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The bilayer hydrophobic thickness l is estimated from the 2H-NMR spectra of acyl chain perdeuteriated membrane phospholipids. Values obtained from individual quadrupolar splittings and those from the first moment of a powder spectrum are in close agreement. In the fluid (La) phase, the thermal variation of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidic acid (DMPA) at pH 4.2 and 8.2, and DMPC: cholesterol (7:3) bilayer thicknesses are compared; 30% cholesterol increases the thickness of dMPC by approximately 4-5 Å, whatever the temperature. DMPC and DMPA (whatever the pH) have the same hydrophobic thickness near their respective Tc, the gel-to-fluid phase transition temperature, and display a very similar l decrease when increasing the temperature. However, at high temperatures (60°C) DMPC is thinner than DMPA by approximately 1 Å. Steric, electrostatic, and temperature constraints are therefore seen to modulate the biomembrane hydrophobic thickness , as easily estimated by 2H-NMR. [ABSTRACT FROM AUTHOR]

Interactions of calcium ions (Ca2+) or poly-L-lysines (PLL) of different molecular weight with negatively charged model membranes (dimyristoylphosphatidic acid (DMPA) dispersions) have been followed by Raman and deuterium (2H) NMR spectroscopies. Results show that both short (m.v.=3 300 to 4 000) and long (m.w.=180 000 to 260 000) PLL increase the degree of ordering of the lipid acyl chains either in gel or fluid phases. This effect is attributed to a neutralizing action of the PLL positive charges on the phospholipid head group negative charges, which leads to a better chain packing. In addition, a 20 °C upshift in the temperature, Tc, of the gel-to-fluid phase transition is promoted by long PLL, whereas no change in Tc is observed with short PLL. These observations are correlated to temperature-induced conformational changes of the polypeptides. As detected by Raman amide I bands, long PLL retain their β-sheet conformation over all the temperature range of the study, whereas short PLL undergo at about Tc, a transition from β-sheet (TTc). Effect of calcium leads to an increase in ordering such that lipids are in their gel state up to 75 °C. Above this temperature a structural modification occurs which is not yet identified but which cannot be attributed to a conventional gel-to-fluid phase transition. [ABSTRACT FROM AUTHOR]

The actions of bee venom melittin and of delta-lysin from Staphylococcus aureus on membranes have been monitored by solid state deuterium NMR. Despite inducing quite similar effects on dipalmitoylphosphatidylcholine, significant differences can be shown depending on temperature and on the lipid to peptide molar ratio, Ri. In the fluid phases, for moderate amounts of toxins and at temperatures T>Tc (Tc gel to fluid transition temperature), analysis of the quadrupolar splittings in terms of chain ordering indicates that both melittin and delta-lysin similarly disorder the membrane. The addition of greater amounts of toxins (Ri=4), results in the occurrence of very small structures. At temperatures above but close to Tc, melittin preferentially orders the center of the bilayer while delta-lysin promotes ordering throughout the entire bilayer thickness. These effects are interpreted as reflecting different locations of the peptides with respect to the membrane surface. In the gel phase, for lipid to peptide molar ratios >15, melittin induces isotropic lines reflecting the presence of small discoidal structures whereas delta-lysin does not. The ability of both toxins to induce bilayer fragmentation is proposed as a possible step in the direct lysis of membranes. [ABSTRACT FROM AUTHOR]