In-situ 2D bacterial crystal growth as a function of protein concentration: An atomic force microscopy study

Crystal Contact Engineering for Enhanced Cross-Linking Efficiency of HheG Crystals

The generation of cross-linked enzyme crystals is a very attractive method for immobilization of enzymes displaying high crystalizability. However, the commonly used cross-linker glutaraldehyde is not always compatible with enzyme activity. Therefore, we previously reported the engineering of halohydrin dehalogenase HheG from Ilumatobacter coccineus to enable thiol-specific cross-linking during CLEC generation by insertion of cysteine residues in the crystal contact. To broaden the applicability of this approach, herein crystal contact engineering of HheG has been performed to incorporate additional lysine residues as defined cross-linking sites for CLEC generation. Using the primary amine-specific cross-linker dithiobis(succinimidyl propionate) (DSP), CLECs of HheG variant V46K were obtained that displayed a high gain in thermal stability compared to wild-type HheG, while using only a low cross-linker concentration. Moreover, respective V46K CLECs exhibited a 10 K higher reaction temperature optimum as well as significantly improved activity and stability at acidic pH and in the presence of organic co-solvents. Overall, our study demonstrates that lysine-specific cross-linkers can also be used as an alternative to glutaraldehyde for stable CLEC generation of halohydrin dehalogenases, and that cross-linking efficiency is significantly improved upon crystal contact engineering.

Protein Needles Designed to Self-Assemble through Needle Tip Engineering

The dynamic process of formation of protein assemblies is essential to form highly ordered structures in biological systems. Advances in structural and synthetic biology have led to the construction of artificial protein assemblies. However, development of design strategies exploiting the anisotropic shape of building blocks of protein assemblies has not yet been achieved. Here, the 2D assembly pattern of protein needles (PNs) is controlled by regulating their tip-to-tip interactions. The PN is an anisotropic needle-shaped protein composed of β-helix, foldon, and His-tag. Three different types of tip-modified PNs are designed by deleting the His-tag and foldon to change the protein-protein interactions. Observing their assembly by high-speed atomic force microscopy (HS-AFM) reveals that PN, His-tag deleted PN, and His-tag and foldon deleted PN form triangular lattices, the monomeric state with nematic order, and fiber assemblies, respectively, on a mica surface. Their assembly dynamics are observed by HS-AFM and analyzed by the theoretical models. Monte Carlo (MC) simulations indicate that the mica-PN interactions and the flexible and multipoint His-tag interactions cooperatively guide the formation of the triangular lattice. This work is expected to provide a new strategy for constructing supramolecular protein architectures by controlling directional interactions of anisotropic shaped proteins.

Millimeter-Scale Zn(3-ptz)2 Metal-Organic Framework Single Crystals: Self-Assembly Mechanism and Growth Kinetics

The solvothermal synthesis of metal-organic frameworks (MOFs) often proceeds through competing crystallization pathways, and only partial control over the crystal nucleation and growth rates is possible. It challenges the use of MOFs as functional devices in free-space optics, where bulk single crystals of millimeter dimensions and high optical quality are needed. We develop a synthetic protocol to control the solvothermal growth of the MOF [Zn(3-ptz)2] n (MIRO-101), to obtain large single crystals with projected surface areas of up to 25 mm2 in 24 h, in a single reaction with in situ ligand formation. No additional cooling and growth steps are necessary. We propose a viable reaction mechanism for the formation of MIRO-101 crystals under acidic conditions, by isolating intermediate crystal structures that directly connect with the target MOF and reversibly interconverting between them. We also study the nucleation and growth kinetics of MIRO-101 using ex situ crystal image analysis. The synthesis parameters that control the size and morphology of our target MOF crystal are discussed. Our work deepens our understanding of MOF growth processes in solution and demonstrates the possibility of building MOF-based devices for future applications in optics.

Atomic Force Microscopy Meets Biophysics, Bioengineering, Chemistry, and Materials Science

Briefly, herein the use of atomic force microscopy (AFM) in the characterization of molecules and (bioengineered) materials related to chemistry, materials science, chemical engineering, and environmental science and biotechnology is reviewed. First, the basic operations of standard AFM, Kelvin probe force microscopy, electrochemical AFM, and tip-enhanced Raman microscopy are described. Second, several applications of these techniques to the characterization of single molecules, polymers, biological membranes, films, cells, hydrogels, catalytic processes, and semiconductors are provided and discussed.

Life under Continuous Streaming: Recrystallization of Low Concentrations of Bacterial SbpA in Dynamic Flow Conditions

The well-known bacterial S-layer protein SbpA from Lysinibacillus sphaericus CCM2177 induces spontaneous crystal formation via cooperative self-assembly of the protein subunits into an ordered supramolecular structure. Recrystallization occurs in the presence of divalent cations (i.e., Ca2+) and finally leads to producing smooth 2-D crystalline coatings composed of squared (p4) lattice structures. Among the factors interfering in such a process, the rate of protein supply certainly plays an important role since a limited number of accessible proteins might turn detrimental for film completion. Studies so far have mostly focused on high SbpA concentrations provided under stopped-flow or dynamic-flow conditions, thus omitting the possibility of investigating intermediate states, in which dynamic flow is applied for more critical concentrations of SbpA (i.e., 25, 10, and 5 µg/mL). In this work, we have characterized both physico-chemical and topographical aspects of the assembly and recrystallization of SbpA protein in such low concentration conditions by means of in situ Quartz Crystal Microbalance with Dissipation (QCMD) and atomic force microscopy (AFM) measurements, respectively. On the basis of these experiments, we can confirm how the application of a dynamic flow influences the formation of a closed and crystalline protein film from low protein concentrations (i.e., 10 µg/mL), which otherwise would not be formed.

Novel biodegradable and non-fouling systems for controlled-release based on poly(ε-caprolactone)/Quercetin blends and biomimetic bacterial S-layer coatings

Quercetin is a strong antioxidant with low bioavailability due to its high crystallinity. A further drawback is that Quercetin has potentially toxic effects at high concentrations. To improve this low water solubility, as well as control the concentration of the flavonoid in the body, Quercetin is incorporated into a polymeric matrix to form an amorphous solid dispersion (ASD) stable enough to resist the recrystallization of the drug. For this purpose, miscible poly(ε-caprolactone) (PCL) and Quercetin (Q) blends are prepared, provided that they have complementary interacting groups. For compositions in which the flavonoid remains in an amorphous state thanks to the interactions with polymer chains, various PCL/Q drug release platforms are fabricated: micrometric films by solvent casting, nanometric films by spin coating, and nanofibers by electrospinning. Then, the potential use of bacterial S-layer proteins as release-preventive membranes is tested on PCL–Quercetin blends, due to their ability to construct a biomimetic coating including nanometric pores. For all the platforms, the SbpA coating can maintain a stable release under the toxicity level of Quercetin. Accordingly, a PCL/Q system with an S-layer coating allows the design of versatile bioavailable Quercetin eluting devices that prevent toxicity and biofouling issues.

S-Layer coating on PCL–Quercetin miscible blends acts as a release controller from films and scaffolds, as well as antifouling agent.

In-situ 2D bacterial crystal growth as a function of protein concentration: An atomic force microscopy study

The interplay between protein concentration and (observation) time has been investigated for the adsorption and crystal growth of the bacterial SbpA proteins on hydrophobic fluoride-functionalized SiO2 surfaces. For this purpose, atomic force microscopy (AFM) has been performed in real-time for monitoring protein crystal growth at different protein concentrations. Results reveal that (1) crystal formation occurs at concentrations above 0.08 µM and (2) the compliance of the formed crystal decreases by increasing protein concentration. All the crystal domains observed presented similar lattice parameters (being the mean value for the unit cell: a = 14.8 ± 0.5 nm, b = 14.7 ± 0.5 nm, γ = 90 ° ± 2). Protein film formation is shown to take place from initial nucleation points which originate a gradual and fast extension of the crystalline domains. The Avrami equation describes well the experimental results. Overall, the results suggest that protein-substrate interactions prevail over protein-protein interactions. RESEARCH HIGHLIGHTS: AFM enables to monitor protein crystallization in real-time. AFM high-resolution determines lattice parameters and viscoelastic properties. S-layer crystal growth rate increases with protein concentration. Avrami equation models protein crystal growth.