Fig. 3.

¹³C’{³¹P} REDOR data of (a) crystalline POPC at 293 K and (b) TRE-POPC at 263 K. The chemical structure of POPC is shown at the top, with the dashed lines indicating the distances measured here. (a) Best-fit distances for crystalline POPC, whose ¹³C’ spectrum is shown in the inset, are 5.3 Å (dashed line) for the two downfield peaks, and 6.2 Å and 6.7 Å (solid lines) for the two upfield peaks. (b) Inset is the S₀ (solid line) and S (dashed line) spectra of TRE-POPC at t_m = 12.8 ms. Best-fit distances are 5.3 ± 0.6 Å for the downfield peaks and 5.8 ± 1.0 Å and 6.8 ± 1.2 Å for the upfield peaks. Simulations in (a, b) used an intensity scaling factor of 90% to account for the effects of pulse imperfection [15]. (c) ¹³C’-³¹P distance distribution in liquid-crystalline POPC bilayers from MD simulations. The average sn-1 ¹³C’ - ³¹P distance (blue) is 6.2 Å with a standard deviation of 0.5 Å. The average sn-2 ¹³C’-³¹P distance (red) is 5.5 ± 0.5 Å. Black arrows: REDOR-extracted ¹³C’-³¹P distances in crystalline POPC. Green arrows: ¹³C’-³¹P distances in the DMPC crystal structure [11]. (d) ¹³C’-³¹P distance distribution in gel-phase POPC bilayers from MD simulations [12]. The average sn-1 ¹³C’-³¹P distance (blue) is 6.7 ± 0.6 Å. The average sn-2 ¹³C’-³¹P distance (red bars) is 4.7 ± 0.6 Å. Black arrows: measured ¹³C’-³¹P distances in the TRE-POPC membrane.