Deuterium NMR study of intermolecular interactions in lamellar phases containing palmitoyllysophosphatidylcholine

Abstract

Theoretical models of phospholipid systems have indicated that both intramolecular and intermolecular forces are important in governing their acyl chain order. Knowledge of the nature and magnitude of these interactions is central to understanding the balance of forces present in lipid lamellar phases, which in turn is related to their microscopic and macroscopic behavior. It is possible to explore the contribution of intermolecular interactions using lipid systems with the same headgroup and acyl chain identity by variation of the ratio of the headgroups to acyl chains. In this paper, deuterium (2H) NMR spectroscopy has been used to gain information on the orientational order of an acyl chain perdeuterated lipid, l-perdeuteriopalmitoyl-sn-Bly~3-phosphocholine (PaLpc-d31), in various molecular environments. The orientational order of PaLpGd31 was studied in four different lamellar phases, including pure PaLPC-d31 (containing 10 wt % H20), diplmitoylphosphatidyl~holine/PaLPC-d3~ (3:1), palmitic a~id/PaLPC-d~~ (l:l), and ch~lesterol/PaLPC-d~~ (1:l) (each containing 50 wt % HzO). ' H NMR spectra were obtained for the low-temperature and liquid-crystalline (L,) states of each of these mixtures. In the low-temperature state, the f i t three systems yielded ZH NMR spectra characteristic of all-trans chains undergoing axial diffusion, with the methyl groups rotating about their C3 axes. The molecular order, as judged by the presenoe of spectral discontinuities and moment analysis, was found to be almost identical in the low-tempratwe phases. A different behavior was observed for the ch~lesterol/PaLPC-d~~ (1:l) sample in that the maximum splitting was close to the all-trans rotating value, with a profile of quadrupolar splittings due to inmased disorder near the chain ends. The f i t three systems underwent orderdisorder phase transitions near the same midpoint temperature (range of T, values 40-48 "C), whereas the ch~lesterol/PaLPC-d~~ (1:l) sample did not display a transition over the temperature range studied. In the L, phase, where order profdm were determined as a function of acyl chain segmglt position, the segmental ordering d i f f e r e d significantly among the samples. The differences were interpreted using a simple diamond lattice model for the acyl chain configurational statistics, as a means of comparing the effective lengths, (L), projected along the bilayer normal and estimated chain cross-sectional areas, (A), of PaLPC-d31 in the various mixtures. The derived values of (L) and (A) can be understood qualitatively in terms of average packing parameters related to the balance of forces in the headgroup and acyl chain regions, or alternatively the curvature free energy of the membrane lipid-water interface. In lamellar phases of pure P~L P C -C~~~ the curvature stnap is potentially large, and interdigitation of the acyl chains of the appoaed monolayers may occur. However, in mixtures of PaLPC-d31 with 1 , 2 -d i p a l m i t o y l -s n -B l y~~3 -p h o s p~~~e(DPPC), the curvature elaatic stress is apparently...

Chain Length of Free Fatty Acids Influences the Phase Behavior of Stratum Corneum Model Membranes

“…This limited rotational diffusion was reminiscent of that observed from the spectrum of gel-phase DPPC-d 62 bilayers. 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 mixtures were compared at 64 o C, a temperature for which both mixtures existed mainly in a fluid phase (Fig. 3).…”

“…The spectrum recorded at 46 °C was similar to that of pure DPPC-d 62 at 25 °C whereas that at 58 °C resembled to that of DPPC-d 62 in the presence of lysopalmitoylPC (PaLPC) (at 3:l molar ratio) reported in a previous study. 29 In that study, these spectra could be nicely reproduced using as a model an all-trans acyl chains undergoing axial rotation at 2.8 x 10 6 rad/s (440 kHz) for DPPC-d 62 and at 6.8x10 8 rad/s (108 MHz) for DPPC with PaLPC.…”

Area per Lipid and Cholesterol Interactions in Membranes from Separated Local-Field 13 C NMR Spectroscopy

“…Note that the volumetric thickness, D C , and the mean area, hAi, are inversely related by the assumption of constant volume, meaning that the bilayer core has approximately the density of liquid hydrocarbon (77,80). However, D C is not the same as the mean projected acyl length, as discussed by Jansson et al (79). Due to end effects of the acyl chains, the mean travel away from the aqueous interface is less than the distance to the bilayer midplane, as required for the well-established assumption of constant volume to apply (35).…”

“…To avoid complications from chain upturns (35,70), for estimating the average area per lipid instead of the area of the entire hydrocarbon chain, one can consider relatively ordered acyl segments near the headgroup region. For solid-state 13 C NMR (43) the largest RDC corresponds to the plateau region of the 2 H NMR order profiles, where it is reasonable to assume that the segmental cross-sectional area and projected length are inversely correlated, as discussed in Jansson et al (79). The average cross-sectional area per lipid is thus given as (35) …”

Cholesterol-induced suppression of membrane elastic fluctuations at the atomistic level

“…The microscopic observables from 2 H NMR spectroscopy can then be related to the nano- or microstructure of the membrane lipid assembly. Structural quantities of interest for the lamellar state correspond to the mean interfacial area 〈 A 〉, together with the average thickness D C of the bilayer hydrocarbon region, and the mean aqueous distance separating the lamellae (Brown, 1996; Jansson et al, 1992; Mallikarjunaiah et al, 2011; Nagle and Tristram-Nagle, 2000; Pastor et al, 2002; Thurmond et al, 1991). Clearly, the area per lipid molecule plays an important role in molecular dynamics (MD) simulations of lipid membranes (Huber et al, 2002; Klauda et al, 2008a; Klauda et al, 2008b; Klauda et al, 2010; Klauda et al, 2008c; Meinhardt et al, 2013; Pastor et al, 2002; Toppozini et al, 2014; Venable et al, 2014).…”