Location of melittin fragment carrying spiropyran in phospholipid bilayer membrane determined by thermal isomerization

Melittin fragments carrying spiropyran were synthesized, and their distribution in phospholipid bilayer membrane was studied by using spiropyran as a probe. Spiropyran was connected to the side chain of a Glu residue (Glu(OSp)), and the residue was replaced for the fourth position of melittin (1–7) fragment (M7Sp). M7Sp showed a high affinity for phospholipid membrane. The spiropyran group of M7Sp was converted to a merocyanin group by UV irradiation, which reduced the amount of the peptide bound to the membrane to the half of the initial amount. The location of the merocyanin group of M7Sp in the membrane was evaluated by the rate of thermal isomerization from merocyanin to spiropyran, which is sensitive to the microenvironment of merocyanin. A large fraction of the merocyanin group isomerized rapidly back to a spiropyran form, indicating that M7Sp is located in a relatively hydrophobic region of the membrane. Although the interaction of the peptide with phospholipid membrane is affected by photoisomerization of the spiropyran substituent, spiropyran was shown to be a useful tool to evaluate the location of the peptide in the lipid membrane.