Simultaneous assessment of membrane bilayer structure and drug insertion by 19F solid-state NMR

Fluorine-19 is an ideal nucleus for studying biological systems using NMR due to its rarity in biological environments and its favorable magnetic properties. In this work, we used a mixture of monofluorinated palmitic acids (PAs) as tracers to investigate the molecular interaction of the fluorinated drug rosuvastatin in model lipid membranes. More specifically, PAs labeled at the fourth and eighth carbon positions of their acyl chains were coincorporated in phospholipid bilayers to probe different depths of the hydrophobic core. First, the 19F chemical shift anisotropy (CSA), indicative of membrane fluidity, was simultaneously determined for fatty acids (FAs) and the fluorinated drug using either slow magic-angle spinning (MAS) 1D 19F solid-state NMR (SS-NMR) or MAS 2D 19F-19F SS-NMR with CSA recoupling. Membrane heterogeneity and selective partitioning of rosuvastatin into fluid regions could thus be evidenced. We then examined the possibility of mapping intermolecular distances in bilayers, in both the fluid and gel phases, using 19F-19F and 1H-19F correlation experiments by SS-NMR using MAS. Spatial correlations were evidenced between the two PAs in the gel phase, while contacts between the statin and the lipids were detected in the fluid phase. This work paves the way to mapping membrane-active molecules in intact membranes, and stresses the need for new labeling strategies for this purpose.

19F solid-state NMR approaches to probe antimicrobial peptide interactions with membranes in whole cells

To address the global problem of bacterial antibiotic resistance, antimicrobial peptides (AMPs) are considered promising therapeutic candidates due to their broad-spectrum and membrane-lytic activity. As preferential interactions with bacteria are crucial, it is equally important to investigate and understand their impact on eukaryotic cells. In this study, we employed 19F solid-state nuclear magnetic resonance (ssNMR) as a novel approach to examine the interaction of AMPs with whole red blood cells (RBCs). We used RBC ghosts (devoid of hemoglobin) and developed a protocol to label their lipid membranes with palmitic acid (PA) monofluorinated at carbon positions 4, 8, or 14 on the acyl chain, allowing us to probe different locations in model and intact RBC ghost membranes. Our work revealed that changes in the 19F chemical shift anisotropy, monitored through a CF bond order parameter (SCF), can provide insights into lipid bilayer dynamics. This information was also obtained using magic-angle spinning 19F ssNMR spectra with and without 1H decoupling, by studying alterations in the second spectral moment (M2) as well as the 19F isotropic chemical shift, linewidth, T1, and T2 relaxation times. The appearance of an additional isotropic peak with a smaller chemical shift anisotropy, a narrower linewidth, and a shorter T1, induced by the AMP caerin 1.1, supports the presence of high-curvature regions in RBCs indicative of pore formation, analogous to its antimicrobial mechanism. In summary, the straightforward incorporation of monofluorinated FAs and rapid signal acquisition offer promising avenues for the study of whole cells using 19F ssNMR.

Progress in the synthesis of fluorinated phosphatidylcholines for biological applications

Fluorinated phospholipids have attracted a lot of interest over the past 40 years. While mono- and polyfluorinated analogs are mostly designed to be used as ¹⁹F NMR probes, highly fluorinated phospholipids are mainly developed as drug delivery devices and oxygen carriers. This review describes their synthetic pathways, their properties and potential applications.

Scalable Synthesis and Purification of Acetylated Phosphatidyl Choline Headgroup

The acetylated headgroup of the most abundant mammalian phospholipid, 1,2-diacetyl-3-sn-phosphatidyl choline (DAcPC), has several important applications in research. For instance, it can be dissolved in the same amount of water as in the fluid PC bilayer, to create a surrogate of a PC headgroup stratum, for studying solvation of small molecules and the influence of their structure on the process. In contrast to PC derivatives with longer acyl chains, DAcPC does not self-aggregate, rendering the aqueous solution homogeneous and suitable for simplified analyses of interactions of molecules with the headgroups. Several studies have been published where DAcPC was used in a crudely purified form. Here we describe a one-step preparation of DAcPC from commercially available bulk chemicals and purification of the product by crystallization and washing. The process gives a good yield and is easily scalable. The availability of enantiopure, crystalline DAcPC could open the door to more extensive biochemical, pharmacological, and nutritional studies of this interesting chemical.