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The tilt angle of helical peptides reconstituted in non-oriented membranes can be derived by 15N powder patterns line-shape analysis. This approach allows more physiological-like conditions compared with conventional oriented SS-NMR spectroscopy, but spectral distortions at the isotropic chemical shift compromise the precision and the interpretation of the measure. Our new method, RODEO, recovers the theoretical powder pattern line-shape by ROtor-Directed Exchange of Orientations Cross-Polarization. Firstly, RODEO was tested on designed peptides in unoriented model membranes. Successively, we applied it to the antimicrobial peptide PLAH4 in extracted lipid mixtures, and, for the first time, in vivo, in Escherichia coli.
In the second part of this work, we present some unexpected solid-state NMR, oriented circular dichroism and X-ray scattering results of the antimicrobial peptide LAH4 in the presence of citrate. It was previously shown that the LAH4-helix adopts an in-plane orientation in acidic conditions, while, at neutral pH, the peptide adopts a trans-membrane orientation. In contrast, we found that when citrate buffer is added to regulate the pH at 5, the peptide inserts in a transmembrane manner. Some possible explanations are suggested.