Mixing of single-chain amphiphiles in two-chain lipid bilayers. 2. Characteristics of chlorophyll a and α-tocopherol incorporation in unilamellar phosphatidylcholine vesicles

Abstract

As a step toward the elucidation of the biological significance of the isoprenic chains found unbiquitously 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 α-tocopherol (αT) in unilamellar phosphatidylcholine (PC) vesicles. We observed that while Chl a is added to the PC bilayers in a simple, almost linear way, the inclusion of αT is a sigmoid-like function characterized first by a slow variation of αT incorporation followed by a rather steep increase till saturation occurs at initial mol% αT of approx. 5%. Owing to the likeness of the isoprenic chains of Chl a and αT 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 αT in the membranes results from adsorption differences brought about by the extent of penetration of the molecules into the monolayers of the unilamellar vesicles.