Bilayer structural destabilization by low amounts of chlorophyll a

The present study shows that small admixtures of one chlorophyll a (Chla) molecule per several hundred lipid molecules have strong destabilizing effect on lipid bilayers. This effect is clearly displayed in the properties of the Lα–HII transformations and results from a Chla preference for the HII relative to the Lα phase. Chla disfavors the lamellar liquid crystalline phase Lα and induces its replacement with inverted hexagonal phase HII, as is consistently demonstrated by DSC and X-ray diffraction measurements on phosphatidylethanolamine (PE) dispersions. Chla lowers the Lα–HII transition temperature (42 °C) of the fully hydrated dipalmitoleoyl PE (DPoPE) by ∼ 8 °C and ∼ 17 °C at Chla/DPoPE molar ratios of 1:500 and 1:100, respectively. Similar Chla effect was recorded also for dielaidoyl PE dispersions. The lowering of the transition temperature and the accompanying significant loss of transition cooperativity reflect the Chla repartitioning and preference for the HII phase. The reduction of the HII phase lattice constant in the presence of Chla is an indication that Chla favors HII phase formation by decreasing the radius of spontaneous monolayer curvature, and not by filling up the interstitial spaces between the HII phase cylinders. The observed Chla preference for HII phase and the substantial bilayer destabilization in the vicinity of a bilayer-to-nonbilayer phase transformation caused by low Chla concentrations can be of interest as a potential regulatory or membrane-damaging factor.