124‐kDa PHYTOCHROME IN MODEL MEMBRANE SYSTEMS: STUDIES OF THE Ii₇₀₀ INTERMEDIATES WITH THE PROTEIN COVALENTLY BOUND TO PREFORMED LIPOSOMES

We have described the covalent binding of 124‐kDa oat phytochrome to large unilamellar liposomes composed of either dioleoyl phosphatidylcholine or dipalmkoyl phosphatidylcholine or soybean lecithin, without affecting the photochromic properties of the protein. These phytochrome‐liposome systems have now been studied by laser flash photolysis. The liposomes, independent of their membrane rigidity (liquid‐crystal vs gel‐like phase), do not influence the ratio and reactivity of the two primary photoproducts, I i 700 ‐ of the red absorbing form of phytochrome, P l Thus, the lifetimes of the I i 700 intermediates and the activation parameters associated with I i 700 I i bl are the same as those measured for nonbound phytochrome in buffer solution. The temperature increase from about 273 K. to 297 K lowers the population of the shorter‐lived I i 700 intermediate to the same extent both in the liposome‐P l and in nonbound P l , whereas it does not affect the relative population of the I i 700 intermediates from non‐bound P l in the presence of 25% ethylene glycol added to the buffer solution (ionic strength 0.17).