Evidence for cooperative effects in the binding of polyvalent metal ions to pure phosphatidylcholine bilayer vesicle surfaces

Adela Chrzeszczyk, Arnold Wishnia, Charles S. Springer

Research output: Contribution to journalArticlepeer-review

44 Scopus citations

Abstract

An internal NMR monitor for the study of lanthanide ion (Ln3+) binding to phospholipid bilayer membranes has been developed. The dimethylphosphate anion, DMP-, forms labile complexes with Ln3+ in aqueous solution and in solutions also containing bilayer dispersions. The hyperfine shift in the DMP- resonance induced by Pr3+ ions has been used to determine the overall thermodynamic formation constants for the Pr(DMP)2+ and Pr(DMP)2+ complexes: 81 (M-1) and 349 (M-2) at 52°C; the limiting hyperfine shift (31P) at 52°C is 91.5 ppm downfield. These parameters, applied to the observed DMP- hyperfine shift in the presence of the membrane, establish both the free Pr3+ concentration and the amount of Pr3+ bound to the phospholipid surface. Extensive data for the binding of Pr3+ to the outer surfaces of sonicated vesicles yield a limiting hyperfine shift per Pr3+ of 181.6 ppm downfield for the dipalmitoylphosphatidylcholine 31P resonance at 52°C, clearly demonstrating that the binding stoichiometry is two DPPCs per Pr3+. A Hill analysis indicates that the binding data are more anti-cooperative than a realistic Langmuir isotherm, yet more cooperative than a Stern isotherm incorporating electrostatic considerations at the Debye-Hückel level. Fittings to specific models lead to a cooperative model in which tense (T) sites, with low affinity for Pr3+, present in the absence of metal ions, quickly give way to relaxed (R) sites (two DPPCs per site), with much higher affinity for Pr3+, as the amount of Pr3+ bound to the surface increases. The intrinsic equilibrium constants for the binding of Pr3+ to DPPC vesicles are 2 M-1 and 3 000 M-1 for the T and R sites, respectively, at 52°C. The distribution coefficient between these sites ([R]/[T]) in the absence of Ln3+ is 0.14 at 52°C. We picture the binding site conversion as a head-group conformational change involving mostly the choline moiety. Sketchy results for binding on the inside vesicle surface indicate that the overall affinity for Pr3+ is significantly greater and suggest that the site stoichiometry may be different.

Original languageEnglish (US)
Pages (from-to)28-48
Number of pages21
JournalBBA - Biomembranes
Volume648
Issue number1
DOIs
StatePublished - Oct 20 1981
Externally publishedYes

Keywords

  • Cooperative binding
  • NMR
  • Phosphatidylcholine
  • Phospholipid bilayer
  • Polyvalent metal ion

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Cell Biology

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