Biocatalysis of large-sized substrates finds wide applications. Immobilizing the involved enzymes on solid supports improves biocatalysis yet faces challenges such as enzyme structural perturbation, leaching, and low cost-efficiencies, depending on immobilization strategies/matrices. Carbon nanotubes (CNTs) are attractive matrices but challenged by enzyme leaching (physical adsorption) or perturbation (covalent linking). Zeolitic imidazolate frameworks (ZIFs) overcome these issues. However, our recent study [J. Am. Chem. Soc., 2018, 140, 16032−16036] showed reduced costefficiency as enzymes trapped below the ZIF surfaces cannot participate in biocatalysis; the enzyme−ZIF composites are also unstable under acidic conditions. In this work, we demonstrate the feasibility of using ZIFs to immobilize enzymes on CNT surfaces on two model enzymes, T4 lysozyme and amylase, both of which showed negligible leaching and retained catalytic activity under neutral and acidic conditions. To better understand the behavior of enzymes on CNTs and CNT−ZIF, we characterized enzyme orientation on both matrices using site-directed spin-labeling (SDSL)−electron paramagnetic resonance (EPR), which is immune to the complexities caused by CNT and ZIF background signals and enzyme−matrix interactions. Our structural investigations showed enhanced enzyme exposure to the solvent compared to enzymes in ZIFs alone; orientation of enzymes in matrices itself is directly related to substrate accessibility and, therefore, essential for understanding and improving catalytic efficiency. To the best of our knowledge, this is the first time ZIFs and onepot synthesis are employed to anchor large-substrate enzymes on CNT surfaces for biocatalysis. This is also the first report of enzyme orientation on the CNT surface and upon trapping in CNT−ZIF composites. Our results are essential for guiding the rational design of CNT−ZIF combinations to improve enzyme stabilization, loading capacity, and catalytic efficiency.

Gold nanoparticles (AuNPs) are advancing a number of areas. Meanwhile, the inevitable contact of AuNPs with surrounding biomolecules has been realized to “negatively” impact the nanoparticle (NP) function, biological environment, and even public health. Understanding the mechanisms of the NP–biomolecule interaction often leads to revealing the pathways toward the negative, deleterious effects. It is therefore essential to know the undesirable pathways so that these unwanted effects can be reduced by blocking or rerouting these pathways. A fundamental step to understand the interaction mechanism is to elucidate the changes in protein structure and dynamics and in NP properties upon the contact of AuNPs and protein, at the molecular level. However, it is challenging to probe these areas experimentally due to the large size and the complexity of the protein–nano interface. Here we report the use of a unique approach, electron paramagnetic resonance (EPR) spectroscopy, to probe the structure and dynamics of a model protein, T4 lysozyme (T4L), upon interaction with AuNPs. In combination with circular dichroism (CD) spectroscopy, UV–vis spectroscopy, and a protein activity assay, we found that T4L triggered the AuNPs to aggregate but remained in its native structure. Remarkably, our results also revealed the pathways of how T4L triggers AuNP aggregation, the potential applications of which were discussed. Lastly, this work demonstrated the usefulness of the EPR spectroscopy in probing the complexes formed by nanomaterials and macrobiomolecules, opening a new window to probe the nano–bio interface in the native state.

Silica nanoparticles (SiNPs) are important nano-sized, solid-state carriers/hosts to load, store, and deliver biological or pharmaceutical cargoes. They are also good potential solid supports to immobilize proteins for fundamental protein structure and dynamics studies. However, precaution is necessary when using SiNPs in these areas because adsorption might alter the activity of the cargoes, especially when enzymes are loaded. Therefore, it becomes important to understand the structural basis of the cargo enzyme activity changes, if there is any. The high complexity and dynamics of the nano-bio interface present many challenges. Reported here is a comprehensive study of the structure, dynamics, and activity of a model enzyme, T4 lysozyme, upon adsorption to a few surface-modified SiNPs using several experimental techniques. Not surprisingly, a significant activity loss on each studied SiNP was found. The structural basis of the activity loss was identified based on results from a unique technique, the Electron Paramagnetic Resonance (EPR) spectroscopy, which probes structural information regardless of the complexity. Several docking models of the enzyme on SiNPs with different surfaces, at different enzyme-to-SiNP ratios are proposed. Interestingly, we found that the adsorbed enzyme can be desorbed via pH adjustment, which highlighted the potential to use SiNPs for enzyme/protein delivery or storage due to the high capacity. In order to use SiNPs as enzyme hosts, minimizing the enzymatic activity loss upon adsorption is needed. Lastly, the work outlined here demonstrate the use of EPR in probing structural information on the complex (inorganic)nano-bio interface.

Gold nanoparticles (AuNPs) and nanorods (AuNRs) find broad applications due to their unique optical and chemical properties. In biological applications, the contact of AuNPs/AuNRs with proteins is inevitable, resulting in the formation of a "protein corona", protein−particle agglomerates, or particle precipitation. While nonspecific adsorption or particle precipitation should be avoided, controllable protein adsorption and agglomerate formation via surface modification find applications in protein immobilization and therapeutics. Therefore, it becomes essential to understand the influences of particle surfaces on protein adsorption. Recently, we found a "problematic" globular protein, T4 lysozyme (T4L), precipitating AuNPs [Neupane et al. J. Phys. Chem. C. 2017, 121, 1377−1386. Herein, we systematically investigated the effects of surface modification on the adsorption of T4L. We found that both positively charged and neutral polymer coatings are effective in preventing such precipitation. In addition, for AuNPs with negative coatings, T4L could form either a stable protein corona or agglomerates, depending on the coating. For T4L and negatively coated AuNRs, only coronas were formed regardless of coating thickness. In all cases, we utilized EPR to detect protein rotational tumbling and backbone dynamics, which revealed the local environment that T4L experiences in these complexes. Such information is important for guiding future designs of gold nanomaterial−protein complexes with desired functions. Our findings demonstrate the importance of coatings on AuNP/AuNR functions in biological environments. With negative coatings, AuNPs/AuNRs can serve as immobilizers for carrying positively charged proteins. Furthermore, with proper coatings, a "precipitation-causing" protein could facilitate the formation of AuNP-based agglomerates which can have thermotherapeutic applications.

Polymer structure and conformational dynamics are essential to polymer macroscopic properties, but are challenging to probe. We report here a synthetic pathway to chemically add a nitroxide moiety onto block polymers in a mild, aqueous environment and demonstrate its use in a series of polymeric micelles using Electron Paramagnetic Resonance (EPR) spectroscopy. The micelles were characterized with several analytical approaches and EPR findings were in general consistent with other approaches. Upon exposure to organic solvents, the line shape changes reflected the micelle swelling and EPR spectral simulations revealed structural information of the swelled micelles. The label introduced via our method can be cleaved and replaced with other probes to report different information site‐specifically. The mild conditions facilitate the future use of EPR in solving biopolymer problems. In combination with other labeling approaches, one can perform polymer spin labeling with different chemistry, so that various information about polymers can be obtained site‐specifically. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1770–1782