The effect of the bilayer phase transition on the carbonyl carbon-13 chemical shift anisotropy

Abstract

Proton enhanced (PE), natural abundance carbon-13 magnetic resonance spectra have been obtained of the carbonyl groups in hydrated dispersions of 1,2-dimyristoyl- sn-glycero-3-phosphocholine. A four-fold change in the overall linewidth results on passing from the fluid to crystalline phase. The carbonyl resonance provides a sensitive measure of the changes in mobility experienced by the lipid molecule above and below the phase transition temperature. The spectral shapes derived from both the fluid ( T = 45°C) and crystalline ( T = 15°C) phases indicate that even in the crystalline phase sufficient molecular motion is present to average the chemical shielding tensor. It is suggested that this motion in the L β′ phase is a result of dislocations and packing faults diffusing in the plane of the bilayer. Because of the small size of the chemical shielding interaction (approx. 3 kHz for ω 0 = 22.63 MHz) lipid diffusion coefficients of order 10 −10 cm 2/sec observed in the L β′ phase [1] are effective in averaging the shielding tensor. A comparison is made with the perturbation suffered by the carbonyl groups in the L α phase in the presence of substantial amounts of cholesterol or the polypeptide gramicidin A.