Phosphate Ions Promoting Association between Peptide and Modeling Cell Membrane Revealed by Sum Frequency Generation Vibrational Spectroscopy

Phosphate ion is one of the most important anions present in the intracellular and extracellular fluid. It can form strongly hydrogen-bonded and salt-bridged complexes with arginine and lysine to activate the voltage gated channel protein. A molecular-level insight into how the phosphate anions mediate the interaction between peptides and cell membrane is critical to understand membrane-bound peptide actions. In this study, sum frequency generation vibrational spectroscopy (SFG-VS) has been applied to characterize interactions between mastoparan (MP, a G-protein-activating peptide) and different charged lipid bilayers in situ. It is found that phosphate ions can greatly promote the association of MP with lipid bilayers and accelerate the conformation transition of membrane-bound MP from aggregation into alpha-helical structure. In phosphate buffer solution, MP can insert not only into negatively and neutrally charged lipid bilayers but also into positively charged lipid bilayers. In neutrally and negatively charged lipid bilayers, the tilt angle of alpha-helical structure becomes smaller with increasing buffer concentration, while MP adopts a multiple orientation distribution in the positively charged lipid bilayer. MP interacts with lipid bilayers in the salt solution environment most likely by formation of toroidal pores inside the bilayer matrix. Results from our studies will provide insight into the MP action mechanism and offer some ideas to deliver exogenous protein into the cytosol.