Interaction between lipids and antimicrobial oligomers studied by solid-state NMR

Abstract

Antimicrobial peptides and their synthetic analogues are well known to interact with the cell membrane, which has complex distributions of lipids. The phase behavior of DOPE/DOPG mixed lipids and the interaction between the lipids and several synthetic amphiphilic antimicrobial oligomers (AMOs) were studied by solid-state nuclear magnetic resonance (NMR). A phase diagram of the lipids over a broad window of water content was constructed. There are large areas in the phase diagram where multiple phases coexist, and the fraction of each phase at a given state is dependent on the sample's preparation and thermal history. The comparable stability of the different phases implies that even slight changes in the lipid condition could result in substantial changes to the phase structure, which may be utilized by living organisms to achieve many membrane functions. Nuclear Overhauser spectroscopy (NOESY) and several other NMR experiments indicated that the AMO primarily resides in the head group region of the lipids and that DOPE, the negative intrinsic curvature lipid, does not selectively enrich in the inverted hexagonal phase.

Figure 1

Phase diagram of DOPE/DOPG (80/20) mixed lipids detected by ³¹P wide-line NMR spectra. The experimental data are represented by large symbols. The continuous areas in which each phase is found are indicated by different shades (dots: L_α; small circles: H_II; grids: Q_II). The boundaries are approximate and only serve as a guide to the eye.

Figure 2

Effect of AMO1, AMO2, and DMSO on the DOPE/DOPG (80/20) mixed lipid phase structure; (a) lipid + DMSO only (same amount of DMSO as that in b and c), (b) lipid + small dosage of AMO1/DMSO solution, (c) lipid + small dosage of AMO2/DMSO solution, (d) lipid + DMSO only (same amount of DMSO as that in e and f), (e) lipid + large dosage of AMO1/DMSO solution, (f) lipid + large dosage of AMO2/DMSO solution. The lipid/AMO ratio is 220:1 (mol/mol) in (b) and (c) and is 20:1 in (e) and (f).

Figure 3

NOESY spectra of DOPE/DOPG mixed lipid (75/25) at 302 K. The MAS speed was 9 kHz. Mixing times are indicated on each spectrum.

Figure 4

(a) Wide-line and (b) MAS spectra of a mixed lipid sample in the L_α/H_II coexisting state; (c) expanded view of the center band in (b).

Figure 5

PG/PE peak intensity ratios of the four largest sidebands (including the center band). The experimental results for a PE/PG = 75/25 (wt/wt; or 24/76 (mol/mol)) mixed lipid (experimental series) are very close to the lipid formulation (indicated by the dotted line). If the PE composition (mol/mol) in the H_II phase increased to 80% (hypothetical series), the sideband intensity ratios would be substantially different from the experimental results. The error bars represents 5% of the peak intensity ratios.