Non-linear van't Hoff behavior in pulmonary surfactant model membranes

Electron paramagnetic resonance spectroscopy on G-protein-coupled receptors: Adopting strategies from related model systems

Membrane proteins, including ion channels, transporters and G-protein-coupled receptors (GPCRs), play a significant role in various physiological processes. Many of these proteins are difficult to express in large quantities, imposing crucial experimental restrictions. Nevertheless, there is now a wide variety of studies available utilizing electron paramagnetic resonance (EPR) spectroscopic techniques that expand experimental accessibility by using relatively small quantities of protein. Here, we give an overview starting from basic strategies in EPR on membrane proteins with a focus on GPCRs, while emphasizing several applications from recent years. We highlight how the arsenal of EPR-based techniques may provide significant further contributions to understanding the complex molecular machinery and energetic phenomena responsible for seamless workflow in essential biological processes.

Exploring the pH-Induced Functional Phase Space of Human Serum Albumin by EPR Spectroscopy

A systematic study on the self-assembled solution system of human serum albumin (HSA) and paramagnetic doxyl stearic acid (5-DSA and 16-DSA) ligands is reported covering the broad pH range 0.7–12.9, mainly using electron paramagnetic resonance (EPR) methods. It is tested to which extent the pH-induced conformational isomers of HSA reveal themselves in continuous wave (CW) EPR spectra from this spin probing approach in comparison to an established spin-labeling strategy utilizing 3-maleimido proxyl (5-MSL). Most analyses are conducted on empirical levels with robust strategies that allow for the detection of dynamic changes of ligand, as well as protein. Special emphasis has been placed on the EPR spectroscopic detection of a molten globule (MG) state of HSA that is typically found by the fluorescent probe 8-Anilino- naphthalene-1-sulfonic acid (ANS). Moreover, four-pulse double electron-electron resonance (DEER) experiments are conducted and substantiated with dynamic light scattering (DLS) data to determine changes in the solution shape of HSA with pH. All results are ultimately combined in a detailed scheme that describes the pH-induced functional phase space of HSA.

Surface Charge and Overlayer pH Influence the Dynamics of Supported Phospholipid Films

Understanding the thermodynamics and kinetics of interactions between model lipid bilayers and planar supports is of critical importance in the furtherance of biosensing and the creation of biomimetic devices. Evaluating these properties can be accomplished through understanding the diffusional properties of the bilayer constituents. In this report, the dynamics of a model DMPC bilayer supported on a phosphorylated silica surface are studied in the presence and absence of interfacial Ca²⁺ as a function of pH of the aqueous overlayer. The data for this system reveal the importance of the balance of ionic interactions between the interfacial species, and the dependence of the diffusional, kinetic and thermodynamic properties of the system on pH. The thermodynamic data suggest that interactions between the bilayer and surface are mediated enthalpically rather than entropically.

Probing the Nanoscopic Thermodynamic Fingerprint of Paramagnetic Ligands Interacting with Amphiphilic Macromolecules

Self-assembly of macromolecules with ligands is an intricate dynamic process that depends on a wide variety of parameters and forms the basis of many essential biological processes. We elucidate the underlying energetic processes of self-assembly in a model system consisting of amphiphilic core-shell polymers interacting with paramagnetic, amphiphilic ligand molecules from temperature-dependent continuous wave electron paramagnetic resonance (CW EPR) spectroscopy subsequent to spectral simulation. The involved processes as observed from the ligands’ point of view are either based on temperature-dependent association constants (K_A,j,k) or dynamic rotational regime interconversion (IC) constants (K_IC,j,k). The interconversion process describes a transition from Brownian (b₁) towards free (b₂) diffusion of ligand. Both processes exhibit non-linear van’t Hoff (lnK vs. T⁻¹) plots in the temperature range of liquid water and we retrieve decisive dynamic information of the system from the energetic fingerprints of ligands on the nanoscale, especially from the temperature-dependent interconversion heat capacity (∆C°_P,IC).