Abstract
The binding of inorganic mercury Hg(II) to phospholipid headgroups has been investigated by phosphorus-31 nuclear magnetic resonance of phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylcholine (PC) in water micellar and multilamellar phases. HgCl2 triggers the aggregation of phospholipid micelles, leading to a lipid-mercury precipitate that is no longer detectable by high-resolution31P-NMR. The remaining signal area corresponds to micelles in the soluble fraction and is a non-linear function of the initial mercury-to-lipid molar ratio. Kinetics of micelle aggregation are exponential for the first 15 min and show a plateau tendency after 120 min. Apparent Hg(H) affinities for phospholipid headgroups are in the order: PE > PS > PC. The same binding specificity is observed when HgCl2 is added to (1:1) mixtures of different micelles (PE + PC; PS + PC). However, mercury binding to mixed micelles prepared with two lipids (PE/PC or PS/PC) induces the aggregation of both lipids. Hg(II) also leads to a31P-NMR chemical shift anisotropy decrease of PC, PS and mixed (1:1) PE/PC multilamellar vesicles and markedly broadens PS spectra. This indicates that HgCl2 binding forces phospholipid headgroups to reorient and that the concomitant network formation leads to a slowing down of PS membrane collective motions. Formation of a gel-like lamellar phase characterized by a broad NMR linewidth is also observed upon HgCl2 binding to PE samples both in fluid (Lα) or hexagonal (HII) phases. The PE hexagonal phase is no longer detected in the presence of HgCl2. Mixed PE/PC dispersions remain in the fluid phase upon mercury addition, indicating that no phase separation occurs. Addition of excess NaCl leads to the appearance of the non-reactive species HgCl sup2−inf4 and induces the reversal of all the above effects.
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Abbreviations
- A(t):
-
time-dependence of peak area
- A40 :
-
peak area at t=40 min
- 1/β:
-
rate of peak area decrease
- Δδ:
-
isotropic chemical shift
- δ:
-
isotropic chemical shift change
- Δσ:
-
chemical shift anisotropy
- DPPC:
-
dipalmitoylphosphatidylcholine
- Hg(II):
-
inorganic mercury
- NMR:
-
nuclear magnetic resonance
- pCl:
-
−log [Cl−]
- PC:
-
phosphatidylcholine
- PE:
-
phosphatidylethanolamine
- PL:
-
phospholipid
- PS:
-
phosphatidylserine
- Ri :
-
mercury-to-lipid molar ratio
- MLV:
-
multilamellar vesicles
- SUV:
-
small unilamellar vesicles
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Girault, L., Lemaire, P., Boudou, A. et al. Interactions of inorganic mercury with phospholipid micelles and model membranes. A31P-NMR study. Eur Biophys J 24, 413–421 (1996). https://doi.org/10.1007/BF00576713
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DOI: https://doi.org/10.1007/BF00576713