ABSTRACT
As a step toward the elucidation of the biological significance of the isoprenic chains found ubiquitously in single-chain lipids involved in electron and energy transfer of chloroplasts and mitochondria, we undertook a comparative study of the incorporation of chlorophyll a (Chl a) and alpha-tocopherol (alpha T) in unilamellar phosphatidylcholine (PC) vesicles. We observed that while Chl alpha is added to the PC bilayers in a simple, almost linear way, the inclusion of alpha T is a sigmoid-like function characterized first by a slow variation of alpha T incorporation followed by a rather steep increase till saturation occurs at initial mol% alpha T of approx. 5%. Owing to the likeness of the isoprenic chains of Chl a and alpha T the simplest interpretation of the data is that the different mixing behaviours of the two lipids in PC vesicles are due to the special arrangement of the tetrapyrrole macrocycle and the chromanol ring in the lipid bilayers. However, no satisfactory explanation of the mechanisms involved can be given as yet. One possibility is that the inclusion of Chl a and alpha T in the membranes results from adsorption differences brought about by the extent of penetration of the molecule into the monolayers of the unilamellar vesicles.
