Label-Free Quantification of Molecular Interaction in Live Red Blood Cells by Tracking Nanometer Scale Membrane Fluctuations

Physicochemical Perspective of Biological Heterogeneity

The vast majority of chemical processes that govern our lives occur within living cells. At the core of every life process, such as gene expression or metabolism, are chemical reactions that follow the fundamental laws of chemical kinetics and thermodynamics. Understanding these reactions and the factors that govern them is particularly important for the life sciences. The physicochemical environment inside cells, which can vary between cells and organisms, significantly impacts various biochemical reactions and increases the extent of population heterogeneity. This paper discusses using physical chemistry approaches for biological studies, including methods for studying reactions inside cells and monitoring their conditions. The potential for development in this field and possible new research areas are highlighted. By applying physical chemistry methodology to biochemistry in vivo, we may gain new insights into biology, potentially leading to new ways of controlling biochemical reactions.

Electrospun Fiber Membrane with the Dual Affinity of Chelation and Covalent Interactions for the Efficient Enrichment of Glycoproteins

As important biomarkers of many diseases, glycoproteins are of great significance to biomedical science. It is essential to develop efficient glycoprotein enrichment platforms and investigate their adsorption mechanism. In this work, a conspicuous enrichment strategy for glycoproteins was developed by using an electrospun fiber membrane wrapped with polydopamine (PDA) and modified with 3-aminophenylboronic acid and nickel ions, named PAN/DA@PDA@APBA/Ni. The enrichment characteristics of PAN/DA@PDA@APBA/Ni toward glycoproteins were explored through adsorption behavior. Thanks to the existence of two sites of interaction (metal ion chelation and boronate affinity), PAN/DA@PDA@APBA/Ni exhibited significant enrichment capacity for glycoproteins, ovalbumin (604.6 mg/g), and human immunoglobulin G (331.0 mg/g). The adsorption kinetic results of glycoprotein ovalbumin on PAN/DA@PDA@APBA/Ni conform to the pseudo-first-order kinetic model in the first adsorption stage, while the second half adsorption stage is more in line with the pseudo-second-order kinetic model. Moreover, the physical characteristics of PAN/DA@PDA@APBA/Ni and subsequent adsorption experiments on electrospun fiber modified with only phenylboronic acid or nickel ions both confirmed two sites of interaction (metal ion chelation and boronate affinity, respectively). Furthermore, a stepwise elution method with dual-affinity interaction was designed and successfully applied to enrich glycoproteins in real biological samples. This work provides an idea for sample pretreatment, especially for the design of dual-affinity materials in glycoproteins enrichment.

Polarization-Sensitive Asymmetric Scattering at the Single-Particle Scale via Surface Plasmon Resonance Microscopy

Surface plasmon resonance microscopy (SPRM), based on the scattering of single molecules at the interface, is a highly efficient analytical platform widely used in the fields of biology and chemistry. Due to the interference scattering, the imaging pattern exhibits typical parabolic tail and phase transition features, providing a quantitative means of observing the changes in the physical and chemical properties of single molecules. In this work, we reported another unique asymmetric parabolic distribution pattern resulting from polarization conversion in the experiment based on SPRM. This microscopic-level feature is derived from the switching between SPR resonant and nonresonant states. Starting from energy flux theory, we constructed an analysis model and conducted full-wave numerical simulations to verify the experimental results. Furthermore, we demonstrate that the optical rotation induced by chiral thin films can be directly measured through imaging with asymmetric features, providing valuable insights into the field of chiral materials. The quantitative interpretation of asymmetric scattering not only advances the fundamental understanding of the imaging mechanism of SPRM, but also opens up possibilities for utilizing this polarization-sensitive characteristic for single-particle detection and sensing in the future.

Ligand Binding-Induced Cellular Membrane Deformation is Correlated with the Changes in Membrane Stiffness

Study interaction between ligands and protein receptors is a key step for biomarker research and drug discovery. In situ measurement of cell surface membrane protein binding on whole cells eliminates the cost and pitfalls associated with membrane protein purification. Ligand binding to membrane protein was recently found to induce nanometer-scale cell membrane deformations, which can be monitored with real-time optical imaging to quantify ligand/protein binding kinetics. However, the insight into this phenomenon has still not been fully understood. We hypothesize that ligand binding can change membrane stiffness, which induces membrane deformation. To investigate this, cell height and membrane stiffness changes upon ligand binding are measured using atomic force microscopy (AFM). Wheat germ agglutinin (WGA) is used as a model ligand that binds to the cell surface glycoprotein. The changes in cell membrane stiffness and cell height upon ligand bindings are determined for three different cell lines (human A431, HeLa, and rat RBL-2H3) on two different substrates. AFM results show that cells become stiffer with increased height after WGA modification for all cases studied. The increase in cell membrane stiffness is further confirmed by plasmonic scattering microscopy, which shows an increased cell spring constant upon WGA binding. Therefore, this study provides direct experimental evidence that the membrane stiffness changes are directly correlated with ligand binding-induced cell membrane deformation.

Measuring Single Bacterial Viability in Optical Traps with a Power Sweeping Technique

Assessing bacterial viability is crucial in public health, food safety, environmental microbiology, and other relevant fields. The classical agar plate counting method and the popular dye-based assays have shown their strengths, but they also have limitations including high time consumption, relatively complex sample preparations, and cytotoxicity. In this work, we present a new bacterial viability assay based on optical tweezers utilizing a power sweeping strategy. By monitoring and analyzing bacterial nanomotion in optical traps under different trapping laser powers, the slope of the proportionality between the quantified extent of motion and the trapping laser power was defined as the mobility restriction coefficient (MRC) to quantify bacterial viability. We first established a firm correlation between the viability and MRC by measuring alive and dead Escherichia coli and Photobacterium phosphoreum. Then the capability of real-time long-term characterization of the assay was validated by measuring the viability of individual P. phosphoreum while regulating the viability with an inactivation light. Notably, a 'spinning-induced stabilization' mechanism was proposed to explain the surprising increase of apparent bacterial mobility after inactivation. Overall, the assay was proved to be a reliable label-free bacterial viability assay at a single-cell level, which holds potential in antibiotic susceptibility testing, drug screening, and rapid diagnostics.