The effect of POPC acyl chains packing by aromatic amino acid methyl esters investigated by ATR-FTIR combined with QM calculations

Aromatic residues play important roles in modulating the function of proteins and binding of proteins to biomembranes. However, the effect of small molecules which contain aromatic residues on the packing of unsaturated lipid acyl chains and the dynamics properties in membrane bilayers remains unclear. The membrane interactions of a series of amino acid methyl esters have been investigated to establish how different amino acids influence lipid acyl chains packing. Attenuated total reflectance-Fourier transform infrared spectroscopic (ATR-FTIR) measurements show that the CH and CC stretching vibrational bands of 2-oleoyl-1-pamlitoyl-sn-glyecro-3-phosphocholine (POPC) present evident red-shifts when aromatic amino acid methyl esters are mixed into membrane bilayers. Quantum mechanical (QM) calculations further demonstrate that the red shifts of the CH and CC stretching modes for POPC acyl chains are not through a direct interaction between the side chain groups and the HCCH motif but are mainly caused by the conformational change of POPC acyl chains, in which the HCCH motif is packed closed to the other one of an adjacent acyl chain with enhancement by dispersion interactions. The implication of these findings is that the packing of POPC acyl chains can be significantly influenced by aromatic amino acid methyl esters. This work may provide valuable insight into the effect on the lipid acyl chains packing by small molecules interacting with membranes and offer possibilities for the dynamic control of membrane fusion and binding of proteins to the membrane.