Study of Membrane Polymorphism by 31P-NMR

For a phase-separated region to exist, lipids have to move into and out of various phases. The lateral diffusion constant in liquid crystalline bilayers is about l(T8cm2/s, which corresponds to an exchange frequency between lipid-lipid nearest neighbors of about 106/s. A necessary precondition for the detection of phases by NMR technique is that the proportion of observable species in the phase is sufficiently large. 

Not all NMR-active nuclei may therefore be suitable for the study of phase separation in phospholipids. Limited information can be gained on phospholipid phase transformation from 1H- and UC-NMR because of problems in resolution. Only some signals, for example the choline methyl groups of phospholipids in the outer and inner leaflets of unilamellar bilayers, can be identified by 1H- and 13C-NMR when chemical shift reagents are used. However, 13C-NMR can be applied to the study of phospholipid phase transition when the lipid is specifically enriched at the sn-2, carbonyl position [91]. 

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