Photobuforin II, a fluorescent photoswitchable peptide

Antimicrobial peptide buforin II translocates across the cell membrane and binds to DNA. Its sequence is identical to a portion of core histone protein H2A making it a highly charged peptide. Buforin II has a proline residue in the middle of its sequence that creates a helix-hinge-helix motif which has been found to play a key role in its ability to translocate across the cell membrane. To explore the structure-function relationship of this proline residue this study has replaced P11 with a meta-substituted azobenzene amino acid (Z). The resultant peptide, photobuforin II, retained the secondary structure and membrane activity of the naturally occurring peptide while gaining new spectroscopic properties. Photobuforin II can be isomerized from its trans to cis isomer upon irradiation with ultra-violet (UV) light and from its cis to trans isomer upon irradiation with visible (VL). Photobuforin II is also fluorescent with an emission peak at 390 nm. The intrinsic fluorescence of the peptide was used to determine binding to the membrane and to DNA. VL-treated photobuforin II has a 2-fold lower binding constant compared to UV-treated photobuforin and causes 11-fold more membrane leakage in 3:1 POPC:POPG vesicles. Photobuforin II provides insights into the importance of structure function relationships in membrane active peptides while also demonstrating that azobenzene can be used in certain peptide sequences to produce intrinsic fluorescence.