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(alpha)-H(II) transformations and results from a Chla preference for the H(II) relative to the L(alpha) phase. Chla disfavors the lamellar liquid crystalline phase L(alpha) 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(alpha)-H(II) transition temperature (42 degrees C) of the fully hydrated dipalmitoleoyl PE (DPoPE) by approximately 8 degrees C and approximately 17 degrees 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.