Modifications of the structure and dynamics of dimyristoylphosphatidic acid model membranes by calcium ions and poly-L-lysines. A Raman and deuterium NMR study

Abstract

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, T c , of the gel-to-fluid phase transition is promoted by long PLL, whereas no change in T c 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 T c , a transition from β-sheet (T<T c ) to random coil (T>T c ).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.Key wordsDMPAmodel membranescalciumpoly-L-lysinesRaman2H-NMR