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
, of the gel-to-fluid phase transition is promoted by long PLL, whereas no change in T
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
, a transition from β-sheet (T<T
) to random coil (T>T
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