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 (S_CF), 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 T_1, 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.