Transverse nuclear spin relaxation in phosphatidylcholine bilayers containing gramicidin

Comparison between the dynamics of lipid/gramicidin A systems in the lamellar and hexagonal phases: a solid-state 13C NMR study

We have investigated the effect of gramicidin A on the dynamics of two model membranes: dimyristoylphosphatidylcholine (DMPC) in the lamellar phase at a lipid-to-peptide molar ratio of 10:1 and dioleoylphosphatidylcholine (DOPC) in the hexagonal HII phase at a lipid-to-peptide molar ratio of 5:1. Natural abundance 13C nuclear magnetic resonance (NMR) spectroscopy was used in combination with magic angle spinning to increase the spectral resolution, therefore allowing the different regions of the lipid bilayers to be investigated from the same spectra. 31P NMR was also used to detect and confirm the formation of the DOPC HII phase in the presence of gramicidin A. In order to examine the effect of gramicidin A on both the fast and slow motions of DMPC and DOPC, the 1H spin-lattice relaxation times in the laboratory frame (HT1) as well as the 1H spin-lattice relaxation times in the rotating frame (HT1rho) were calculated for each resolved protonated lipid resonance in the 13C spectra. For both DMPC and DOPC, we found that the presence of gramicidin A does not significantly affect the fast motions of the lipid acyl chains but increases slightly the fast motions of the polar head group. However, the HT1rho are significantly decreased, this effect being more pronounced for DOPC most likely due to a decrease in the rate of the lipid lateral diffusion.

Effect of calcium on phospholipid interaction with pulmonary surfactant protein C

Porcine pulmonary surfactant-associated protein SP-C was incorporated into bilayers of chain-perdeuterated dipalmitoylphosphatidylglycerol (DPPG-d62) and chain-perdeuterated dipalmitoyl-phosphatidylcholine (DPPC-d62) and into bilayers containing 70 mol% dipalmitoyl-phosphatidylcholine (DPPC) and 30 mol% DPPG-d62 or 70 mol% DPPC-d62 and 30 mol% dipalmitoylphosphatidylglycerol (DPPG). The effect of SP-C on the phase behavior, lipid chain order, and dynamics in these bilayers was examined by using deuterium nuclear magnetic resonance. SP-C was found to have a similar effect on the chain order and phase behavior of DPPC-d62 and DPPG-d62 in bilayers with a single lipid component. In gel phase DPPC/DPPG (7:3) bilayers with one or the other lipid component chain-perdeuterated, SP-C was found to affect first spectral moment more strongly for DPPG-d62 than for DPPC-d62. This may indicate that SP-C induced a nonrandom lateral distribution in the mixed lipid bilayer. SP-C was also found to influence motions responsible for deuteron transverse relaxation in both the gel and liquid crystalline phases. The presence of 5 mM Ca2+ in the aqueous phase substantially altered the effect of SP-C on transverse relaxation in the bilayer.

Thermal history alters cholesterol effect on transition of 1-palmitoyl-2-linoleoyl phosphatidylcholine

The effect of cholesterol on the bilayer phase behavior of heteroacid phosphatidylcholines with one unsaturated fatty acid depends on the nature of the unsaturated chain. Previous differential scanning calorimetry (DSC) studies showed that 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (16:0-18:2 PC) had a broad, weak transition at about -18 degrees C, which was effectively eliminated by less than 15 mol% cholesterol. Phospholipids with greater and lesser degrees of unsaturation displayed stronger phase transitions and less sensitivity to cholesterol. In this work, deuterium nuclear magnetic resonance has been used to examine the phase behavior of 1-perdeuteriopalmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (16:0-18:2 PC-d31) alone, and with 15 mol % cholesterol. The behavior is found to be sensitive to sample thermal history. Moderately fast cooling (1 degree/h) results in a continuous phase change from a fluid to an ordered phase in the pure lipid. Under similar cooling conditions, the sample containing cholesterol displays increased chain order and a continuous phase change with no apparent isothermal transition. However, when these systems are cooled at a reduced rate (0.3 degree/h), the continuous phase change is pre-empted by a sharp transition into a more ordered phase that gives a deuterium spectrum having intensity at a value of the quadrupole-splitting characteristic of a rigid lattice system. In the pure lipid, this transition effectively coincides with the center of the continuous phase change. Addition of 15 mol % cholesterol lowers the temperature of this sharp transition by about 3 degrees C. These observations provide some insights into the behavior of this system seen using differential scanning calorimetry. Results of deuteron transverse relaxation measurements under these conditions are also reported.

Effects of hydrostatic pressure on bilayer phase behavior and dynamics of dilauroylphosphatidylcholine

Deuterium nuclear magnetic resonance spectroscopy was used to study the thermotropic phase behavior of dilauroylphosphatidylcholine (DLPC) bilayers at pressures up to 221 MPa. Pressure was found to separate the liquid crystal to gel transition from the gel to ordered crystalline phase transition. The jump in chain order observed on cooling through the transition into the gel phase was found to be small and thus consistent with the trend in longer chain saturated diacyl phosphatidylcholines. On cooling, DLPC was observed to enter an unusual state above the transition into the gel phase. This unusual state displayed fluid-like conformational order but short transverse relaxation times. It was found to be much better pronounced and to span a broader temperature range at elevated pressure than at lower pressures. Transverse relaxation measurements of deuterons on the chain alpha-carbons revealed a substantial slowing of molecular motions within the temperature range of the unusual fluid phase. The observation of such a phase at high pressure appears to be consistent with recent reports of an unusual fluid phase, Lx, in DLPC at ambient pressure.

Hydrostatic pressure-induced conformational changes in phosphatidylcholine headgroups: a 2H NMR study

The effects of pressure and temperature on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dimyristoyl-sn-glycero-3-phosphocholine headgroup conformations were examined using deuterium nuclear magnetic resonance. Isothermal compression was found to produce a decrease in the choline alpha deuteron quadrupole splitting and increases in the choline beta and gamma deuteron quadrupole splittings. A similar counterdirectional change, seen in the presence of positive surface charge, has been attributed to tilting of the headgroup away from the bilayer surface in response to the torque exerted on the phosphocholine dipole by positive surface charges. The direction of the change in headgroup deuteron quadrupole splitting is consistent with the pressure-induced reduction in area per lipid in the liquid crystalline phase, which can be inferred from the ordering of phospholipid acyl chains under comparable conditions. The temperature dependences of the headgroup deuteron quadrupole splittings were also examined. It was found that at elevated pressure, the alpha splitting was insensitive to temperature, whereas the beta and gamma splittings decreased. The response of the beta deuteron splitting to temperature was found to be weaker at elevated pressure than at ambient pressure.

Smoothed acyl chain orientational order parameter profiles in dimyristoylphosphatidylcholine-distearoylphosphatidylcholine mixtures: a 2H-NMR study

The accommodation of chain-length mismatch in liquid crystal phase bilayers was examined by using deuterium nuclear magnetic resonance to obtain smoothed orientational order parameter profiles for acyl chains of both components in binary lipid mixture bilayers. Mixtures of dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) covering a range of compositions were prepared with either DSPC acyl chains or DMPC acyl chains perdeuterated. Orientational order parameters in the plateau regions of the smoothed profiles for both components were found to increase smoothly with increasing DSPC concentration. The orientational order parameters in the DSPC-smoothed profile were found to be slightly higher than corresponding values for DMPC over a wide range of bilayer composition. The shapes of the smoothed profiles for both components were found to be sensitive to bilayer composition. At low DSPC concentration, DSPC methylene deuterons near the bilayer center display a secondary plateau at low orientational order. At high DSPC concentration, the plateau of the DMPC-smoothed profile is stretched slightly. The concentration dependence of the smoothed profiles at low DSPC concentration appears to be consistent with a picture in which the last few segments of the DSPC chain cross the bilayer midplane, on average, but remain very disordered.

The structure of an integral membrane peptide: a deuterium NMR study of gramicidin

Solid state deuterium NMR was employed on oriented multilamellar dispersions consisting of 1,2-dilauryl-sn-glycero-3-phosphatidylcholine and deuterium (2H) exchange-labeled gramicidin D, at a lipid to protein molar ratio (L/P) of 15:1, in order to study the dynamic structure of the channel conformation of gramicidin in a liquid crystalline phase. The corresponding spectra were used to discriminate between several structural models for the channel structure of gramicidin (based on the left- and right-handed beta 6.3 LD helix) and other models based on a structure obtained from high resolution NMR. The oriented spectrum is complicated by the fact that many of the doublets, corresponding to the 20 exchangeable sites, partially overlap. Furthermore, the asymmetry parameter, eta, of the electric field gradient tensor of the amide deuterons is large (approximately 0.2) and many of the amide groups are involved in hydrogen bonding, which is known to affect the quadrupole coupling constant. In order to account for these complications in simulating the spectra in the fast motional regime, an ab initio program called Gaussian 90 was employed, which permitted us to calculate, by quantum mechanical means, the complete electric field gradient tensor for each residue in gramicidin (using two structural models). Our results indicated that the left-handed helical models were inconsistent with our observed spectra, whereas a model based on the high-resolution structure derived by Arseniev and coworkers, but relaxed by a simple energy minimization procedure, was consistent with our observed spectra. The molecular order parameter was then estimated from the motional narrowing assuming the relaxed (right-handed) Arseniev structure. Our resultant order parameter of SZZ = 0.91 translates into an rms angle of 14 degrees, formed by the helix axis and the local bilayer normal. The strong resemblance between our spectra (and also those reported for gramicidin in 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) multilayers) and the spectra of the same peptide incorporated in a lyotropic nematic phase, suggests that the lyotropic nematic phase simulates the local environment of the lipid bilayer.

Chain-length dependence of lipid bilayer properties near the liquid crystal to gel phase transition

The temperature dependence of the mean orientational order parameter in the vicinity of the liquid crystal to gel phase transition is obtained from the first moment M1 of deuterium nuclear magnetic resonance spectra for bilayers of chain perdeuterated phosphatidylcholines with acyl chains of 12, 14, 16, and 18 carbons. The data clearly show an increasing temperature dependence of the orientational order parameter in the vicinity of the transition, with the effect becoming more pronounced with decreasing chain length. Assuming a linear relationship between the mean orientational order parameter and the extension of the acyl chain, estimates of the change in area of the membrane at the transition are shown to be consistent with those obtained from other measurements. It is shown that the transition may be modeled in terms of a Landau expansion of the free energy involving a small number of phenomenological parameters. From this it is shown that the behavior of these systems in the temperature range of interest is, in large part, controlled by the close proximity of a spinodal to the transition temperature.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

The effect of changes in gramicidin conformation on bilayer lipid properties

The effects of two different gramicidin conformations on lipid phase behaviour and dynamics are compared. Samples of chain-perdeuterated dimyristoylphosphatidylcholine containing gramicidin were first prepared with gramicidin in a state having a circular dichroism spectrum generally identified as corresponding to the non-channel conformation. The effects, on bilayer lipid properties, of gramicidin in this conformation were then determined using deuterium nuclear magnetic resonance measurements of acyl chain orientational order and transverse relaxation times as a function of temperature. These samples were then incubated at 65 degrees C to convert the gramicidin to a state with a circular dichroism spectrum of the type generally identified with the channel conformation. The nuclear magnetic resonance measurements were then repeated. In the gel phase, it was found that transverse relaxation time and chain orientational order of the lipid were insensitive to gramicidin conformation. In the liquid crystalline phase, gramicidin in the channel conformation was found to have a slightly larger effect on transverse relaxation and orientational order than gramicidin in the non-channel conformation. The perturbation of the phase behavior by gramicidin was found to be relatively insensitive to gramicidin conformation.

Interaction between perdeuterated dimyristoylphosphatidylcholine and low molecular weight pulmonary surfactant protein SP-C

A low molecular weight hydrophobic protein was isolated from porcine lung lavage fluid using silicic acid and Sephadex LH-20 chromatography. The protein migrated with an apparent molecular weight of 5000-6000 on SDS-PAGE under reducing and nonreducing conditions. Gels run under reducing conditions also showed a minor band migrating with a molecular weight of 12,000. Amino acid compositional analysis and sequencing data suggest that this protein preparation contains intact surfactant protein SP-C and about 30% of truncated SP-C (N-terminal leucine absent). The surfactant protein was combined with perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) in multilamellar vesicles. The protein enhanced the rate of adsorption of the lipid at air-water interfaces. The ability of the protein to alter normal lipid organization was examined by using high-sensitivity differential scanning calorimetry (DSC) and 2H nuclear magnetic resonance spectroscopy (2H NMR). The calorimetric measurements indicated that the protein caused a decrease in the temperature maximum (Tm) and a broadening of the phase transition. At a protein concentration of 8% (w/w), the enthalpy change of transition was reduced to 4.4 kcal/mol compared to 6.3 kcal/mol determined for the pure lipid. NMR spectral moment studies indicated that protein had no effect on lipid chain order in the liquid-crystal phase but reduced orientational order in the gel phase. Two-phase coexistence in the presence of protein was observed over a small temperature range below the pure lipid transition temperature. Spin-lattice relaxation times (T1) were not substantially affected by the protein. Transverse relaxation time (T2e) studies suggest that the protein influences slow lipid motions.