An expression for the C-C bond order parameter, SCC, of membrane hydrocarbon chains has been derived from the observed C-D bond order parameters. It allows calculation of the probability of each of the C-C bond rotamers and, consequently, the number of gauche defects per chain as well as their projected average length onto the bilayer normal, thus affording the calculation of accurate hydrophobic bilayer thicknesses. The effect of temperature has been studied on dilauroyl-, dimyristoyl-, and dipalmitoylphosphatidylcholine (DLPC, DMPC, DPPC) membranes, as has the effect of cholesterol on DMPC. The salient results are as follows: 1) an odd-even effect is observed for the SCC versus carbon position, k, whose amplitude increases with temperature; 2) calculation of SCC, from nonequivalent deuterons on the sn-2 chain of lipids, SCC2, leads to negative values, indicating the tendency for the C1-C2 bond to be oriented parallel to the bilayer surface; this bond becomes more parallel to the surface as the temperature increases or when cholesterol is added; 3) calculation on the sn-2 chain length can be performed from C1 to Cn, where n is the number of carbon atoms in the chain, and leads to 10.4, 12.2, and 13.8 A for DLPC, DMPC, and DPPC close to the transition temperature, TC, of each of the systems and to 9.4, 10.9, and 12.6 for T-TC = 30-40 degrees C, respectively; 4) separation of intra- and intermolecular motions allows quantitation of the number of gauche defects per chain, which is equal to 1.9, 2.7, and 3.5 for DLPC, DMPC, and DPPC near TC and to 2.7, 3.5, and 4.4 at T-TC = 30-40 degrees C, respectively. Finally, the validity of our model is discussed and compared with previously published models.

The effect of sterols from mammals, plants, fungi, and bacteria on model and natural membrane dynamics are reviewed, in the frame of ordering-disordering properties of membranes. It is shown that all sterols share a common property: the ability to regulate dynamics in order to maintain membranes in a microfluid state where it can convey important biological processes. Depending on the sterol class, this property is modulated by molecular modifications that have occurred during evolution. The role of sterols in rafts, antibiotic complexes, and in protecting membranes from the destructive action of amphipathic toxins is also discussed.

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Phospholipid head group dynamics have been studied by pulsed phosphorus-31 nuclear magnetic resonance (31P-NMR) of unoriented and macroscopically aligned dimyristoylphosphatidylcholine model membranes in the temperature range, 203-343 K. Lineshapes and echo intensities have been recorded as a function of interpulse delay times, temperature and macroscopic orientation of the bilayer normal with respect to the magnetic field. The dipolar proton-phosphorus (1H-31P) contribution to the transverse relaxation time, T2E, and to lineshapes was eliminated by means of a proton spin-lock sequence. In case of longitudinal spin relaxation, T1Z, the amount of dipolar coupling was evaluated by measuring the maximum nuclear Overhauser enhancement. Hence, the results could be analyzed by considering chemical shift anisotropy as the only relaxation mechanism. The presence of various minima both in T1Z and T2E temperature plots as well as the angular dependence of these relaxation times allowed description of the dynamics of the phosphate head group in the 31P-NMR time window, by three different motional classes, i.e., intramolecular, intermolecular and collective motions. The intramolecular motions consist of two hindered rotations and one free rotation around the bonds linking the phosphate head group to the glycerol backbone. These motions are the fastest in the hierarchy of time with correlation times varying from less than 10(-12) to 10(-6) s in the temperature range investigated. The intermolecular motions are assigned to phospholipid long axis rotation and fluctuation. They have correlation times ranging from 10(-11) s at high temperatures to 10(-3) s at low temperatures. The slowest motion affecting the 31P-NMR observables is assigned to viscoelastic modes, i.e., so called order director fluctuations and is only detected at high temperatures, above the main transition in pulse frequency dependent T2ECP experiments. Comprehensive analysis of the phosphate head group dynamics is achieved by a dynamic NMR model based on the stochastic Liouville equation. In addition to correlation times, this analysis provides activation energies and order parameters for the various motions, and a value for the bilayer elastic constant.

The African parasite Trypanosoma brucei gambiense accounts for 97% of human sleeping sickness cases. T. b. gambiense resists the specific human innate immunity acting against several other tsetse-fly-transmitted trypanosome species such as T. b. brucei, the causative agent of nagana disease in cattle. Human immunity to some African trypanosomes is due to two serum complexes designated trypanolytic factors (TLF-1 and -2), which both contain haptoglobin-related protein (HPR) and apolipoprotein LI (APOL1). Whereas HPR association with haemoglobin (Hb) allows TLF-1 binding and uptake via the trypanosome receptor TbHpHbR (ref. 5), TLF-2 enters trypanosomes independently of TbHpHbR (refs 4, 5). APOL1 kills trypanosomes after insertion into endosomal/lysosomal membranes. Here we report that T. b. gambiense resists TLFs via a hydrophobic β-sheet of the T. b. gambiense-specific glycoprotein (TgsGP), which prevents APOL1 toxicity and induces stiffening of membranes upon interaction with lipids. Two additional features contribute to resistance to TLFs: reduction of sensitivity to APOL1 requiring cysteine protease activity, and TbHpHbR inactivation due to a L210S substitution. According to such a multifactorial defence mechanism, transgenic expression of T. b. brucei TbHpHbR in T. b. gambiense did not cause parasite lysis in normal human serum. However, these transgenic parasites were killed in hypohaptoglobinaemic serum, after high TLF-1 uptake in the absence of haptoglobin (Hp) that competes for Hb and receptor binding. TbHpHbR inactivation preventing high APOL1 loading in hypohaptoglobinaemic serum may have evolved because of the overlapping endemic area of T. b. gambiense infection and malaria, the main cause of haemolysis-induced hypohaptoglobinaemia in western and central Africa.

The interactions between the B3 (catechin-4alpha,8-catechin) red wine tannin and the human salivary protein fragment IB7(14) (SPPGKPQGPPPQGG) were monitored by (1)H magic angle spinning NMR, circular dichroism, electrospray ionization mass spectrometry, and molecular modeling. It is found that the secondary structure of IB7(14) is made of a type II helix (collagen helix) and random coil. The central glycine 8 appears to act as a flexible rotula separating two helix II regions. Three tannin molecules tightly complex the peptide, without modifying its secondary structure, but seem to reduce its conformational dynamics. The binding dissociation constant is in the millimolar range. B3 tannins with a "tweezers" conformation bind to the hydrophilic side of the saliva peptide, suggesting that the principal driving forces toward association are governed by hydrogen bonding between the carbonyl functions of proline residues and both the phenol and catechol OH groups. These findings are further discussed in the frame of an astringency phenomenon.

Proton decoupled deuterium NMR spectra of oriented bilayers made of DMPC and 30 mol % deuterated cholesterol acquired at 76.8 MHz (30 degreesC) have provided a set of very accurate quadrupolar splitting for eight C-D bonds of cholesterol. Due to the new precision of the experimental data, the original analysis by. Biochemistry. 23:6062-6071) had to be reconsidered. We performed a systematic study of the influence on the precision and uniqueness of the data-fitting procedure of: (i) the coordinates derived from x-ray, neutron scattering, or force field-minimized structures, (ii) internal mobility, (iii) the axial symmetry hypothesis, and (iv) the knowledge of some quadrupolar splitting assignments. Good agreement between experiment and theory could be obtained only with the neutron scattering structure, for which both axial symmetry hypothesis and full order parameter matrix analysis gave satisfactory results. Finally, this work revealed an average orientation of cholesterol slightly different from those previously published and, most importantly, a molecular order parameter equal to 0.95 +/- 0.01, instead of 0.79 +/- 0.03 previously found for the same system at 30 degreesC. Temperature dependence in the 20-50 degreesC range shows a constant average orientation and a monotonous decrease of cholesterol Smol, with a slope of -0.0016 K-1. A molecular order parameter of 0.89 +/- 0.01 at 30 degreesC was determined for a DMPC/16 mol % of cholesterol.

Mixtures of dimyristoylphosphatidylcholine (DMPC) and dicaproylphosphatidylcholine (DCPC) were investigated by solid-state 31P and 2H NMR. By variation of the mole fraction, X, of DMPC in DCPC, the temperature, and the water content, the conditions at which small discoidal particles, the so-called “bicelles”, self-orient in the magnetic field were determined. The bicellar region has an ellipsoidal shape and is delineated by compositions and temperatures ranging from X = 65−87% and T = 25−45 °C, at 80% hydration in the presence of 100 mM KCl. In the absence of salt, the bicelles still orient in the field but for a narrower range of composition (X ≈ 72−87%). The degree of macroscopic orientation increases in the presence of salt. Reducing hydration to 40% increases the breadth of the bicelle domain. On the contrary, above 95% water content, bicelles are no longer detected by NMR. In the bicelle domain, water shows residual ordering which linearly increases with the decrease in water content. Analysis of the water behavior in terms of bound and free water suggests that water bound to bicelles has the same surface properties as in pure lipid membranes and that it promotes swelling. Perdeuteration of the DMPC lipid chains indicates a reduction in local ordering in the bicelle core, relative to pure lipid dispersions, for corresponding temperatures. As a consequence, the bilayer would be slightly thinner in the bicelle.