Influence of metal ions on phosphatidylcholine–bovine serum albumin model membrane, an FTIR study

FTIR spectroscopy was used to study the interaction of K+, Ca2+ and Eu3+ ions and the Phosphatidylcholine (PC)–bovine serum albumin (BSA) complex. First, a PC–BSA interaction system was constructed. The analytical results of transmission electron microscope (TEM), quasi-elastic light scattering (QELS) techniques and FTIR-ATR spectroscopy indicated that PC molecules interacted with BSA in aqueous solutions. However, IR inspection was limited for aqueous solutions. Solid experimental condition was then employed, and FTIR spectra showed that the PC and BSA molecules incorporated with each other, which could represent their interactions in solutions. Then, the influence of metal ions on PC–BSA system was studied in solid experimental conditions, and FTIR spectroscopy was used in this study. The spectral results showed that: (1) K+, Ca2+ and Eu3+ ions all decreased the rigidities of acyl chains of PC in PC–BSA systems. (2) The interactions between Ca2+, Eu3+ ions and the hydrophilic phosphate ester and carbonyl ester groups of PC were stronger than that of K+ ions, while the influent modes of Ca2+ and Eu3+ ions on these regions were different. (3) When the relative molar content of Eu3+ ions to PC (Ri/p) reached 2, the coordination effect between Eu3+ ions and PO2− groups of PC was saturated. (4) The addition of these ions increased the content of α-helix structures of BSA, and decreased the content of β-turn structures. By comparing these results with the interactions of K+, Ca2+, Eu3+ ions with phospholipid system in the absence of protein, some special characters were discovered in the acyl regions of PC, while their interactions results with the hydrophilic regions of PC were alike. It might be interpreted that these metal ions influenced the acyl chains of PC mediated from BSA molecules, and coordinated directly with the hydrophilic regions of PC. As for biological membrane was a system included both phospholipid and proteins, these characters suggested that phospholipid–protein mixture system could be a better model in the studies of interaction between metal ions and biological membranes using FTIR spectroscopy.