The interaction of vitamin K1 with phospholipid vesicles

Vitamin K1 is a component of the electron transport chain in chloroplasts and also an activator of carboxylases present in microsomes of different tissues. In order to understand its mechanism of action it is necessary to know its interactions, localization and organization in the phospholipid bilayer. We have studied this question using reconstituted systems of vitamin K1 incorporated in DPPC multibilayer vesicles, using DSC, DPH anisotropy and FT-IR spectroscopy. DSC shows that the pretransition is modified and disappears as the concentration of vitamin K1 in the bilayer increases. The main transition is also affected, with a decrease of Tm and delta H with increasing concentrations of vitamin K1. Furthermore, a second peak appears at high concentrations of vitamin K1, which is indicative of a lateral phase separation of a phase rich in vitamin K1. Fluorescence measurements using DPH show, in agreement with the calorimetric measurements, that the phase transition is shifted to lower temperatures and the anisotropy is increased above Tm but, interestingly, not below Tm. FT-IR spectroscopic measurements are also in good agreement with the calorimetric and fluorescence results, indicating that vitamin K1 induced a broadening and a shift to lower temperatures in the phase transition. It is deduced from the variation of the frequency parameter of the CH2 stretching vibration band with temperature that vitamin K1 perturbs the average number of gauche and all-trans conformers of DPPC, only during the phase transition interval but neither at temperatures above nor below the phase transition. However, the bandwidth parameter of this vibration indicated a perturbation above, but not below, the phase transition. The possible relationship between these observations and those coming from fluorescence depolarization of DPH are discussed. Finally, it is concluded from the observation of the C=O stretching mode that vitamin K1 does not produce a very strong perturbation of the interfacial region of the membrane of the type given by, for example, cholesterol.