Krzysztof Górny

This study combines broadband dielectric spectroscopy experiments with molecular dynamics (MD) simulations to investigate the impact of nanoparticle (NP) inclusions on pretransitional phenomena in liquid-crystal (LC) systems, specifically focusing on the relationship between nanoparticles, topological defects, and prenematic behavior. Our experimental results, using SiO_{2}-doped 4-Cyano-4'-pentylbiphenyl composites, demonstrate that while NP additions do not significantly alter the isotropic-nematic transition temperature (T_{c}), prenematic effects exhibit universal behavior, confirmed by identical critical exponents across all samples. This indicates that the fundamental character of prenematic fluctuations remains unperturbed by the nanoparticles at these concentrations. Crucially, the observed systematic decrease in dielectric permittivity with increasing NP concentration is elucidated by MD simulations. These simulations reveal that nanoparticles act as "seeds" for topological defects, specifically forcing the surrounding LC molecules into a "hedgehog" configuration. This static, defect-induced structure leads to a local antiparallel alignment and cancellation of molecular dipoles. This provides a direct microscopic mechanism for the macroscopic dielectric response, successfully bridging the micro-macro scales and highlighting the nanoparticle-induced local ordering as a key factor in modifying the dielectric properties of the composite system.

In this study, we performed molecular dynamics simulations to investigate the lubricating properties of graphene situated between phospholipid membranes. Four membrane models were analyzed: 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) with 10% or 20% admixture of cholesterol, pure DMPC, and pure 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE). The simulations explored how varying the thickness of the water layer and the sliding speeds of graphene affect its interactions with the membranes. The results show that the presence of water and cholesterol significantly reduces the shear stress required to move graphene. In contrast, van der Waals interactions between graphene and lipids depend on membrane composition. These findings contribute to a better understanding of the potential of graphene as a biolubricant in biomedical applications.

The lubrication mechanism in synovial fluid and joints is not yet fully understood. Nevertheless, intermolecular interactions between various neutral and ionic species including large macromolecular systems and simple inorganic ions are the key to understanding the excellent lubrication performance. An important tool for characterizing the intermolecular forces and their structural consequences is molecular dynamics. Albumin is one of the major components in synovial fluid. Its electrostatic properties, including the ability to form molecular complexes, are closely related to pH, solvation, and the presence of ions. In the context of synovial fluid, it is relevant to describe the possible interactions between albumin and hyaluronate, taking into account solution composition effects. In this study, the influence of Na⁺, Mg²⁺, and Ca²⁺ ions on human serum albumin–hyaluronan interactions were examined using molecular dynamics tools. It was established that the presence of divalent cations, and especially Ca²⁺, contributes mostly to the increase of the affinity between hyaluronan and albumin, which is associated with charge compensation in negatively charged hyaluronan and albumin. Furthermore, the most probable binding sites were structurally and energetically characterized. The indicated moieties exhibit a locally positive charge which enables hyaluronate binding (direct and water mediated).

Due to the semi-liquid nature and uneven morphologies of biological membranes, indentation may occur in a range of non-ideal conditions. These conditions are relatively unstudied and may alter the physical characteristics of the process. One of the basic challenges in the construction of nanoindenters is to appropriately align the nanotube tip and approach the membrane at a perpendicular angle. To investigate the impact of deviations from this ideal, we performed non-equilibrium steered molecular dynamics simulations of the indentation of phospholipid membranes by homogeneous CNT and non-homogeneous SiCNT indenters. We used various angles, rates, and modes of indentation, and the withdrawal of the relative indenter out of the membrane in corresponding conditions was simulated.

Hyaluronan is an essential physiological bio macromolecule with different functions. One prominent area is the synovial fluid which exhibits remarkable lubrication properties. However, the synovial fluid is a multi-component system where different macromolecules interact in a synergetic fashion. Within this study we focus on the interaction of hyaluronan and phospholipids, which are thought to play a key role for lubrication. We investigate how the interactions and the association structures formed by hyaluronan (HA) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are influenced by the molecular weight of the bio polymer and the ionic composition of the solution. We combine techniques allowing us to investigate the phase behavior of lipids (differential scanning calorimetry, zeta potential and electrophoretic mobility) with structural investigation (dynamic light scattering, small angle scattering) and theoretical simulations (molecular dynamics). The interaction of hyaluronan and phospholipids depends on the molecular weight, where hyaluronan with lower molecular weight has the strongest interaction. Furthermore, the interaction is increased by the presence of calcium ions. Our simulations show that calcium ions are located close to the carboxylate groups of HA and, by this, reduce the number of formed hydrogen bonds between HA and DPPC. The observed change in the DPPC phase behavior can be attributed to a local charge inversion by calcium ions binding to the carboxylate groups as the binding distribution of hyaluronan and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine is not changed.

Synthesis of graphene in 2004 stimulated wide interest in potential application of 2D materials in catalysis, optoelectronics, biotechnology and construction of sensing devices. In the presented study interactions between graphene sheets and phospholipid bilayers are examined using steered molecular dynamics simulations. Graphene sheets of different sizes were inserted into bilayer and subsequently withdrawn from it at two different rates (1 m/s and 2 m/s). In some cases, nanoindentation led to substantial damage of the phospholipid bilayer, however an effective self-sealing process occurred even after significant degradation. The average force and work, deflection of the membrane during indentation, withdrawal processes, and structural changes caused by moving sheets are discussed. These quantities are utilized to estimate the suitability of graphene sheets for targeted drug delivery or other nanomedicine tools. The results are compared with those obtained for other nanostructures such as homogeneous and heterogeneous nanotubes.