Calcium mediated interaction of calf-thymus DNA with monolayers of distearoylphosphatidylcholine: a neutron and X-ray reflectivity study

Abstract

X-ray and neutron reflection studies, the latter in conjunction with contrast variation, have been combined to study the interaction of calf thymus DNA (ctDNA) with monolayers of distearoylphosphatidylcholine (DSPC) in the presence of 20 mM Ca²⁺ ions, at the air–liquid interface as a function of surface pressure (10, 20, 30 and 40 mN m⁻¹). Analysis of the X-ray and neutron reflection data showed that, regardless of the surface pressure of the monolayer, a layer of ctDNA was present below the DSPC lipid head groups and that this ctDNA-containing layer (thickness ∼12.5 to 15 Å) was separated from the DSPC head groups by a layer of water of ∼9 Å thickness. The thickness of the ctDNA-containing layer was thinner than that reported for monolayers of cationic lipid at the air–water interface (18–25 Å) although in these monolayers no water layer separating the lipid head groups from the layer containing ctDNA has been reported. At all surface pressures the amount of ctDNA present in the layer was in the range 30–40% by volume. As no significant re-arrangement of the DSPC film was required to accommodate the presence of the ctDNA, this suggests that the distribution of charges in the lipid film matches well the charge spacing of ctDNA. Brewster angle microscopy measurements of DSPC on water in the absence of Ca²⁺ showed the presence of a continuous film containing small, regular shaped domains at all four surface pressures examined. When Ca²⁺ ions were present in the sub-phase, although the film was still continuous, the domains comprising the film were more irregular in appearance while the presence of Ca²⁺ ions and ctDNA resulted in the domains becoming smaller and more regularly packed on the surface.