The Impact of Model Peptides on Structural and Dynamic Properties of Egg Yolk Lecithin Liposomes - Experimental and DFT Studies

Electron spin resonance (ESR), H-1-NMR, voltage and resistance experiments were performed to explore structural and dynamic changes of Egg Yolk Lecithin (EYL) bilayer upon addition of model peptides. Two of them are phenylalanine (Phe) derivatives, Ac-Phe-NHMe (1) and Ac-Phe-NMe2 (2), and the third one, Ac-(Z)-Phe-NMe2 (3), is a derivative of (Z)-,-dehydrophenylalanine. The ESR results revealed that all compounds reduced the fluidity of liposome's membrane, and the highest activity was observed for compound 2 with N-methylated C-terminal amide bond (Ac-Phe-NMe2). This compound, being the most hydrophobic, penetrates easily through biological membranes. This was also observed in voltage and resistance studies. H-1-NMR studies provided a sound evidence on H-bond interactions between the studied diamides and lecithin polar head. The most significant changes in H-atom chemical shifts and spin-lattice relaxation times T-1 were observed for compound 1. Our experimental studies were supported by theoretical calculations. Complexes EYLAc-Phe-NMe2 and EYLAc-(Z)-Phe-NMe2, stabilized by NHO or/and CHO H-bonds were created and optimized at M06-2X/6-31G(d) level of theory in vacuo and in H2O environment. According to our molecular-modeling studies, the most probable lecithin site of H-bond interaction with studied diamides is the negatively charged O-atom in phosphate group which acts as H-atom acceptor. Moreover, the highest binding energy to hydrocarbon chains were observed in the case of Ac-Phe-NMe2 (2).