Influence of Temperature on Molecular Adsorption and Transport at Liposome Surfaces Studied by Molecular Dynamics Simulations and Second Harmonic Generation Spectroscopy

Molecular simulation-based insights into dye pollutant adsorption: A perspective review

Growing concerns about environmental pollution have highlighted the need for efficient and sustainable methods to remove dye contamination from various ecosystems. In this context, computational methods such as molecular dynamics (MD), Monte Carlo (MC) simulations, quantum mechanics (QM) calculations, and machine learning (ML) methods are powerful tools used to study and predict the adsorption processes of dyes on various adsorbents. These methods provide detailed insights into the molecular interactions and mechanisms involved, which can be crucial for designing efficient adsorption systems. MD simulations, detailing molecular arrangements, predict dyes' adsorption behaviour and interaction energies with adsorbents. They simulate the entire adsorption process, including surface diffusion, solvent layer penetration, and physisorption. QM calculations, especially density functional theory (DFT), determine molecular structures and reactivity descriptors, aiding in understanding adsorption mechanisms. They identify stable adsorption configurations and interactions like hydrogen bonding and electrostatic forces. MC simulations predict equilibrium properties and adsorption energies by sampling molecular configurations. ML methods have proven highly effective in predicting and optimizing dye adsorption processes. These models offer significant advantages over traditional methods, including higher accuracy and the ability to handle complex datasets. These methods optimize adsorption conditions, clarify adsorbent functionalization roles, and predict dye removal efficiency under various conditions. This research explores MD, MC, QM, and ML approaches to connect molecular interactions with macroscopic adsorption phenomena. Probing these techniques provides insights into the dynamics and energetics of dye pollutants on adsorption surfaces. The findings will aid in developing and optimizing new materials for dye removal. This review has significant implications for environmental remediation, offering a comprehensive understanding of adsorption at various scales. Merging microscopic data with macroscopic observations enhances knowledge of dye pollutant adsorption, laying the groundwork for efficient, sustainable removal technologies. Addressing the growing challenges of ecosystem protection, this study contributes to a cleaner, more sustainable future.

Facilitating flip-flop: Structural tuning of molecule-membrane interactions in living bacteria

The first barrier that a small molecule must overcome before trespassing into a living cell is the lipid bilayer surrounding the intracellular content. It is imperative, therefore, to understand how the structure of a small molecule influences its fate in this region. Through the use of second harmonic generation, we show how the differing degrees of ionic headgroups, conjugated system, and branched hydrocarbon tail disparities of a series of four styryl dye molecules influence the propensity to "flip-flop" or to be further organized in the outer leaflet by the membrane. We show here that initial adsorption experiments match previous studies on model systems; however, more complex dynamics are observed over time. Aside from probe molecule structure, these dynamics also vary between cell species and can deviate from trends reported based on model membranes. Specifically, we show here that the membrane composition is an important factor to consider for headgroup-mediated small-molecule dynamics. Overall, the findings presented here on how structural variability of small molecules impacts their initial adsorption and eventual destinations within membranes in the context of living cells could have practical applications in antibiotic and drug adjuvant design.

Monitoring membranes: The exploration of biological bilayers with second harmonic generation

Nature's seemingly controlled chaos in heterogeneous two-dimensional cell membranes stands in stark contrast to the precise, often homogeneous, environment in an experimentalist's flask or carefully designed material system. Yet cell membranes can play a direct role, or serve as inspiration, in all fields of biology, chemistry, physics, and engineering. Our understanding of these ubiquitous structures continues to evolve despite over a century of study largely driven by the application of new technologies. Here, we review the insight afforded by second harmonic generation (SHG), a nonlinear optical technique. From potential measurements to adsorption and diffusion on both model and living systems, SHG complements existing techniques while presenting a large exploratory space for new discoveries.

Understanding the different cross-membrane transport kinetics of two charged molecules on the DOPG lipid surface with second harmonic generation and MD simulation

A clear physical picture of the dynamic behavior of molecules on the surface of the lipid membrane is highly desired and has attracted great attention from researchers. In this study, a step forward in this direction based on previous studies was presented with second harmonic generation (SHG) and molecular dynamic (MD) simulation. Specifically, details on the orientation flipping and cross-membrane transport of two charged molecules, 4-(4-diethylaminostyry)-1-methyl-pyridinium iodide (D289) and malachite green (MG), on the surface of 2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DOPG) lipids were presented. Firstly, the orientation flipping of the two molecules on the surface of lipids before their cross-membrane transport was confirmed by the MD simulation. Then, the concentration dependent rate of the cross membrane transport for MG/D289 was analyzed. It was found that a simplified model could satisfactorily interpret the faster cross-membrane transport of MG under higher bulk concentrations. A different concentration dependent dynamics was observed with D289 and the reason behind it was also discussed. With this investigation, the surface structures and dynamics of D289 and MG on the DOPG lipid surface were clearly presented.

Influence of Acetaminophen on Molecular Adsorption and Transport Properties at Colloidal Liposome Surfaces Studied by Second Harmonic Generation Spectroscopy

Time-resolved second harmonic generation (SHG) spectroscopy is used to investigate acetaminophen (APAP)-induced changes in the adsorption and transport properties of malachite green isothiocyanate (MGITC) dye to the surface of unilamellar 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes in an aqueous colloidal suspension. The adsorption of MGITC to DOPC liposome nanoparticles in water is driven by electrostatic and dipole-dipole interactions between the positively charged MGITC molecules and the zwitterionic phospholipid membranes. The SHG intensity increases as the added MGITC dye concentration is increased, reaching a maximum as the MGITC adsorbate at the DOPC bilayer interface approaches a saturation value. The experimental adsorption isotherms are fit using the modified Langmuir model to obtain the adsorption free energies, adsorption equilibrium constants, and the adsorbate site densities to the DOPC liposomes both with and without APAP. The addition of APAP is shown to increase MGITC adsorption to the liposome interface, resulting in a larger adsorption equilibrium constant and a higher adsorption site density. The MGITC transport times are also measured, showing that APAP decreases the transport rate across the DOPC liposome bilayer, especially at higher MGITC concentrations. Studying molecular interactions at the colloidal liposome interface using SHG spectroscopy provides a detailed foundation for developing potential liposome-based drug-delivery systems.