Electrostatics, Mechanics, and Dynamics of Biological Interfaces at and out of Equilibrium

Biological interfaces are not just boundaries separating two bulk phases, but also possess structures and functions. The primary aim of this thesis is the quantitative determination of electrostatics, mechanics, and dynamics of biological interfaces in the absence and presence of external perturbation by the combination of various physical techniques. In chapter 4, I investigated the mechanism how the combination of charged surfactants and alcohol synergistically destroys the electrostatic barrier protecting the surface of bacteria on molecular level. To quantitatively model the surface of bacteria, a monolayer of complex glycolipids was prepared at the air-water interface. Structural changes in the direction perpendicular to the membrane were investigated with specular X-ray reflectivity in sub-Å resolution. To determine the concentration profiles of individual ions in the vicinity of the surface in Å resolution, X-ray fluorescence was measured under grazing incidence near the angle of total reflection. The obtained data indicated that the permeation of cationic surfactants into the membrane was blocked by a condensed layer of Ca2+ ions cross-linking the charged saccharide groups. Remarkably, the addition of aromatic alcohol causes a significant increase of roughness between hydrocarbon chains and saccharide headgroups, leading to a disturbance of structural integrity of the membrane. These results demonstrate that the combination of X-ray reflectivity and grazing incidence X-ray fluorescence unravelled, how charged surfactants and alcohol, the main ingredients of sanitisers, synergistically act on the outer surface of bacteria. Chapter 5 focuses on the quantitative investigation of the mechanical properties and non-equilibrium dynamics of living cells on substrates precisely mimicking cellular microenvironments. As biological system, I chose human haematopoietic stem cells in collaboration with Prof. A. D. Ho and Prof. C. Müller-Tidow (Internal Medicine V, University Hospital Heidelberg), and investigated the impact of clinical agents for leukaemia therapy on adhesion function and spatio-temporal dynamics. Area of tight cell contact and adhesion strength were measured by label-free micro-interferometry and a cell-detachment assay, which is based on shockwaves induced by pico-second laser pulses. Dynamics of cells were characterised by power spectral analysis of membrane deformation from live cell imaging data, yielding the energy dissipation due to active deformation. Finally, the spatio-temporal dynamics of cells were theoretically modelled by adding periodic deformation forces and friction to the Ohta-Okuma model for self-propelled, deformable particles. Although this model is not able to include chemical reactions inside the cells, the simulations could describe the deformation and motion of human stem cells in the absence and presence of drugs as modulation of adhesion and active deformation