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 L_α-H_II transformations and results from a Chla preference for the H_II relative to the L_α phase. Chla disfavors the lamellar liquid crystalline phase L_α and induces its replacement with inverted hexagonal phase H_II, as is consistently demonstrated by DSC and X-ray diffraction measurements on phosphatidylethanolamine (PE) dispersions. Chla lowers the L_α-H_II 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 H_II phase. The reduction of the H_II phase lattice constant in the presence of Chla is an indication that Chla favors H_II phase formation by decreasing the radius of spontaneous monolayer curvature, and not by filling up the interstitial spaces between the H_II phase cylinders. The observed Chla preference for H_II 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.