Solid‐state NMR spectroscopic studies on the interaction of sorbic acid with phospholipid membranes at different pH levels

Abstract

2H, 31P, and 1H-magic-angle-spinning (MAS) solid-state NMR spectroscopic methods were used to elucidate the interaction between sorbic acid, a widely used weak acid food preservative, and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers under both acidic and neutral pH conditions. The linewidth broadening observed in the 31P NMR powder pattern spectra and the changes in the 31P longitudinal relaxation time (T1) indicate interaction with the phospholipid headgroup upon titration of sorbic acid or decanoic acid into DMPC bilayers over the pH range from 3.0 to 7.4. The peak intensities of sorbic acid decrease upon addition of paramagnetic Mn2+ ions in DMPC bilayers as recorded in the 1H MAS NMR spectra, suggesting that sorbic acid molecules are in close proximity with the membrane/aqueous surface. No significant 2H quadrupolar splitting (Δ γQ) changes are observed in the 2H NMR spectra of DMPC-d54 upon titration of sorbic acid, and the change of pH has a slight effect on Δ γQ, indicating that sorbic acid has weak influence on the orientation order of the DMPC acyl chains in the fluid phase over the pH range from 3.0 to 7.4. This finding is in contrast to the results of the decanoic acid/DMPC-d54 systems, where Δ γQ increases as the concentration of decanoic acid increases. Thus, in the membrane association process, sorbic acids are most likely interacting with the headgroups and shallowly embedded near the top of the phospholipid headgroups, rather than inserting deep into the acyl chains. Thus, antimicrobial mode of action for sorbic acid may be different from that of long-chain fatty acids.

Distinct Effects of Sorbic Acid and Acetic Acid on the Electrophysiology and Metabolism of Bacillus subtilis

“…Future experiments should establish to what extent these and other preservatives have uncoupling properties and what the quantitative effect of ⌬ depletion on the growth rate is. It is likely that their ability to cross the membrane barrier (log K ow ) into the aqueous phase will play a role, as seems to be the case with sorbic acid and acetic acid (42,43).…”

Probing the interaction of Arg9Cys mutated phospholamban with phospholipid bilayers by solid-state NMR spectroscopy

“…2 H quadrupolar splittings of 2 H labels on the entire acyl chain of the POPC-d 31 lipids reflect dynamics of the lipid bilayer hydrophobic region [34, 35, 42], S CD order parameters calculated from the 2 H quadrupolar splittings yield individual 2 HH dynamics along the acyl chain. 2 H nuclei near the membrane surface have larger order parameters than the ones near the membrane core since 2 H nuclei near the membrane surface are more restricted [41].…”

“…Membrane structure and dynamic changes induced by the incorporation of proteins into the phospholipid bilayer can be revealed using 31 P solid-state NMR spectroscopy [33-35]. Also, 2 H solid-state NMR spectroscopy of 2 H labels along the lipid acyl chain can be used to study the dynamics of the hydrophobic region of the membrane [34-42], In this study, interactions between lipid bilayers and WT-PLB, P-PLB and R9C-PLB are investigated from the membrane perspective using 31 P and 2 H solid-state NMR spectroscopy. These experiments shed light onto the loss of functionality of the R9C mutation from the membrane perspective.…”

To kill or not to kill Bacilli: opportunities for food biotechnology

“…Finally, recent NMR data show that sorbic acid interacts mostly with head groups of membrane phospholipids rather than inserting deeply into the membrane [34 ]. This observation suggests that the compound does not have a detergent-like mode of action, but rather interferes with the membrane surface thus possibly modulating the activity of membrane proteins, that is amino-acid permeases similar to the situation observed in yeast [35] and Aspergillus niger [36].…”