$^{19}$F NMR of biomembranes

In this chapter we give an overview of the use of 19F NMR for the study of biomembranes. Due to its unique sensitivity and small size, fluorine may be considered to bridge the gap between NMR on the one hand, and ESR and fluorescence on the other hand. Conventional NMR isotope labels do not alter the studied molecules but possess only low sensitivity, while ESR and fluorescence labels provide higher sensitivity but also represent a larger modification of the system of interest. The high sensitivity to its environment, large anisotropies and strong dipolar couplings render 19 F a useful NMR label to probe structure and dynamics in biomembranes, and to study intermolecular interactions. Fluorine has been introduced into numerous constituents of biomembranes, such as lipids, sterols as well as membrane associated peptides and integral membrane proteins. In particular, a growing set of fluorine-labeled amino acids has allowed one to address selected structural information in membrane proteins and peptides. The large anisotropy of 19F NMR interactions in the solid-state has been used to measure molecular orientations with respect to the membrane normal, allowing characterization, for example, of the function of membrane-active peptides. ... mehrDistance measurements exploring the large distance range of 19F have been beneficial in the study of intermolecular interactions, to reveal oligomeric architectures of membrane proteins and/or transmembrane peptides. The sensitivity of the 19F chemical shift to the environment makes it possible to probe the molecular environment and detect even solvent isotope effects, this way revealing conformational changes and solvent/lipid accessibilities. Given the constraints in placing fluorine into biomembranes and embedded molecules, 19F NMR may not be a universal technique, but it certainly provides a powerful tool to answer specific questions regarding the structure and dynamics in biomembranes that would be difficult to solve otherwise.