Motional narrowing of the 2H NMR spectra near the chain melting transition of phospholipid/D2O mixtures

Abstract

The reduction in spectral splitting, or motional narrowing, of the deuterium spectra of D2O/phospholipid mixtures near the main chain melting phase transition was studied for palmitoyloleoylphosphatidylcholine (POPC), palmitoyloleoylphosphatidylethanolamine (POPE) and equimolar mixtures of the two at 10% hydration. For POPC the splitting was about 1700 Hz in both the fluid and gel phases, dropping to zero near the phase transition (as reported previously). For POPE the splitting remained approximately constant above the phase transition. Below the phase transition the spectrum showed a single broad line whose linewidth varied between 100 Hz and 800 Hz. This was interpreted as being due to small domains of water within a weakly hydrated crystal. POPC:POPE (1:1) samples exhibited motional narrowing behaviour similar to that for POPC except that the splitting above the phase transition was approximately twice that below the transition. The relatively broad temperature range (approximately 20 K) of the transition is explained using a simple physical model involving lipid fluctuations near the phase transition.

Hydration, Structure, and Molecular Interactions in the Headgroup Region of Dioleoylphosphatidylcholine Bilayers: An Electron Spin Resonance Study

“…5 and 6) and 10 wt % water (Fig. 7) are very similar to those of the temperature variation of the quadrupolar splitting of interbilayer D 2 O in PCs and PEs (Pope et al, 1981;Hawton and Doane, 1987;Bryant et al, 1992), as shown in Fig. 2.…”

“…They ascribed the small Dn of D 2 O to the fast rotation of the O-D bond of the water molecule around an axis that makes nearly a magic angle with the O-D bond. Similarly, we note that Dn of interbilayer D 2 O in PC and PE membranes containing 10 wt % or less water was reported to be \2 kHz (Bryant et al, 1992;Pope et al, 1981), which is much smaller than the static quadrupolar splitting constant of 170 kHz, indicative of a very low ordering of the O-D bond of water. This is inconsistent with the high ordering of headgroups in bilayers of low hydration, e.g., in 10WT and DH DOPC dispersions that we have observed in our present ESR studies.…”

“…A third observation is that 2 H NMR measurements have shown an ''anomalous disordering'' of the interbilayer water around the main phase transition. That is, the quadrupolar splittings (Dn) of interbilayer D 2 O molecules in PC and PE vesicles near T c show a minimum, which is nearly zero (Pope et al, 1981;Hawton and Doane, 1987;Bryant et al, 1992). The data for DMPC, DPPC, and DMPE are shown in Fig.…”

“…1 in Hawton and Doane (1987)). This anomaly was ascribed to a fluctuation phenomenon (Hawton and Doane, 1987;Bryant et al, 1992) or to a pretransition effect (Pope et al, 1981). However, little is known how, if at all, it is related to the anomalous swelling of the bilayers.…”

Hydration Forces Between Lipid Bilayers: A Theoretical Overview and a Look on Methods Exploring Dehydration

“…4.23, is slightly disturbed, most probably due to admixture effects (Pfeiffer et al 2010), the parameters A and B are very similar to the original values published before. An important limitation of this method should be mentioned; the quadrupolar splitting is reduced at and in the range of the main phase transition, a phenomenon which is called motional narrowing (Bryant et al 1992). The motional narrowing proceeds over a relatively wide temperature ranges (10-20 K) and its temperature dependency can mathematically be described by Eq.…”