High resolution 13C NMR spectra on oriented lipid bilayers: from quantifying the various sources of line broadening to performing 2D experiments with 0.2-0.3 ppm resolution in the carbon dimension

¹³C NMR spectra routinely performed on oriented lipid bilayers display linewidth of 1–2 ppm, although T₂ measurements indicate that 0.1–0.2 ppm could be obtained. We have prepared a DMPC – ¹³C₄-cholesterol (7/3) sample, and oriented the lipid bilayers between glass plates so that the bilayer normal makes an angle of 90° (or of the magic angle) with B₀. We have measured T₂s, CSAs, and linewidths for the choline ¹³C--methyl, the cholesterol-C₄ carbons and the lipid head group phosphorus, at both angles and 313 K. The magnetic field distribution within the sample was calculated using the surface current formalism. The line shapes were simulated as a function of B₀ field inhomogeneities and sample mosaic spread. Both effects contribute to the experimental linewidth. Using three signals of different CSA, we have quantified both contributions and measured the mosaic spread accurately. Direct shimming on a sample signal is essential to obtain sharp resonances and ¹³C labelled choline methyl resonance of DMPC is a good candidate for this task. After optimisation of the important parameters (shimming on the choline resonance, mosaic spread of ±0.30° ), ¹³C linewidth of 0.2–0.3 ppm have been obtained. This newly achieved resolution on bilayers oriented at 90°, has allowed to perform two 2D experiments, with a good sensitivity: 2D PELF (correlation of carbon chemical shifts and C-H dipolar couplings) and 2D D-resolved experiment (correlation of carbon chemical shifts and C-C dipolar couplings). A C-C dipolar coupling of 35 ± 2 Hz between the choline methyl carbons was determined.