. The strong interaction of Arg⁺ residue with the CNT is primarily driven by the favorable interactions of the Gdm⁺ group with the CNT wall. The Gdm⁺ group is not as well-hydrated on its flat sides, which binds to the CNT wall. This is consistent with a similar binding of Gdm⁺ ions to a hydrophobic polymer. In contrast, the Nva residue, which lacks the Gdm⁺ group, binds to the CNT weakly. We present details of the free energy of binding, molecular structure, and dynamics of these solutes on the CNT surface. Our results highlight the important role of Arg⁺ residues in protein–CNT or protein-carbon-based material interactions. Such interactions could be manipulated precisely through protein engineering, thereby offering control over protein orientation and structure on CNTs, graphene, or other hydrophobic interfaces

Chaperonin-Inspired pH Protection by Mesoporous Silica SBA-15 on Myoglobin and Lysozyme

While enzymes are valuable tools in many fields of biotechnology, they are fragile and must be protected against denaturing conditions such as unfavorable solution pH. Within living organisms, chaperonins help enzymes fold into their native shape and protect them from damage. Inspired by this natural solution, mesoporous silica SBA-15 with different pore diameters is synthesized as a support material for immobilizing and protecting enzymes. In separate experiments, the model enzymes myoglobin and lysozyme are physically adsorbed to SBA-15 and exposed to a range of buffered pH conditions. The immobilized enzymes’ biocatalytic activities are quantified and compared to the activities of nonimmobilized enzymes in the same solution conditions. It has been observed that myoglobin immobilized on SBA-15 is protected from acidic denaturation from pH 3.6 to 5.1, exhibiting relative activity of up to 350%. Immobilized lysozyme is protected from unfavorable conditions from pH 6.6 to 7.6, with relative activity of up to 200%. These results indicate that the protective effects conferred to enzymes immobilized by physical adsorption to SBA-15 are driven by the enzymes’ electrostatic attraction to the material’s surface. The pore diameter of SBA-15 affects the quality of protection given to immobilized enzymes, but the contribution of this effect at different pH values remains unclear

Mesostructure of Mesoporous Silica/Anodic Alumina Hierarchical Membranes Tuned with Ethanol

Hierarchically structured membranes composed of mesoporous silica embedded inside the channels of anodic alumina (MS-AAM) were synthesized using the aspiration method. Ethanol is shown to have a significant effect on the type and organization of the mesoporous silica phase. Detailed textural analysis revealed that the pore size distribution of the mesoporous silica narrows and the degree of ordering increases with decreasing ethanol concentration used in the synthesis mixture. The silica mesopores were synthesized with pores as small as 6 nm in diameter, with the channel direction oriented in lamellar, circular, and columnar directions depending on the ethanol content. This study reveals ethanol concentration as a key factor behind the synthesis of an ordered mesoporous silica–anodic alumina membrane that can increase its functionality for membrane-based applications

Hierarchical Silicoaluminophosphate Catalysts with Enhanced Hydroisomerization Selectivity by Directing the Orientated Assembly of Premanufactured Building Blocks

The ability to generate nanoscale zeolites and direct their assembly into hierarchical structures offers a promising way to maximize their diffusion-dependent catalytic performance. Herein, we report an orientated assembly strategy to construct hierarchical architectures of silicoaluminophosphates (SAPOs) by using prefabricated nanocrystallites as a precursor. Such a synthesis is enabled by interrupting the dry gel conversion process to prepare nanocrystallites, as crystal growth is shown to proceed predominantly by particle attachment. The orientation of assembly can be controlled to form either a three-dimensional, spongelike morphology or a two-dimensional “house-of-cards” structure, by modifying the additives. Structures with a high degree of control over crystal size, shape, architecture, pore network, and acidic properties are achieved. This versatile technique avoids the more tedious and expensive templating routes that have been proposed previously. The catalytic performance for the hydroisomerization of <i>n</i>-heptane was evaluated for a series of Pt-supported catalysts, and a record isomer yield (79%) was attained for a catalyst with spongelike architecture. The hierarchical architecture influences isomer selectivity for two reasons: expanding the intrinsic-reaction-controlled regime to be able to work at higher temperatures or conversion levels, and enhancing mass transport to reduce cracking of dibranched isomers. Such an acidity–diffusivity interplay indicates that strong acidity favors isomerization operating at temperatures away from the diffusion-limited regime, while crystal size and pore connectivity are key factors for enhancing diffusion. The proposed materials offer tremendous opportunities to realize hierarchical catalyst designs that work under optimal operating conditions