Mechanical measurement of hydrogen bonded host-guest systems under non-equilibrium, near-physiological conditions

Insights into spheroids formation in cellulose nanofibrils and Matrigel hydrogels using AFM-based techniques

The recent FDA decision to eliminate animal testing requirements emphasises the role of cell models, such as spheroids, as regulatory test alternatives for investigations of cellular behaviour, drug responses, and disease modelling. The influence of environment on spheroid formation are incompletely understood, leading to uncertainty in matrix selection for scaffold-based 3D culture. This study uses atomic force microscopy-based techniques to quantify cell adhesion to Matrigel and cellulose nanofibrils (CNF), and cell-cell adhesion forces, and their role in spheroid formation of hepatocellular carcinoma (HepG2) and induced pluripotent stem cells (iPS(IMR90)-4). Results showed different cell behaviour in CNF and Matrigel cultures. Both cell lines formed compact spheroids in CNF but loose cell aggregates in Matrigel. Interestingly, the type of cell adhesion protein, and not the bond strength, appeared to be a key factor in the formation of compact spheroids. The gene expression of E- and N-cadherins, proteins on cell membrane responsible for cell-cell interactions, was increased in CNF culture, leading to formation of compact spheroids while Matrigel culture induced integrin-laminin binding and downregulated E-cadherin expression, resulting in looser cell aggregates. These findings enhance our understanding of cell-biomaterial interactions in 3D cultures and offer insights for improved 3D cell models, culture biomaterials, and applications in drug research.

Optical Tweezers to Force Information out of Biological and Synthetic Systems One Molecule at a Time

Over the last few decades, in vitro single-molecule manipulation techniques have enabled the use of force and displacement as controlled variables in biochemistry. Measuring the effect of mechanical force on the real-time kinetics of a biological process gives us access to the rates, equilibrium constants and free-energy landscapes of the mechanical steps of the reaction; this information is not accessible by ensemble assays. Optical tweezers are the current method of choice in single-molecule manipulation due to their versatility, high force and spatial and temporal resolutions. The aim of this review is to describe the contributions of our lab in the single-molecule manipulation field. We present here several optical tweezers assays refined in our laboratory to probe the dynamics and mechano-chemical properties of biological molecular motors and synthetic molecular devices at the single-molecule level.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Dynamics of Hydrogen Bonding in Three-Component Nanorotors

The dynamics of hydrogen bonding do not only play an important role in many biochemical processes but also in Nature's multicomponent machines. Here, a three-component nanorotor is presented where both the self-assembly and rotational dynamics are guided by hydrogen bonding. In the rate-limiting step of the rotational exchange, two phenolic O-H-N,N(phenanthroline) hydrogen bonds are cleaved, a process that was followed by variable-temperature 1 H NMR spectroscopy. Activation data (ΔG≠ 298 =46.7 kJ mol-1 at 298 K, ΔH≠ =55.3 kJ mol-1 , and ΔS≠ =28.8 J mol-1  K-1 ) were determined, furnishing a rotational exchange frequency of k298 =40.0 kHz. Fully reversible disassembly/assembly of the nanorotor was achieved by addition of 5.0 equivalents of trifluoroacetic acid (TFA)/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) over three cycles.

Hydrogen-bonded host-guest systems are stable in ionic liquids

We show that H-bonded host–guest systems associate in ionic liquids (ILs), pure salts with melting point below room temperature, in which dipole–dipole electrostatic interactions should be negligible in comparison with dipole-charge interactions. Binding constants (K_a) obtained from titrations of four H-bonded host–guest systems in two organic solvents and two ionic liquids yield smaller yet comparable K_a values in ionic liquids than in organic solvents. We also detect the association event using force spectroscopy, which confirms that the binding is not solely due to (de)solvation processes. Our results indicate that classic H-bonded host–guest supramolecular chemistry takes place in ILs. This implies that strong H-bonds are only moderately affected by surroundings composed entirely of charges, which can be interpreted as an indication that the balance of Coulombic to covalent forces in strong H-bonds is not tipped towards the former.

Measuring the Stability of Supramolecular Complexes in the Proximity of Single-Walled Carbon Nanotubes

The decoration of SWNTs with supramolecular motifs is a common strategy for their subsequent noncovalent functionalization. However, due to the lack of a standard methodology, there are no quantitative measurements showing the extent to which the supramolecular equilibria are affected by one of the host-guest couple being anchored to the SWNT. Here, we use a method we initially developed to quantify association of small organic molecules to the walls of SWNTs to compare association constants of two host-guest systems, a Hamilton receptor-cyanuric acid derivative and a crown ether-ammonium couple, in solution and when the host is covalently attached to the SWNTs. Our data show that association does occur, but the stability of the complexes is significantly affected, as reflected in a sizable reduction in their association constant, when compared to solution.

Chaperone-Assisted Host-Guest Interactions Revealed by Single-Molecule Force Spectroscopy

The recent discovery of ultra-high binding affinities in cucurbit[7]uril (CB7)-based host-guest pairs in an aqueous environment has rendered CB7 a rather attractive material in analytical and biomedical applications. Due to the lack of a molecular platform that can follow the same host-guest complex during repetitive mechanical measurements, however, mechanical stabilities of the CB7 system have not been revealed, hindering its potential to rival widely used conjugation pairs, such as streptavidin-biotin. Here, we assembled a DNA template in which a flexible DNA linker was exploited to keep the host (CB7) and guest (adamantane) in proximity. This platform not only increased the efficiency of the single-molecule characterization in optical tweezers but also clearly revealed mechanical features of the same host-guest complex. We found that positively charged adamantane guest demonstrated higher mechanical stability (49 pN) than neutral adamantane (44 pN), a trend consistent with the chemical affinity between guest molecules and the CB7 host. Surprisingly, we found that a hexyl group adjacent to the adamantane served as a chaperone to assist the formation of the adamantane-CB7 pairs. The discovery of an unprecedented chaperone-assisted interaction mechanism provides new approaches to efficiently assemble host-guest-based supramolecules with increased mechanical stabilities, which can be exploited in various biomedical, biosensing, and materials fields.

Dynamics of individual molecular shuttles under mechanical force

Molecular shuttles are the basis of some of the most advanced synthetic molecular machines. In these devices a macrocycle threaded onto a linear component shuttles between different portions of the thread in response to external stimuli. Here, we use optical tweezers to measure the mechanics and dynamics of individual molecular shuttles in aqueous conditions. Using DNA as a handle and as a single molecule reporter, we measure thousands of individual shuttling events and determine the force-dependent kinetic rates of the macrocycle motion and the main parameters governing the energy landscape of the system. Our findings could open avenues for the real-time characterization of synthetic devices at the single molecule level, and provide crucial information for designing molecular machinery able to operate under physiological conditions.

Molecular shuttles are bi-stable and stimuli-responsive systems that are considered potential elements for molecular machinery. Here, the authors use optical tweezers to measure the force dependent real-time kinetics of individual molecular shuttles under aqueous conditions.