Fig. 2.

Calculated deuterium NMR spectra of a hypothetical phospholipid molecule deuterated at a single position on a fatty acid chain. The spectra arise from isotropic distributions of bilayer orientations. Inner, pure phospholipid bilayer (f = 0), quadrupole splitting constant q_b = 0.5. Middle, membrane composed of 50% 1:2 complex and 50% phospholipid (f = 0.5). Outer, membrane composed of only 1:2 complex, quadrupole splitting q_b = 1.0. The spectrum in the middle results from chemical exchange of phospholipids between the bound and free states, given by the rate parameter p = 0.636. For the E–DPPC mixture, this value of p corresponds to a kinetic off-rate constant of 10⁵ s^–1. (For the E–DPPC mixture, q_b is ≈25 kHz). The middle spectrum corresponds to rapid exchange, but a residual broadening is apparent because of the finite exchange rate.