The effect of temperature and lipid on the conformational transition of gramicidin A in lipid vesicles

The Effect of Calcium and Halide Ions on the Gramicidin A Molecular State and Antimicrobial Activity

Gramicidin A (gA) forms several convertible conformations in different environments. In this study, we investigated the effect of calcium halides on the molecular state and antimicrobial activity of gramicidin A. The molecular state of gramicidin A is highly affected by the concentration of calcium salt and the type of halide anion. Gramicidin A can exist in two states that can be characterized by circular dichroism (CD), mass, nuclear magnetic resonance (NMR) and fluorescence spectroscopy. In State 1, the main molecular state of gramicidin A is as a dimer, and the addition of calcium salt can convert a mixture of four species into a single species, which is possibly a left-handed parallel double helix. In State 2, the addition of calcium halides drives gramicidin A dissociation and denaturation from a structured dimer into a rapid equilibrium of structured/unstructured monomer. We found that the abilities of dissociation and denaturation were highly dependent on the type of halide anion. The dissociation ability of calcium halides may play a vital role in the antimicrobial activity, as the structured monomeric form had the highest antimicrobial activity. Herein, our study demonstrated that the molecular state was correlated with the antimicrobial activity.

Electric field driven changes of a gramicidin containing lipid bilayer supported on a Au(111) surface

Langmuir-Blodgett and Langmuir-Schaeffer methods were employed to deposit a mixed bilayer consisting of 90% of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 10% of gramicidin (GD), a short 15 residue ion channel forming peptide, onto a Au(111) electrode surface. This architecture allowed us to investigate the effect of the electrostatic potential applied to the electrode on the orientation and conformation of DMPC molecules in the bilayer containing the ion channel. The charge density data were determined from chronocoulometry experiments. The electric field and the potential across the membrane were determined through the use of charge density curves. The magnitudes of potentials across the gold-supported biomimetic membrane were comparable to the transmembrane potential acting on a natural membrane. The information regarding the orientation and conformation of DMPC and GD molecules in the bilayer was obtained from photon polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) measurements. The results show that the bilayer is adsorbed, in direct contact with the metal surface, when the potential across the interface is more positive than -0.4 V and is lifted from the gold surface when the potential across the interface is more negative than -0.4 V. This change in the state of the bilayer has a significant impact on the orientation and conformation of the phospholipid and gramicidin molecules. The potential induced changes in the membrane containing peptide were compared to the changes in the structure of the pure DMPC bilayer determined in earlier studies.